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What She Knew: A Novel
Ask Again, Yes: A Novel
Psychological Bulletin Copyright 2000 by the American Psychological Association, Inc.
2000, Vol. 126, No. 6, 890-909 0033-2909/00/$5.00 DOI: 10.1037//0033-2909.126.6.890
Emotion, Plasticity, Context, and Regulation:
Perspectives From Affective Neuroscience
Richard J. Davidson, Daren C. Jackson, and Ned H. Kalin
University of Wisconsin--Madison
The authors present an overview of the neural bases of emotion. They underscore the role of the
prefrontal cortex (PFC) and amygdala in 2 broad approach- and withdrawal-related emotion systems.
Components and measures of affective style are identified. Emphasis is given to affective chronometry
and a role for the PFC in this process is proposed. Plasticity in the central circuitry of emotion is
considered, and implications of data showing experience-induced changes in the hippocampus for
understanding psychopathology and stress-related symptoms are discussed. Two key forms of affective
plasticity are described--context and regulation. A role for the hippocampus in context-dependent
normal and dysfunctional erootional responding is proposed. Finally, implications of these data for
understanding the impact on neural circuitry of interventions to promote positive affect and on mecha-
nisms that govern health and disease are considered.
Biobehavioral scientists are increasingly recognizing the impor- change using both nonpharmacological and pharmacological in-
tance of emotion for the fundamental tasks of survival and adap- tervention (Davidson, 1994a).
tation (A. R. Damasio, 1994; Ekman & Davidson, 1994; Pinker, In this article, we provide a broad overview of the underlying
1997). Emotion facilitates decision making, has significant influ- biological bases of emotion with a focus on the neural circuitry at
ence on learning and memory, and provides the motivation for the human level. We begin with some historical background to set
critical action in the face of environmental incentives. Emotion is the stage for modern psychophysiological and neuroscientific re-
also the stuff of individual differences. It is a key component, if not search. The subcomponents of emotion that are suggested by these
the major ingredient, for many of the fundamental dimensions of findings are highlighted. Individual differences in affective style
personality and vulnerability factors that govern risk for psycho- are then considered. The different parameters of affective style that
pathology (see Davidson, Abercrombie, Nitschke, & Putnam, can be objectively measured are identified, and some data are
1999). Some of the most impressive evidence for brain plasticity is reviewed to illustrate how these parameters can be assessed. Plas-
emotional learning (LeDoux, 1996). Plasticity in the neural cir- ticity in this circuitry is then described, and the implications of
cuitry underlying emotion is also likely to play an important role these data for understanding the etiology of psychopathology and
in understanding the impact of early environmental factors in stress-related symptoms are discussed. We then focus upon two
influencing later individual differences and risk for psychopathol- key components involved in the determination of affective style
ogy (Meaney et al., 1996). Moreover, plasticity in the neural
and its plasticity-regulation and context. These terms are un-
circuitry underlying emotion can be recruited for therapeutic
packed, and several varieties of regulation are delineated. The role
of context and the neural substrates underlying context-dependent
emotional responding are described. Dysfunctions in context reg-
ulation and their import for understanding certain forms of psy-
Richard J. Davidson, Departments of Psychology and Psychiatry, Uni-
versity of Wisconsin--Madison; Daren C. Jackson, Department of Psy- chopathology are considered. Finally, several issues in this re-
chology, University of Wisconsin--Madison; Ned H. Kalin, Departments search area that will be important for the future are highlighted.
of Psychiatry and Psychology, University of Wisconsin--Madison.
The research from the Laboratory for Affective Neuroscience and the
Wisconsin Center for Affective Science reported in this article was sup- Historical B a c k g r o u n d
ported by National Institute of Mental Health Grants MH40747, MH43454,
MH46792, MH52354, and P50-MH52354, and Training Grant T32- Modern theoretical and experimental interest in the psychology
MH18931; by Research Scientist Award K05-MH00875; and by a grant and biology of emotion may be properly said to have begun with
from the Research Network on Mind-Body Interaction of the John D. and the American psychologist and philosopher William James.
Catherine T. MacArthur Foundation. We thank the members of the Lab- James's theories of the physiological antecedents and underpin-
oratory for Affective Neuroscience and the members of the Wisconsin nings of emotional experience have influenced psychologists
Center for Affective Science for their sustained contributions to this
through the present day, and these theories are still debated in the
research program.
Correspondence concerning this article should be addressed to Richard scientific literature. In his Principles of Psychology, James (1890/
J. Davidson, University of Wisconsin--Madison, Laboratory for Affective 1981) repeated verbatim from an earlier publication what has
Neuroscience, Department of Psychology, 1202 West Johnson Street, become known as the James-Lange theory of emotion. James
Madison, Wisconsin 53706. Electronic mail may be sent to rjdavids@ suggested that "bodily changes follow directly the perception of
facstaff.wisc.edu. the exciting fact, and that our feeling of the same changes as they
SPECIAL ISSUE: EMOTION, PLASTICITY,AND REGULATION 891
occur IS the emotion" (p. 1065, emphasis in original). 1 James's the experience of emotion. More recently, MacLean (1949, 1952)
theoretical account of the nature of emotion experience contains argued that the hippocampal formation (a term that included the
two important components that can be tested empirically. First, the amygdala, an important structure that has figured prominently in
physiological response to a stimulus is antecedent to the emotional many modern-day theories of emotion) plays a paramount role in
experience (the felt emotion); more properly, the physiological emotional experience. MacLean classified brain structures by pu-
response (sensation followed by motor output) in fact provides the tative evolutionary stage: reptilian (essentially, brain stem and
basis for the emotional experience. Without this physiological cerebellum), paleomammalian, and neomammalian. The paleo-
component, there is no emotion. As we shall see, this idea has mammalian brain, dubbed by MacLean the limbic system, includes
altered the course of emotion research in this century, leading to a the septum, amygdala, hypothalamus, anterior thalamic nucleus,
search for autonomic states specific to different emotions. James's tegmentum, hippocampus, and insular cortex (MacLean, 1952,
second premise, also stated unequivocally in the Principles, is that 1993). Although, as pointed out by LeDoux (1987), the concept of
there are no brain centers or circuits, other than the necessary a unified limbic system is undoubtedly a convenient but imprecise
sensory and motor cortical areas, specifically involved in the shorthand for researchers, it has had an enduring influence in their
experience of emotion. A major weakness of this theory is the attempt to better characterize the neural circuits involved in emo-
failure to provide an account of why certain events trigger tional experience and expression. The effect of the theories of
emotion-relevant peripheral changes whereas other events do not. Papez and MacLean on subsequent emotion research has been
The idea that there are no dedicated brain centers for emotion overwhelming. Although many of the specific tenets of these early
was later questioned by Walter Cannon (1927, 1929). Cannon's emotion theories have since proved unsupportable, they have had
criticisms were many, but the most salient may have been that the the salutary effect of spurting researchers to search for the specific
internal organs implicated by James and Lange in physiological and complex neural circuitry involved in emotion. However, be-
reactivity are ill suited to perform the kinds of tasks that the fore turning our attention to more recent theoretical models un-
James-Lange theory requires of them. Visceral organs are incapa- derscoring the importance of central nervous system activity in
ble of providing the complex information that would be necessary emotion, we examine the experimental legacy prompted by the
for fine distinctions in emotional experience, they are largely ideas of William James. Beginning in the middle part of this
nonspecific in response to different kinds of stimuli, and they are century, this legacy took the forms of a search for autonomic
too slow to account for rapid changes in emotional experience. specificity associated with different emotional states and an ongo-
Two experimental refutations of the James-Lange theory were ing discussion concerning the importance of facial feedback in
presented by Cannon. First, stimulation of the viscera does not emotion experience.
necessarily lead to qualitative changes in emotional state. Second, As reviewed above, early experimental work led Cannon to
surgical separation of the viscera from the central nervous system discount James's ideas about physiological antecedence in emo-
does not alter emotional behavior. This latter criticism was tional processes. In an attempt to test James's hypotheses, later
grounded on experiments performed by Cannon and Phillip Bard researchers attempted at least to show physiological specificity for
in which the "sham rage" response was observed in decorticate different emotional states. In a perhaps unwarranted narrowing of
animals (Bard, 1928, 1929). On the basis of further experimenta- James's notions (Ellsworth, 1994), this research has generally
tion and the extant lesion literature, Bard made a case for a neural taken the form of a search for autonomic specificity. There is some
circuit underlying the autonomic nervous system (ANS) responses evidence to support the notion of such specificity, at least for some
seen in sham rage involving the caudal hypothalamus and can- negative emotions such as fear and anger (see, e.g., Ax, 1953;
doventral thalamus. Sham rage was characterized as a disinhibition Schachter, 1957), though this issue has become one of the more
syndrome, with the removal of inhibitory cortex leading to the contentious ones among emotion researchers, as is elaborated
striking autonomic and behavioral changes seen in these animals below. Schacter and Singer's (1962) now famous article reported
(Bard & Mountcastle, 1948). In terms of the present discussion, the results from several experimental manipulations designed to
perhaps the most important aspect of Cannon and Bard's work is assess the relative contributions of bodily feedback and cognitive
the experimental elucidation of specific neural circuits involved in appraisal to the determination of emotional state. The authors
the expression (and presumably, the experience) of emotion, in hypothesized that participants would label their emotional state
direct opposition to James's notion that there are no specific neural following an epinephrine injection according to available cogni-
centers of emotion. The importance of this idea cannot be overes- tions provided by the context manipulated by the experimenter.
timated, although several of Cannon's specific criticisms of the They found this to be the case: Participants who were either
James-Lange hypothesis have since been disproved. uninformed or misinformed about the effects of the injections
The Cannon-Bard hypothesis of the involvement of dience- tended to feel (according to self-report) and behave similarly to
phalic structures in emotion expression and of cortical structures in confederates who acted angry or euphoric; whereas participants
emotion experience sparked other researchers to posit more spe- who were correctly informed regarding the injection effects did not
cific and elaborate neural circuits of emotion. The work of Papez (Schacter & Singer, 1962). Subsequent emotion theorists have thus
has been particularly influential in this regard (see Ledoux, 1987, used the Schacter and Singer study as evidence for the nonspeci-
for a thorough review). Papez theorized that the functions of
ficity of autonomic activity during emotion. This view holds that
central emotion are instantiated in a complex circuit involving the
hypothalamus, anterior thalamic nucleus, hippocampus, and cin-
gulate cortex (Papez, 1937). The hypothalamus was thought to be 1See Ellsworth (1994) for a view of this passage as an exaggerated and
the structure that imbued incoming stimuli with emotional signif- incomplete account of James's thoughts on emotion. Also see James
icance, whereas the cingulate cortex was thought to be involved in (1894/1994) for a rebuttal of his contemporarycritics.
892 DAVIDSON, JACKSON, AND KALIN
emotion is accompanied by undifferentiated physiological arousal action sequelae recruited by emotion. On this view, a primary
that is then attributed to whatever the most salient contextual cue determinant of autonomic patterning is the nature of the action
might be. Arousal plus cognition are the two necessary ingredients plan or pattern recruited by the emotion in question in that specific
that combine to form specific emotions. The specificity in this context.(see Davidson, 1994b, for a more extended discussion of
view is derived from the cognitions and not from the physiological this issue). Some emotions may be associated with a broader range
change (for a review, see Leventhal & Tomarken, 1986). of action sequelae than others and as a result, may be associated
However, Levenson (1992) has recently argued that the with more variable patterns of autonomic discharge, particularly in
Schachter and Singer study was an invalid test of ANS differen- experimental contexts where action is minimized.
tiation because the study design used ANS activity as an indepen- Of particular note in the literature on autonomic correlates of
dent variable (manipulated by epinephrine injection) rather than as emotion is the fact that no systematic, replicable differences that
a dependent variable. Levenson cited several studies employing distinguish between positive and negative affect have been re-
various emotion elicitation techniques, all of which provide some ported, despite the fact that this valence dimension is an extremely
support for ANS differentiation in emotion. In general, ANS salient one and arises in every major conceptual scheme for the
differentiation has been found primarily for negative emotions. structure of emotion (see, e.g., Watson, Wiese, Vaidya, & Telle-
Heart rate acceleration has been seen in states of sadness, anger, gen, 1999). For example, when pictures designed to elicit positive
and fear, whereas heart rate deceleration has been observed in versus negative affect are compared, minimal ANS differences
states of disgust. This pattern of effects has been seen not only in have been observed between these stimulus categories despite the
Westerners but also in participants from the Minangkabau of West fact that the emotion-modulated startle does reliably distinguish
Sumatra, providing support to the hypothesis that some types of between them (Lang, 1995). In the domain of emotion, the most
ANS specificity are not culturally specific (Levenson, Ekman, consistent correlate of autonomic discharge is the arousal or in-
Heider, & Friesen, 1992). tensity dimension of emotion where many investigators have re-
Overall, there is good evidence for some ANS specificity, ported reliable relations between self-reports of affect intensity and
particularly for negative emotions, although not all findings have magnitude of autonomic output, particularly skin conductance
been supportive (cf. Cacioppo, Klein, Berntson, Hatfield, 1993; (Lang, Greenwald, Bradley, & Hamm, 1993). The association
Leventhal & Tomarken, 1986). Stemmler (1989, 1992) has argued between the magnitude of electrodermal change and the intensity
convincinglyfor the use of stricter constructs of ANS specificity in of emotional experience goes back many decades with many
research of this type and for an increased recognition of the reports of it appearing in the 1930s and 1940s and is compellingly
importance of situational variables in ANS activity during emo- summarized by McCurdy (1950).
tional states. According to Stemmler, findings of significantly Following the pioneering work of Tomkins (1962), Ekman,
different physiological profiles following the induction of qualita- Levenson, and Friesen (1983) developed the Directed Facial Ac-
tively different emotional states in differing situational contexts tion Task in which participants are provided with muscle-by-
are not sufficient for claiming physiological emotion specificity muscle instructions that constitute specific facial expressions as-
(Stemmler, 1992). Thus, the same emotion may lead to different sociated with different emotions. Some of the most impressive
physiological profiles depending on the situational variables evidence for autonomic specificity associated with different dis-
within which the emotion is induced, and different emotions may crete emotions has been derived from this task (see, e.g., Leven-
be associated with the same physiological profiles in some in- son, Ekman, & Friesen, 1990). Although there is controversy about
stances. For example, Stemmler (1989) reported that real-life fear the factors responsible for the patterns of autonomic change asso-
induction (listening to the conclusion of Poe's "The Fall of the ciated with different expression types (see, e.g., Boiten, 1996), this
House of Usher" in a darkened room, with appropriately spooky work has been interpreted by some to provide support for the
background music) led to a statistically different autonomic profile feedback ideas championed by James and subsequently by others.
than did a fear imagery task in which participants were asked to Because voluntarily producing certain facial poses recruits auto-
recollect and speak about a frightening personal event. nomic change, many investigators have assumed that such findings
Given the evidence reviewed above on ANS activity and emo- provide strong support for feedback theories (Strack, Martin, &
tion, what can be said about the role of ANS activity in emotion? Stepper, 1988). This logic holds that the autonomic changes are a
The data certainly cannot support a primary role for autonomic consequence of the feedback from the facial musculature. Unfor-
patterning in the determination of emotional experience because tunately, this is not a logical necessity as autonomic changes can
reported emotional experience is far more differentiated than the presumably arise as a function of the efferent patterns of discharge
autonomic changes that have been observed. Moreover, most associated with different facial productions. In others words, there
investigators would agree that peripheral autonomic changes are might be central loops through which the production of facial
simply too coarse to play a causal role in the determination of poses can influence the neural circuitry of emotion and in turn,
emotional experience. Rolls (1999) recently argued that "the pe- emotion-related information processing. (This alternative account
ripheral changes produced during emotion are not sufficiently is described more fully below.) Indeed, this is the general form of
distinct to be able to carry the information which would enable one the explanation preferred by Ekman (1992), who posited a "cen-
to have subtly different emotional feelings to the vast range of tral, hard-wired connection between the motor cortex and other
different stimuli that can produce different emotions" (pp. 71-72). areas of the brain involved in directing the physiological changes
The ANS provides crucial support for action. It mobilizes the that occur during emotion" (p. 65).
resources necessary for different action sequences, both in prepa- There have been a flurry of reports largely in the social psycho-
ration for action and in action itself. It is likely that autonomic logical literature showing effects of manipulating participants'
changes accompanying emotion provide important support for the faces in ways that putatively do not call attention to changes in
emotional expression (Laird et al., 1994; Strack et al., 1988). These Teuber, 1972). These investigations led to proposals about central
studies have examined the impact of such facial manipulations on feedforward mechamsms that provide the brain with "efferent
a variety of different dependent measures including measures of copy" so that peripheral feedback is not required. The classic case
emotional reactivity, mood and mood-related cognition. Although for this was that described by Teuber for eye movements where the
these data are certainly consistent with the feedback ideas ad- frontal eyefields were hypothesized to produce corollary discharge
vanced originally by James, they fail to provide definitive support to visual regions of the brain that inform these areas of an im-
for this position. In particular, in these types of studies, participants pending eye movement. In this way, the visual world does not
must voluntarily maintain certain facial poses. To accomplish such jump around with each eye movement but rather remains stable as
a task, efferent commands to the facial muscles to assume a a function of corollary discharge produced by the frontal eyefields.
particular pose are required. It is entirely possible that the affective Other research by Taub demonstrated that peripheral feedback
changes attributed to feedback from the periphery arise instead from the somatic musculature was not required for an animal to
from central events. This alternative is treated in detail in subse- perform complex movements (Taub et al. 1966).
quent sections. The upshot of this and related work has been to draw attention
If feedback from the periphery were necessary for differentiated to the importance of the central nervous system and to put men-
emotional experience, blocking skeletal-muscular activity would tation back into the brain rather than postulating schemes that
eliminate emotion. There were several very early reports of ani- would enable it to remain in the periphery. LeDoux (1994), for
mals and humans administered neuromuscular blocking agents, example, has forcefully argued that the place to search for differ-
which produce paralysis of the peripheral musculature. Despite entiation is in the brain rather than the periphery, though he
being unable to move their faces or other parts of their skeletal- acknowledged the potential role of peripheral feedback in modu-
musculature, curarized animals given discrete cortical stimulation lating aspects of emotional intensity. Most of the remainder of this
displayed emotion-related autonomic changes that were compara- article is concerned with the examination of the central circuitry of
ble to those observed without curare (Hoff & Green, 1936); drug- emotion.
induced paralysis also failed to affect autonomic aversive condi-
tioning (Girden, 1943). Studying patients with spinal cord lesions
The Central Circuitry of Emotion
to evaluate the role of peripheral feedback in emotion, Hohmann
(1966) reported that such patients described their emotional feel- The Prefrontal Cortex
ings after their injury as being somewhat diminished in intensity,
though overt emotional behavior continued unabated. However, in Though approaching the topic from very different perspectives,
a more recent, larger, and more systematic study of such patients, a growing body of literature is converging on the idea that there
Bermond, Nieuwenhuyse, Fasotti, and Schuerman (1991) found exist two fundamental systems that underlie approach- and
that overall levels of emotional excitability actually increased withdrawal-related emotion and motivation or certain forms of
rather than decreased following injury. These more recent obser- positive and negative affect (Cacioppo & Gardner, 1999; Davidson
vations again call into question a strong form of the feedback & Irwin, 1999b; Gray, 1994; Lang, Bradley, & Cuthbert, 1990;
hypothesis that features peripheral feedback as necessary for the Schnierla, 1959). The precise description of these systems differs
experience of emotion, though central views of human emotion somewhat across investigators, as does the anatomical circuitry
were already receiving some attention in the early part of the that is featured, but the essential elements are quite similar in each
century (see, e.g., Dana, 1921). It may however still be the case of these different proposals. The approach system has been de-
that feedback from the periphery plays a contributory role in scribed by Davidson and Irwin (1999b) as facilitating appetitive
modulating the intensity of emotional experience. behavior and generating particular types of positive affect that are
The effort to place the source of emotional differentiation in the approach-related, such as the emotion occurring as an organism
periphery is reminiscent of a similar project in the cognitive moves closer toward a desired goal. The withdrawal system, on the
domain. Investigators both in the early part of the century (Jacob- other hand, facilitates the withdrawal of an organism from sources
son, 1930; Shaw, 1938) and more recently (McGuigan, 1966, of aversive stimulation and/or organizes appropriate responses to
1997) have attempted to account for complex cognition as covert cues of threat. This system also generates withdrawal-related neg-
speech. By recording muscle activity from the periphery, particu- ative emotions such as disgust and fear. Note that not all negative
larly from the vocal apparatus, they hoped to detect signals that emotions are classified as withdrawal-related. Anger is a case in
would correlate with specific thought content. It is noteworthy that point because it is often associated with approach behavior. Con-
these efforts, in both the cognitive and affective domains, were sistent with this conjecture are data that indicate that anger does
favored during a period in American psychology that was domi- not follow the usual pattern of right-sided prefrontal activation
nated by behaviorism. Placing the source of mental processes in observed during withdrawal-related negative emotion (see, e.g.,
the periphery was something that would be palatable to the behav- Fox & Davidson, 1988; Harmon-Jones & Allen, 1998). A variety
iorism of the time. However, this project never really made much of evidence indicates that the approach and withdrawal systems are
headway, though some limited association between peripheral implemented in partially separable circuits, and it is to this evi-
muscular activity and various measures of thought was obtained dence that we now turn. Our focus is on two key components of
(see McGuigan, 1966, for a review). Just a little later in this this circuitry--the prefrontal cortex (PFC) and the amygdala. For
century, several neuropsychologists and neuroscientists began in- more extensive discussion of this entire circuitry, including other
vestigating the role of peripheral feedback in sensory and motor regions not considered here, see Davidson and Irwin (1999b).
function and whether such feedback was necessary for complex A large corpus of data at both the animal and human levels
sensory and motor functions (Taub, Ellman, & Berman, 1966; implicates various sectors of the PFC in emotion. The PFC is not
a homogeneous zone of tissue but rather has been differentiated on A growing corpus of evidence in normal intact humans is
the basis of both cytoarchitectonic and functional considerations. consistent with the findings derived from the lesion evidence.
The two major subdivisions of the primate PFC most pertinent to Davidson and his colleagues have reported that induced approach-
this discussion are the dorsolateral and orbitofrontal sectors. This related positive and withdrawal-related negative affective states
latter sector overlaps considerably with the ventromedial PFC. shift the asymmetry in prefrontal brain electrical activity in lawful
These sectors of the PFC are distinguished primarily on the basis ways. For example, film-induced disgust and fear increases rela-
of projection zones from the dorsomedial nucleus of the thalamus. tive right-sided prefrontal and anterior temporal activation (David-
The magnocellular, medial portion of the dorsomedial nucleus son, Ekman, Saron, Senulis, & Friesen, 1990), whereas induced
projects to the orbital surface of the PFC (which includes Brod- positive affect elicits an opposite pattern of asymmetric activation.
mann's areas 12 and 13). This sector of the PFC is typically Similar findings have been obtained by others (e.g., Ahem &
referred to as the orbitofrontal cortex and has historically been Schwartz, 1985; Jones & Fox, 1992; Tucker, Stenslie, Roth, &
closely linked with emotion. The parvocellular, lateral portion of Shearer, 1981). In addition, we review in the next section a body
the dorsomedial nucleus projects to the dorsolateral PFC, which of evidence that supports the conclusion that individual differences
has historically been assigned a primary role in short-term or in baseline levels of asymmetric activation in these brain regions
working memory, though it is likely to also participate in aspects predict differences in dispositional affective style. Using an ex-
of emotional processing. In addition, there appear to be important tended picture presentation paradigm designed to evoke longer
functional differences between the left and right sides within each duration changes in mood (Sutton, Davidson, Donzella, Irwin, &
Dottl, 1997), we measured regional glucose metabolism with
of these sectors.
The case for the differential importance of left and right PFC positron emission tomography (PET) to ascertain whether patterns
sectors for emotional processing was first made systematically in of anterior asymmetry similar to those found using electrophysi-
a series of studies of patients with unilateral cortical damage ological measures would be found using this very different and
(Gainotti, 1972; Robinson, Starr; & Price, 1984; Sackheim et al., more precise method of assessing regional brain activity (Sutton,
1982). Each of these studies compared the mood of patients with Ward, et al., 1997). During the production of negative affect, we
observed right-sided increases in metabolic rate in anterior orbital,
unilateral left- or right-sided brain damage and found a greater
inferior frontal, middle, and superior frontal gyri; the production of
incidence of depressive symptoms following left-sided damage. In
positive affect was associated with a pattern of predominantly
most cases, the damage was fairly gross, likely including more
left-sided metabolic increases in the pre- and postcentral gyri.
than one sector of the PFC and often other brain regions as well.
Using PET to measure regional cerebral blood flow, Hugdahl and
The general interpretation that has been placed on these studies is
his colleagues (Hugdahl, 1998; Hugdahl et al., 1995) reported a
that depressive symptoms are increased following left-sided ante-
widespread zone of increased blood flow in the right PFC, includ-
rior PFC damage because this brain territory participates in a
ing the orbitofrontal and dorsolateral cortices and inferior and
circuit that underlies certain forms of positive affect and when
superior cortices, during the extinction phase after learning had
damaged, leads to deficits in the capacity to experience positive
occurred compared with the habituation phase, prior to the pre-
affect, a hallmark feature of depression (Watson et al., 1995).
sentation of the experimental contingencies.
Though most of the extant lesion data are consistent with this
Other investigators have used clinical groups to induce a stron-
general picture (see Robinson & Downhill, 1995, for a review),
ger form of negative affect in the laboratory than is possible with
some inconsistencies have also appeared (see, e.g., Gainotti, Calta-
normal controls. One common strategy for evoking anxiety among
girone, & Zoccolotti, 1993; House, Dennis, Mogridge, Hawton, &
anxious patients in the laboratory is to present them with specific
Warlow, 1990). Davidson (1993) has reviewed these studies in types of stimuli that are known to provoke their anxiety (e.g.,
detail and has addressed a number of critical methodological and pictures of spiders for spider phobics, making a public speech for
conceptual concerns in this literature. The most important of these social phobics). Davidson, Marshall, Tomarken, and Henriques
issues is that according to the diathesis-stress model of anterior (2000), in a study using brain electrical activity measures, have
activation asymmetry proposed by Davidson and colleagues (e.g., recently found that when social phobics anticipate making a public
Davidson, 1995, 1998b; Henriques & Davidson, 1991), individual speech, they show large increases in right-sided anterior activation.
differences in anterior activation asymmetry, whether lesion- Pooling across data from three separate anxiety disordered groups,
induced or functional, represent a diathesis. As such, they alter the Rauch, Savage, Alpert, Fischman, and Jenike (1997) found two
probability that specific forms of emotional reactions will occur in regions of the PFC that were consistently activated across groups:
response to the requisite environmental challenge. In the absence the right inferior PFC and the right medial orbital PFC.
of such a challenge, the pattern of asymmetric activation should The ventromedial PFC has been implicated in the anticipation of
simply reflect a propensity but should not necessarily culminate in future positive and negative affective consequences. The case of
differences in mood or symptoms. Phineas Gage is probably the most well-known in both the neu-
In a recent study with the largest sample size to date (N = 193) rological (Harlow, 1868) and popular (A. R. Damasio, 1994)
for a study of mood sequelae in patients with unilateral lesions, literatures. Gage was injured when a tamping rod used to compress
P. L. Morris, Robinson, Raphael, and Hopwood (1996) found that explosives for the purpose of creating rail passes through moun-
among stroke patients, it was only in those with small-sized lesions tainous regions caused an explosion that sent the rod up through
that the relation between left PFC damage and depressive symp- his cheekbone, into his orbitofrontal cortex and then out of his
toms was observed. It is likely that larger lesions intrude on other skull. Remarkably, he survived the accident, but as has now been
brain territories and mask the relation between left PFC damage well documented, his affective behavior and personality changed
and depression. dramatically. He became quite impulsive and exhibited extreme
forms of emotional instability. More than 100 years later, using the ual's capacity to anticipate future affective outcomes and conse-
preserved skull of Gage, H. Damasio and her colleagues (H. quently results in an inability to guide behavior in an adaptive
Damasio, Grabowski, Frank, Galaburda, & Damasio, 1994) recon- fashion. Such damage is not likely to disrupt an individual's
structed Gage's brain volume and the path of the tamping rod. responding to immediate cues for reward and punishment, only the
Using these image reconstruction methods, H. Damasio et al. were anticipation before and sustainment after an affective cue is pre-
able to demonstrate that the injury Gage sustained was primarily in sented. This proposal can be tested using current neuroimaging
the orbitofrontal/ventromedial PFC. Shimamura (1999) reported methods (e.g., functional magnetic resonance imaging [fMRI]) but
on another case from the mid-1800s, that of the famous photog- has not yet been rigorously evaluated. Note that our conception of
rapher and inventor Eadweard J. Muybridge. In the summer of affective working memory is different from Rolls's (1999) pro-
1860, Muybridge boarded a stage coach in San Francisco bound posal that the orbitofrontal cortex plays a role in the maintenance
for St. Louis. In northeastern Texas, the driver of the coach lost of reinforcement associations. According to his proposal, synaptic
control of the horses, and the coach sped down the side of a modification occurs in the orbitofrontal cortex, which enables the
mountain and crashed. One man on board was killed, and all the organism to retain the reward value of a large number of stimuli.
others were injured. From a variety of evidence in the historical These associations can be stored for long periods of time and
record, Shimamura concluded that Muybridge suffered from or- recalled whenever a learned stimulus is encountered again in the
bitofrontal damage. He showed many of the same affective and future. Our conception of affective working memory involves the
personality changes first described in Phineas Gage. maintenance of actual emotion during periods when emotional
In the modern era, Damasio, Bechara, and colleagues have most stimuli are no longer present. This process is conceptualized as
systematically studied the emotional behavior of patients with playing a critical role in guiding behavior in the absence of
damage to this sector of the PFC. For example, Bechara, Damasio, immediately available incentives.
Damasio, and Anderson (1994) have reported that patients with With regard to the different functional roles of the dorsolateral
bilateral lesions of the ventromedial PFC have difficulty anticipat- and ventromedial sectors of the PFC, Davidson and Irwin (1999b)
ing future positive or negative consequences, although immedi- suggested on the basis of considering both human and animal
ately available rewards and punishments do influence their behav- studies, that the latter sector is most likely involved in the repre-
ior. Such patients show decreased levels of electrodermal activity sentation of elementary positive and negative affective states in the
in anticipation of a risky choice compared with controls, whereas absence of immediately present incentives, whereas the former
controls exhibit such autonomic change before they explicitly sector is most directly involved in the representation of goal states
know that the choice is risky (Bechara, Damasio, Damasio, & Lee, toward which these more elementary positive and negative states
1999; Bechara, Damasio, Tranel, & Damasio, 1997; Bechara, are directed. On the basis of mostly nonhuman primate neurophys-
Tranel, Damasio, & Damasio, 1996). iological data, Rolls (1999) has cogently argued that the orbito-
The findings from the lesion method when effects of small frontal sector of the PFC implements rapid stimulus-reinforcer
unilateral lesions are examined and from neuroimaging studies in association learning and the correction of these associations when
normal participants and patients with anxiety disorders converge the contingencies of reinforcement change. Rolls has also mar-
on the conclusion that increases in right-sided activation in various shaled considerable evidence to support the notion that the orbito-
sectors of the PFC are associated with increased negative affect. 2 frontal cortex is the major zone that represents primary reinforcers.
Less evidence is available for the domain of positive affect be- The animal literature has not systematically examined whether the
cause positive affect is much harder to elicit in the laboratory and representations of reward versus punishment reinforcers are dif-
because of the negativity bias" (see Cacioppo & Gardner, 1999; ferentially lateralized.
Taylor, 1991). This latter phenomenon refers to the general ten-
dency of organisms to react more strongly to negative compared
The Amygdala
with positive stimuli, perhaps as a consequence of evolutionary
pressures to avoid harm. The findings from Bechara et al. (1994) A large corpus of research at the animal--mostly rodent--level
on the effects of ventromedial PFC lesions on the anticipation of has established the importance of the amygdala for emotional
future positive and negative affective consequences are based on processes (see, e.g., Aggleton, 1993; Cahill & McGaugh, 1998;
studies of patients with bilateral lesions. It will be of great interest LeDoux, 1996). Because many reviews of the animal literature
in the future to examine patients with unilateral ventromedial have appeared recently, a detailed description of these studies is
lesions to ascertain whether valence-dependent asymmetric effects not presented here. LeDoux (1993) and his colleagues have mar-
are present for this sector of the PFC as well.
Systematic studies designed to disentangle the specific roles
played by various sectors of the PFC in emotion are lacking. Many z It should be noted that the assessment of asymmetric activations in
theoretical accounts of emotion assign it an important role in neuroimagingstudies has typically not been rigorously performed. Most
image analysismethods reveal voxels of significantactivationwhen com-
guiding action and organizing behavior toward the acquisition of
paring between two conditions and/or groups. If an activation reaches
motivationally significant goals (see, e.g., Frijda, 1994; Levenson,
statistical threshold in one hemisphere but fails to reach threshold in the
1994). This process requires that the organism have some means of opposite hemisphere,it is consideredasymmetric.However, the proper test
representing affect in the absence of immediately present rewards of an asymmetriceffect is through a formal evaluationof the Condition(or
and punishments and other affective incentives. Such a process Group) X Hemisphere interaction. Many of the effects that have been
may be likened to a form of affective working memory. It is likely reported as asymmetricin the literatureprobablywould not survivesuch an
that the PFC plays a key role in this process (see, e.g., Watanabe, interactiontest. See Davidsonand Irwin (1999a, 1999b)for a more detailed
1996). Damage to certain sectors of the PFC impairs an individ- discussion of this issue.
shaled a large corpus of compelling evidence to suggest that the was observed in response to aversive versus neutral stimuli. These
amygdala is necessary for the establishment of conditioned fear. findings suggest that the amygdala might be necessary for the
Whether the amygdala is necessary for the expression of that fear expression of already learned negative affect.
following learning and whether the amygdala is the actual locus of Since 1995, a growing number of studies using PET and fMRI
where the learned information is stored are still matters of some to investigate the role of the amygdala in emotional processes have
controversy (see Cahill, Weinberger, Roozendaal, & McGaugh, begun to appear. Many studies have reported activation of the
1999; Fanselow & LeDoux, 1999). Also not resolved is the extent amygdala detected with either PET or fMRI when anxiety-
to which the amygdala participates in the learning of both negative disordered patients have been exposed to their specific anxiety-
and positive stimulus-incentive associations, as well as whether provoking stimuli compared with control stimuli (see, e.g., Breiter,
there are functional differences between the left and right amyg- Ranch, et al., 1996; Ranch, van der Kolk, Fisler, & Alpert, 1996).
dala (Davidson & Irwin, 1999b). The classic view of amygdala When social phobics were exposed to neutral faces, they showed
damage in nonhuman primates resulting in major affective distur- activation of the amygdala comparable to what was observed in
bances (as expressed in the Kluver-Bucy syndrome where the both the phobics and controls in response to aversive compared
animal exhibits abnormal approach, hyper-orality and sexuality, as with neutral odors (Birbaumer et al., 1998). Consistent with the
well as little fear) is now thought to be a function of damage human lesion data, a number of studies have now reported activa-
elsewhere in the medial temporal lobe. When very selective exci- tion of the amygdala in response to facial expressions of fear
totoxic lesions of the amygdala that preserve fibers of passage are compared with neutral, happy, or disgust control faces (Breiter,
made, nothing resembling the Kluver-Bucy syndrome is observed Etcoff, et al., 1996; P. L. Morris et al., 1996; Phillips et al., 1997).
(Kalin, Shelton, & Davidson, 2000). The upshot of this diverse In the Breiter, Etcoff, et al. (1996) fMRI study, they observed rapid
array of findings is to suggest a more limited role for the amygdala habituation of the amygdala response, which may provide an
in certain forms of emotional learning, though the human data important clue to the time-limited function of the amygdala in the
imply a more heterogeneous contribution than is apparent from the stream of affective information processing. In a recent study,
animal evidence. Whalen and his colleagues (Whalen et al., 1998) observed activa-
Although the number of patients with discrete lesions of the tion of the amygdala in response to masked fear faces that were not
amygdala is small, they have provided unique information on the consciously perceived. Unpleasant compared with neutral and
role of this structure in emotional processing. A number of studies pleasant pictures have also been found to activate the amygdala
have now reported specific impairments in the recognition of facial (Irwin et al., 1996; Lane et al., 1997). Finally, a number of studies
expressions of fear in patients with restricted amygdala damage have reported activation of the amygdala during early phases of
(Adolphs, Damasio, Tranel, & Damasio, 1996; Adolphs, Tranel, aversive conditioning (Buchel, Morris, Dolan, & Friston, 1998;
Damasio, & Damasio, 1995; Broks et al., 1998; Calder et al., LaBar, Gatenby, Gore, LeDoux, & Phelps, 1998; J. S. Morris,
1996). Recognition of facial signs of other emotions was found to
Ohman, & Dolan, 1998). Amygdala activation in response to
be intact. In a study that required participants to make judgments
several other experimental procedures for inducing negative affect,
of trustworthiness and approachability of unfamiliar adults from
including unsolvable anagrams of the sort used to induce learned
facial photographs, patients with bilateral amygdala damage
helplessness (Schneider et al., 1996), aversive olfactory cues (Zald
judged the unfamiliar individuals to be more approachable and
& Pardo, 1997), and aversive gustatory stimuli (Zald, Lee, Fluegel,
trustworthy than did controls (Adolphs, Tranel, & Damasio, 1998).
& Pardo, 1998), has been reported. Other data on individual
Recognition of vocalic signs of fear and anger was found to be
differences in amygdala activation and their relation to affective
impaired in a patient with bilateral amygdala damage (Scott et al.,
style are treated in the next section.
1997), suggesting that this deficit is not restricted to facial expres-
The findings on the role of the amygdala in affective processes
sions. Other researchers (Bechara et al., 1995) have demonstrated
from both the lesion studies and neuroimaging raise a number of
that aversive autonomic conditioning is impaired in a patient with
important questions about the functional significance of amygdala
amygdala damage despite the fact that the patient showed normal
activation and the precise role this structure may play in human
declarative knowledge of the conditioning contingencies. Collec-
tively, these findings from patients with selective bilateral destruc- emotion. One key question is whether the amygdala is implicated
tion of the amygdala suggest specific impairments on tasks that tap in all emotion, negative affect in particular, or fear most specifi-
aspects of negative emotion processing. Most of the studies have cally. Most neuroimaging studies that have induced actual emotion
focused on the perceptual side (i.e., the perception of emotional find greater amygdala activation to negative compared with posi-
cues in contrast to the production of emotion) where the data tive elicitors, though it is also the case that most laboratory-
clearly show the amygdala to be important for the recognition of induced negative affect is more intense than positive affect, despite
cues of threat or danger. The conditioning data also indicate that considerable efforts to insure that they are matched. 3 In a study of
the amygdala may be necessary for acquiring new implicit auto- the effects of cocaine on cocaine addicts, Breiter et al. (1997)
nomic learning of stimulus-punishment contingencies. In one of reported significant deactivation in the amygdala during self-
the few studies to examine the role of the amygdala in the expres-
sion of already learned emotional responses, Angrilli and col-
3 Davidsonet al. (1990) have offered a set of methodologicaldesiderata
leagues (Angrilli et al., 1996) reported on a patient with a benign for physiologicalstudies of emotion. Includedamongthese is the necessity
tumor of the right amygdala in an emotion-modulated startle study. of matching the intensities of positive and negative affect so that any
Among control participants, they observed the well-known effect differencesfoundbetweenthem can be attributedto valenceand not simply
of startle potentiation during the presentation of aversive stimuli. to intensity. Although many investigators recognize the need for this
In the patient with right amygdala damage, no startle potentiation control, it is often difficult to implementand to verify.
reported highs following the administration of cocaine. Of the Affective Style
studies that have examined amygdala activation in response to
facial expressions, all have consistently found greater activation in Davidson (1992, 1998a) has used the term affective style to refer
response to fear compared with other emotional expressions, to the broad range of individual differences in "different subcom-
though a complete range of other emotions has not been sampled. ponents of affective reactivity and dispositional mood. This is a
Whalen (1999) has interpreted these data within a model that very global term, and it is imperative to specify with more preci-
assigns a primary role for the amygdala in the detection of ambi- sion which particular system one is measuring affective reactivity
guity. According to this model, preferential activation of the amyg- in and which subcomponent of reactivity is being targeted for
dala is observed in response to fear versus anger faces because the study. For example, one could measure affective reactivity by
former convey threat though the source of the threat is ambiguous using startle magnitude, fMRI signal change in the amygdala, or
whereas angry faces convey a threat whose source is unambiguous. ratings on a self-report scale. Each of these obviously reflects
Although some data are consistent with this view, other data activity in very different systems, and activation in these systems
indicating that bilateral destruction of the amygdala impairs rec- does not necessarily cohere. What is meant by subcomponent of
ognition of both fear and anger vocal expressions (Scott et al., reactivity has been articulated in detail in Davidson (1998a) and
1997) are not. includes the following parameters: tonic level, threshold to re-
Another important question raised but not answered by the new spond, peak or amplitude of response, rise time to peak of re-
findings on the amygdala is whether there are reliable functional sponse, and recovery time. These are not meant to necessarily
asymmetries in this region. During the experimental arousal of reflect an exhaustive list of subcomponents; they are merely of-
negative affect, some investigators have reported changes in acti- fered as examples. Each of these subcomponents can potentially be
vation in the left amygdala (see, e.g., Schneider et al., 1997), some studied in different response systems, leading to many parameters
have reported changes in the right amygdala (see, e.g., Ranch et of affective style. Virtually nothing is known about the psycho-
al., 1996), and some have reported bilateral changes (see, e.g., metric characteristics of measures of these different parameters,
Irwin et al., 1996). There are data at the rodent level that suggest except for self-report measures (for two recent efforts examining
that there might be important functional differences between left different subcomponents of affective style in two different physi-
versus right amygdala lesions (Coleman-Mesches & McGaugh, ological response systems, see Larson, Ruffalo, Nietert, & David-
son, 2000; Tomarken, Davidson, Wheeler, & Kinney, 1992),
1995a, 1995b). One crucial issue in the human neuroimaging
though this information is crucial if researchers are to develop
literature is the need to perform the proper statistical comparisons
rigorous measures of these constructs. In this section, we review
to ascertain whether true asymmetric effects are present, This
data on the contributions of individual differences in prefrontal and
requires a test of the Condition × Hemisphere interaction. Virtu-
amygdala function to affective style.
ally none of the studies in the human neuroimaging literature have
In 2 decades of previous research, we have performed a large
performed this crucial test (see Davidson & Irwin, 1999a, for an
number of studies designed to examine the role of activation
extensive discussion of this issue).
asymmetries in the PFC and other anterior cortical zones in aspects
Finally, an issue left unaddressed in the human data is
of affective style. This work has been reviewed recently (David-
whether the amygdala is required for the ongoing expression of
son, 1995, 1998a), and only highlights are presented here. Using
negative affect or whether it is specifically involved in only the
measures of scalp-recorded brain electrical activity, we found that
initial acquisition of aversive learning. The fact that amygdala
indices of activation asymmetry based on power spectral measures
activation is present during early phases of conditioning and
were stable over time and exhibited excellent internal consistency
then appears to rapidly habituate (Buchel et al., 1998; LaBar et
reliability (Tomarken, Davidson, Wheeler, & Kinney, 1992), thus
al., 1998) is consistent with the idea that the amygdala may be fulfilling a number of important psychometric criteria for an index
required only in the initial stages of learning. We (Kalin et al., of a traitlike construct. In a series of studies, we found that there
2000) have recently performed studies in Rhesus monkeys were large individual differences in the magnitude and direction of
tested before and after very discrete excitotoxic lesions of the baseline asymmetric activation in brain electrical activity measures
amygdala that preserve fibers of passage and destroy only cell obtained from prefrontal scalp regions4 in both infants (Davidson
bodies. Complete destruction of the amygdala in these animals & Fox, 1989) and adults (Davidson & Tomarken, 1989). In 10-
resulted in a dramatic attenuation of behavioral signs of fear in month-old infants, we found that those with greater relative right-
response to a snake. However, such lesions did not have any sided prefrontal activation in prefrontal scalp regions were more
noticeable impact on freezing in response to a human intruder likely to cry in response to a brief period of maternal separation
paradigm (see Kalin & Shelton, 1989), nor did the lesions affect
any of the biological correlates that have been found to be
associated with an anxious endophenotype, including right pre- 4 It is importantto note that in referring to the brain electrical activity
frontal electroencephalographic (EEG) activation or baseline findings, we state that we are examining activity from prefrontal scalp
cortisol (Kalin, Larson, Shelton, & Davidson, 1998). Collec- regions. When using measures of brain electricalactivity,definitiveinfer-
ences about the sources of the scalp-recorded surface potentialsare diffi-
tively, these findings imply that the amygdala may be crucial
cult to make. Thus, we qualify our descriptions by noting that such
for learning new stimulus-threat contingencies and may be
measures reflect scalp topography. Whether the scalp topographic differ-
important in the expression of cue-specific fear. However, the ences actually reflect different underlying sources must be addressed in
amygdala does not appear to be necessary for the expression of future research with simultaneousmeasuresof both brain electricalactivity
already acquired individual differences in temperament or af- and either blood flow or metabolism(see Larson et al., 1998, for a recent
fective style. example).
compared with their left-activated counterparts (Davidson & Fox, the structural anatomy is not symmetrical, particularly for cortical
1989). In toddlers and young children, we have observed that those tissue, it is very difficult to extract homologous regions for asym-
individuals with greater relative right-sided prefrontal activation metry analyses. The size of the regions may differ on the two sides
show more behavioral inhibition and wariness measured through of the brain, the anatomical homologue may not be in exactly the
laboratory-based behavioral observation (Davidson & Rickman, same location in each hemisphere and the shape of the cortical
1999). In adults, we have found that individual differences in such territory on each side of the brain is often different. These facts.
measures predict dispositional mood (Tomarken, Davidson, present formidable methodological obstacles when using neuroim-
Wheeler, & Doss, 1992), self-report measures of behavioral acti- aging to make inferences about patterns of asymmetric activation.
vation and inhibition (Sutton & Davidson, 1997), repressive de- Irwin et al., using formal methods for assessing asymmetry, found
fensiveness (Tomarken & Davidson, 1994), reactivity to positive that fMRI activation differences in regions of the PFC did predict
and negative emotion elicitors (Tomarken, Davidson, & Hen- dispositional negative emotion. Individuals with greater right-
riques, 1990; Wheeler, Davidson, & Tomarken, 1993), baseline compared with left-sided fMRI signal change in response to neg-
immune function (Kang et al., 1991), and reactivity of the immune ative compared with neutral pictures reported more dispositional
system to emotional challenge (Davidson, Coe, Dolski, & Don- negative affect. These findings are consistent with those using
zeUa, 1999). In very recent work (Larson, Sutton, & Davidson, brain electrical activity measures (see, e.g., Tomarken, Davidson,
1998) we found that individual differences in electrophysiological Wheeler, & Kinney, 1992).
measures of prefrontal asymmetry predicted the magnitude of The data from the Larson et al. (1998) study referred to above
recovery following a negative affective stimulus. These data sug- indicated that individuals with greater relative left-sided prefrontal
gest that the PFC may play a role in the regulation of the time activation at baseline have greater recovery of startle potentiation
course of emotional responding and/or in the active inhibition of following the offset of a negative stimulus. Moreover, the measure
negative affect. We return to these issues later in the article. of asymmetric prefrontal activation accounted for more variance in
We have also found that individual differences in these brain the magnitude of startle post-negative-stimulus offset (i.e., startle
electrical measures of anterior asymmetry are associated with recovery) than it did during the stimulus. These findings imply that
mood and anxiety disorders. In particular, we found that depressed individual differences in prefrontal activation asymmetry may play
subjects and individuals who were currently euthymic but had a a role in regulating the time course of emotional responding and
history of past depression exhibited less left prefrontal activation that those individuals with more left-sided prefrontal activation
compared with never-depressed controls (Henriques & Davidson, may recover more quickly from negative affect or stress than their
1990, 1991). We also found that when social phobics anticipated right-activated counterparts.
making a public speech, they showed large increases in right-sided A clue to the mechanism that may underlie this consequence of
prefrontal activation though they did not differ from controls at left prefrontal activation is provided by a study from LeDoux's
baseline (Davidson et ai., 2000). laboratory that found that rats with lesions of the medial PFC
In a series of studies with Kalin (Davidson, Kalin, & Shelton, showed dramatically slower extinction of a learned aversive re-
1992, 1993; Kalin, Larson, et al., 1998), we have demonstrated sponse compared with sham-operated controls (Morgan, Roman-
that similar activation asymmetries could be measured in Rhesus ski, & LeDoux, 1993; but see Gewirtz, Falls, & Davis, 1997).
monkeys and that they predicted types of behavior and biology These findings imply that there is a descending pathway between
comparable to those we observed in humans. In the most recent the medial PFC and the amygdala (Amaral, Price, Pitkanen, &
effort of this kind, we (Kalin, Larson, et al., 1998) found that Cannichael, 1992) that is inhibitory and thus represents an active
animals with greater relative right-sided prefrontal activation ex- component of extinction. In the absence of this normal inhibitory
hibited higher basal levels of the stress hormone cortisol. Similar input, the amygdala remains unchecked and continues to remain
data have recently been reported in humans (Buss, Dolski, Malm- activated. Whether this inhibitory input from the medial PFC is an
stadt, Davidson, & Goldsmith, 1997). important component of the prominent habituation observed in the
A number of our original EEG observations have now been amygdala remains to be clarified. Davidson (1998a) has suggested
independently replicated by others (Ahem & Schwartz; 1985; that in humans and possibly other primates, the major inhibitory
Allen, Iacono, Depue, & Arbisi, 1993; Dawson, Klinger, Panagio- influence on the amygdala may derive from the left PFC. Consis-
tides, Hill, & Spieker, 1992; Fox, 1991; Harmon-Jones & Allen, tent with this idea, recent PET findings suggest that in normal
1997; Jacobs & Synder, 1996; Wiedemann et al., 1999), though a human participants, glucose metabolism in the left medial and
few studies reporting only partial replications of aspects of our lateral PFC is reciprocally coupled to metabolic activity in the
original findings have appeared (Hagemann, Naumann, Becker, amygdala, such that those participants with increased left prefron-
Maier, & Bartussek, 1998; Reid, Duke, & Allen, 1998). Davidson tal metabolic rate have decreased metabolic rate in the amygdala
(1998b) has called attention to a number of crucial methodological (Abercrombie et al., 1996). We propose that this mechanism may
and conceptual issues in these replication attempts and suggested be responsible for the dampening of negative affect and the short-
that the difficulties in replication are mostly a function of signif- ening of its time course in those individuals who appear to be more
icant methodological limitations. Moreover, few studies using resilient. Such an affective style may also facilitate the mainte-
neuroimaging to address the role of prefrontal asymmetries in nance of approach-related positive affect.
affective processes have appeared. As noted by Davidson and The two key features of the circuitry underlying positive and
Irwin (1999a, 1999b), only a very small handful of studies using negative affect highlighted in this article are the PFC and the
PET or fMRI have conducted the proper statistical comparison to amygdala. In the section above, we detailed studies on the basic
uncover asymmetry effects in their data. Irwin et al. (2000) com- function of the amygdala in affective behavior. Here, we ask about
mented on the complexity of performing these analyses. Because individual differences in amygdala function and its relation to
SPECIAL ISSUE: EMOTION, PLASTICITY, AND REGULATION 899
affective style. Although most research on the amygdala has em- 14
•phasized its phasic function, there is a tonic level of activation in
r=0.63 •
the amygdala that can be assessed with PET measures of regional ~12
glucose metabolism, Using MRI-based coregistration, one can
draw regions of interest around the amygdala on an MRI scan
coregistered to the PET image and extract metabolic activity in
such small regions without using any spatial filtering of the PET
image. This provides higher resolution than could ordinarily be
achieved using conventional cross-participant aggregation meth-
ods that require spatial smoothing of the images (see Abercrombie
et al., 1998; Schaefer et al., 2000). Using such procedures, we have
found that individual differences in metabolic activity in the right
amygdala, in particular, predict dispositional negative affect on the
Positive and Negative Affect Schedule (PANAS; Watson, Clark,
& Tellegen, 1988) in a group of depressed patients (see Figure 1).
Using the same measure of negative affect, we (Irwin, Anderle, Rank of Negative Affect Score
Sutton, Kalin, & Davidson, 2000) have also found that fMRI signal
change in the amygdala in response to negative versus neutral Figure 2. Scatter plot of the relation between dispositional negative affect
stimuli accounts for a substantial amount of variance in PANAS in normal participants assessed with the Positive and Negative Affect
trait negative affect scores (r = .63; see Figure 2). Other research- Schedule trait negative affect scale (Watson et al., 1988) and fMRI signal
change in the right amygdala in response to negative versus neutral
ers have found that individual differences in right amygdala glu-
pictures. N = 12. From "Activation of the Right Amygdala Detected With
cose metabolic rate in response to emotional films predict the Functional MRI Predicts Dispositional Negative Affect," by W. Irwin,
recall of negative emotional films assessed 3 weeks following the M. J. Anderle, S. K. Sutton, N. H. Kalin, and R. J. Davidson, 2000,
PET procedure. Those individuals with higher levels of glucose manuscript in preparation. Copyright 2000 by W. Irwin. Adapted with
metabolism in the right amygdala recalled more of the negative permission.
film clips (Cahill et al., 1996). Other investigators using both PET
(Furmark, Fischer, Wik, Larsson, & Fredrikson, 1997) and fMRI
(LaBar et al, 1998) reported that those participants with greater response to the drug had significantly greater activation of the
activation in the amygdala during classical aversive conditioning amygdala compared with those exhibiting a euphoric response.
showed greater evidence of electrodermal conditioning. Ketter et Moreover, amygdala blood flow correlated positively with fear and
al. (1996), using the anesthetic procaine as a pharmacological negatively with euphoria on self-report measures of emotional
challenge, reported that those individuals who had a dysphoric intensity.
Some of the data reviewed above on relations between amyg-
dala activation and dispositional negative affect appear at least on
the surface to be inconsistent with the animal and human neuro-
imaging data reviewed above implying that the amygdala is im-
portant only in the initial learning of stimulus-threat associations
but not in the expression of preexisting temperamental variation,
such as behavioral inhibition. For example, in our own data using
.4-,
i PET-derived measures of glucose metabolism in the amygdala
(Abercrombie et al., 1998), we found that participants with greater
z@ metabolic rate in the right amygdala reported higher levels of
dispositional negative affect as assessed by the PANAS. A similar
10 J R e •I I I association was found using the identical affect measure with
fMRI where participants showing larger fMRI signal increases in
-1 -0.5 0 0.5 1 the amygdala in response to negative versus neutral pictures re-
Residualized Right Amygdalar rCMRglu ported higher levels of dispositional negative affect. The PANAS
requires participants to rate a series of single-word adjectives on a
Figure 1. Scatter plot of the relation between dispositional negative affect 1-5-point scale to indicate the extent to which that emotion is
in depressed patients assessed with the Positive and Negative Affect present during their dally life. Thus, in these experiments, it
Schedule trait negative affect scale (Watson et al., 1988) and glucose appears that activation levels in the amygdala are associated with
metabolism in the right amygdala, rCMRglu refers to regional cerebral the expression of a preexisting affective style. We believe the key
metabolic rate for glucose (assessed with MRI-coregistered regions of to resolving this apparent inconsistency among these findings lies
interest and position emission tomography), residualized for global meta-
in a more in-depth understanding of the strategies people use to
bolic rate. N = 17, r = .56, p = .02. From "Metabolic Rate in the Right
Amygdala Predicts Negative Affect in Depressed Patients," by H. C. respond to questionnaires like the PANAS. When participants are
Abercrombie, S. M. Schaefer, C. L. Larson, T. R. takes, K. A. Lindgren, asked to make global inferences about the affective dispositions
J. E. Holden, S. B. Perlman, P. A. Turski, D. D. Krahn, R. M. Benca, and that are extended in time, they are not veridical integrators of the
R. J. Davidson, !998, NeuroReport, 9, p. 3505. Copyright 1996 by Lip- momentary affective states that unfolded over the period in ques-
pincott Williams & Wilkins. Adapted with permission. tion. Rather, as a number of commentators have forcefully argued,
900 DAV1DSON; JACKSON, AND KALIN
they exhibit systematic heuristic biases that reflect the information structure can be obtained along with measures of environmental
that is accessible at the time (see Kahneman, 1999; Schwarz & change and behavioral measures of emotional reactivity.
Strack, 1999). In particular, in a series of elegant studies, Kahne- For now, most of the extant data directly relevant to this issue is
man (1999) has demonstrated that individuals tend to adopt what at the animal level. In a series of studies, Meaney and his col-
he refers to as the peak-end rule for forming these retrospective leagues (Caldji et al., 1998; Liu et al., 1997; Meaney, Aitken, van
affective evaluations. Thus, although an individual might be asked Berkel, Bhatnagar, & Sapolsky, 1988; see Francis & Meaney,
to rate how "nervous" he was during the past month, he is likely 1999, for a recent review) have demonstrated that an early envi-
to weight excessively information about the peak episode of ner- ronmental manipulation in rats' frequency of maternal licking/
vousness during this period, as well as his level of nervousness grooming and arched-back nursing produces in the offspring a
very recently. The peak intensity of the emotion in question may cascade of biological changes that shape the central circuitry of
be especially related to amygdala activation because it is likely to emotion and consequently alter the animal's behavioral and bio-
represent a response to a particularly threatening or novel episode. logical responsivity to stress. This work provides an elegant model
Such complexities in measuring subjective aspects of emotion for plasticity in this central circuitry and important clues for the
underscore the need to develop more objective measures that do study of similar influences at the human level, In particular,
not depend on self-report and that can better capture the time Meaney and his colleagues found that the adult offspring of high
course of emotional responding or what Davidson (1998a) has licking/grooming and arched-back nursing mothers displayed sub-
referred to as affective chronometry. stantially reduced behavioral signs of fearfulness to novelty com-
The fact that there exist reliable individual differences in base- pared with the offspring of mothers low in these maternal charac-
line metabolic rate in the amygdala also requires comment in light teristics. In addition, the offspring of mothers high in these traits
of the earlier discussion about the amygdala's role in phasic showed increased central benzodiazepine receptor densities in
affective processes. There is clearly intrinsic neural activity in the various subnuclei of the amygdala as well as in the locus ceruleus
amygdala, even during sleep (Maquet et al., 1996). As a number of (LC), increased a 2 adrenoreceptor density in the LC, and de-
studies have now shown, baseline nontask (resting) levels of creased corticotrophin releasing hormone (CRH) receptor density
activation in the amygdala are associated with dispositional neg- in the LC (Caldji et al., 1998). In other research, they found that
ative affect (Abercrombie et al., 1998) and depression (Drevets et rats exposed to high licking/grooming mothers exhibited a perma-
al., 1992). Whether these baseline differences in amygdala activa- nent increase in concentrations of receptors for glucocorticoids in
tion reflect activation in response to the PET environment or both the hippocampus and the PFC (Liu et al., 1997; Meaney et al.,
whether such differences predict the magnitude of task-induced 1996, 1988). All of these changes induced by early maternal
'activation in the amygdala in response to emotion elicitors are licking/grooming and related behavior involve alterations in the
questions that must be addressed in future research. We believe central circuitry of emotion that result in decreased responsivity to
that when PET is used to measure baseline differences in amygdala stress later in life.
activation, at least for the right amygdala, it likely reflects an The increased benzodiazapine receptor expression in the amyg-
important influence of the experimental situation itself. This claim dala and LC likely play a role in the increased facility in inhibiting
is made on the basis of the fact that our recent evidence (Schaefer or regulating these systems following exposure to stress. Exposure
et al., 2000) using MRI coregistration to extract glucose metabolic to stress provokes an increase in the firing rate of neurons in the
rate in several subcortical regions revealed that test-retest reliabil- LC. Activity within the noradrenergic cell bodies in the LC is
ity over a 6-month period is excellent for all subcortical regions we subject to inhibitory feedback regulation via c~z adrenoreceptors.
examined (hippocampus, caudate, thalamus, left amygdala) except Thus, tx2 adrenoreceptor agonists decrease the firing rate of LC
for the right amygdala. These findings are consistent with the idea neurons and norepinephrine release at terminal sites. The increase
that situational influences are important in modulating activation in a 2 adrenoreceptor density and decrease in CRH receptor density
in the right amygdala. in the LC (CRH infusion has been shown to alter the firing rate of
LC neurons and to increase anxiety; see Weiss et al., 1994, for a
Plasticity in the Central Circuitry of Emotion review) are both likely to decrease fearfulness in the offspring of
the high licking/grooming mothers. The increase in glucocorticoid
As we noted in the discussion of the central circuitry of emotion, receptors in both hippocampus and PFC is likely to play an
the amygdala is clearly a site of plasticity in the brain. It is important role in enhanced negative feedback efficacy of the
involved in emotional learning, whether or not it is itself the site of adrenocortical axis. Among rats with less efficient negative feed-
storage of new stimulus-threat associations. Recent work by Le- back, elevated secretion of glucocorticoids was observed in re-
Doux and his colleagues is beginning to characterize the molecular sponse to stress, and later in life, basal elevations of this system
changes in the amygdala that accompany newly acquired aversive occurred, leading to greater cumulative exposure to glucocorti-
learning (Schafe, Nadel, Sullivan, Harris, & LeDoux, 1999; Weiss- colds. A variety of research indicates that exposure to elevated
kopf & LeDoux, 1999). These findings underscore the fact that levels of glucocorticoids can accelerate hippocampal neuron loss
although there are stable individual differences in activation pat- and produce cognitive and affective impairments (McEwen, 1998).
tems in the central circuitry of emotion, there is also pronounced There are several important implications of these animal data for
plasticity in this circuitry. One of the major challenges for human understanding the role of plastic changes in the circuitry of emo-
affective neuroscience in the next century will be to better under- tion in humans. The findings briefly described above indicate the
stand the environmental forces that shape the circuitry of emotion. PFC, amygdala, and hippocampus are all sites where plasticity is
Answers to this question will depend on the findings from longi- known to occur. One of the interesting puzzles in the human
tudinal studies where sensitive measures of both brain function and literature on prefrontal asymmetries associated with affective style
is that although measures of baseline prefrontal asymmetries are that might be experienced are perfectly normal emotions. They are
stable in adults, they are not stable during early childhood (Da- simply expressed at inappropriate times in nonnormative contexts.
vidson & Rickman, 1999). When we examined brain electrical Thus, the fear that a social phobic might experience in the course
measures of baseline prefrontal activation asymmetry over an of interacting with a group of people is not in itself pathological.
8-year period from 3 to 11 years of age in a cohort of approxi- What makes the expression of the fear pathological is the fact that
mately 65 children, we found little evidence of stability (Davidson it is expressed in contexts in which most other individuals do not
& Rickman, 1999). This is a period during which pronounced experience such fear.
plasticity is likely to occur in the central circuitry of emotion, In an effort to investigate context-inappropriate emotion in
particularly in the PFC, which is still undergoing important devel- nonhuman primates, Kalin (Kalin & Shelton, 2000) examined 100
opmental change at least until puberty (Huttenlocher, 1990). An Rhesus monkeys with a series of behavioral tests called the Human
important challenge for future research will be to obtain better Intruder Paradigm that has been used extensively in his laboratory
measures of life course events, parental influence and other im- (see Kalin & Shelton, 1989). These tests involve exposure of a
portant environmentalfactors and to relate the occurrences of these monkey to three specific conditions. In one condition, the animal
to shifts in patterns of prefrontal activation. is alone (the alone condition); in a second condition, a human
The animal data on the influence of environmental factors in the enters the room and exposes his profile to the animal (the no eye
regulation of glucocorticoid receptor densities in the hippocampus contact condition); in a third condition, the human enters the room
provide some basis for understanding the possible effects of and stares at the animal (the stare condition). Each of these
trauma on hippocampal structure and function. A number of stud- conditions is presented for 10 rain. During each condition, various
ies have reported hippocampal atrophy on MRI in patients with behaviors of the animal are coded. In previous work, Kalin has
posttraumatic stress disorder (Bremner et al., 1995,1997; Stein, demonstrated the differential sensitivity of the different behaviors
Koverola, Hanna, Torchia, & McClarty, 1997). In addition, recent to specific pharmacological manipulations (Kalin & Shelton,
studies have also found hippocampal atrophy in patients with 1989). Normatively, monkeys freeze when exposed to the human
major depression (Sheline, Sanghavi, Mintun, & Gado, 1999; profile. Interestingly, there are individual differences in freezing
Sheline, Wang, Gado, Csernansky, & Vannier, 1996) and alcohol- duration, and these differences are stable over time (Kalin, Shel-
ism (Agartz, Momenan, Rawlings, Kerich, & Hommer, 1999). Of ton, Rickman, & Davidson, 1998). In response to both the alone
course, it is not possible from these studies to tease preexisting and the stare conditions, the normative response of monkeys is to
conditions apart from the influence of environmental stress or display little if any freezing, with other behaviors increasing in
insult and conclusively establish causal influences. In addition, the frequency during these conditions. When a very large group of
functional significance of hippocampal volume reduction that is animals was tested (N = 100), the pattern of normative behavior
proportional to reductions in whole brain volume is not known at previously observed was confirmed at the group level (Kalin &
the present time (Agartz et al., 1999). Nevertheless, these points of Shelton, 2000). There is a highly significant difference in freezing
convergence between the animal studies and human research are duration among the conditions such that freezing is significantly
critical in developing a mechanistic understanding of how early higher during the no eye contact condition compared with the other
stressful life events might influence the central circuitry of emo- two conditions. However, there are also marked individual differ-
tion, which in turn shapes patterns of emotional reactivity later in ences, not only in the duration of freezing during the normative
life. condition (no eye contact) but also in the duration of freezing
during the other conditions. There were 3 animals in this group of
100 that displayed levels of freezing during the stare condition that
Context and Regulation:
were very high and indistinguishable from their freezing duration
Two Instances of Affective Plasticity
during the normative no eye contact condition (see Figure 3). Note
The role of context in the regulation of affective reactivity is that the freezing of this small subgroup of 3 was high during the no
relatively understudied, particularly at the human level. Numerous eye contact condition but that there were quite a few other animals
studies at the animal level have demonstrated the importance of who displayed levels of freezing that were comparable during this
context in the regulation of affective behavior and have high- condition. However, all but these 3 animals turned off this re-
lighted the role of the hippocampus (Anagnostaras, Maren,& sponse during the other conditions. These 3 animals can be said to
Fanselow, 1999) and interconnected structures (bed nucleus of the have displayed context-inappropriate freezing. In analyses of bio-
stria terminalis; Davis & Lee, 1998) in this type of process. logical data (prefrontal activation asymmetry and cortisol), it was
Various forms of psychopathology that involve disorders of affect found that these 3 animals had markedly more extreme patterns
may be best conceptualized as disorders of the context-regulation than their counterparts who froze for an identical duration of time
of affect. For example, both mood and anxiety disorders typically during the normative context (Kalin & Shelton, 2000).
involve the expression of normal emotion in inappropriate con- Although a mechanistic understanding of such context-
texts. That is, the emotion expressed in these disorders would be inappropriate affective responding is not yet available, there are
normative and appropriate in certain contexts. Per the diagnostic several issues that warrant comment. First, given the role of the
criteria for major depression following bereavement, the depres- hippocampus and bed nucleus of the stria terminalis that have been
sion must persist for more than 2 months to be labeled major featured in rodent models of context conditioning (Fanselow,
depression (American Psychiatric Association, 1994). In other 2000; Davis & Lee, 1998), it is likely that these brain regions play
words, the continued expression of depressed affect beyond the a role in the context-regulation of affective responding in humans.
context in which it is deemed appropriate is central to the diagno- It is noteworthy that in several disorders that are known to involve
sis. In many of the anxiety disorders, the fear and other emotions context-inappropriate affective responding, morphometric study of
The second issue that deserves emphasis here is the implications
of the work on context and its role in shaping affective responding
for assessing certain behavioral traits. We use behavioral inhibition
as our example here, in part because the studies we have conducted
in nonhuman primates have been designed to model human be-
havioral inhibition. In the developmental literature on human in-
fants and toddlers, this temperamental characteristic has typically
been assessed by observing behavioral signs of fearfulness and
wariness in contexts of novelty and unfamiliarity (see, e.g., Kagan,
Reznick, & Snidman, 1988). Thus, for example, behavioral inhi-
bition (one measure of which is freezing) has been coded when
toddlers are approached by unfamiliar strangers or bizarre-looking
robots. These are situations where it is normative to show some
wariness. In fact, the display of high levels of approach behavior
in this context is the nonnormative, context-inappropriate re-
NEC ST sponse. As we noted above, there appear to be important differ-
ences between monkeys who express identically high levels of
Figure 3. Freezing duration in seconds (out of a total of 600 s) in
response to the no eye contact (NEC--exposure of the monkey to a profile freezing during a normative context (exposure to the human pro-
of a human) and the stare (ST) conditions in four groups of animals: One file) but differ in their duration of freezing during a nonnormative
group showed very long durations of freezing during the normative context context (exposure to the human staring). Had the assessment
(NEC) but then froze little during ST. Another group showed moderately period with these monkeys been restricted to the normative con-
elevated freezing during NEC and then showed little during ST. A third text, the two groups of high-freezing animals would have been
group showed virtually no freezing during either NEC or ST. Finally, the classified identically. Only by including an assessment of their
fourth group exhibited high levels of freezing in response to both NEC and behavior in a nonnormative context were behavioral differences
ST. This is the group that displayed out-of-context freezing (i.e., during
revealed, which helped to account for some of the variance in basal
ST). The group of 3 animals who showed the out-of-context freezing were
the only animals in a group of 100 to exhibit this pattern. From "The levels of prefrontal activation asymmetry and cortisol (Kalin &
Regulation of Defensive Behaviors in Rhesus Monkeys: Implications for Shelton, 2000). These findings imply that our assessments of
Understanding Anxiety Disorders," by N. H. Kalin and S. E. Shelton, in human behavioral inhibition may not be nearly as sensitive as they
Anxiety, Depression, and Emotion (p. 63), edited by R. J. Davidson, 2000, might. Rather than performing such assessments in the context-
New York: Oxford University Press. Copyright 2000 by the Oxford Uni- appropriate conditions of novelty and unfamiliarity, perhaps we
versity Press. Reprinted with permission. should be measuring these temperamental characteristics in non-
normative contexts. We hypothesize that individuals who habitu-
ally fail to regulate their affective responses in a context-sensitive
hippocampal volume with high resolution MRI has revealed sig-
nificant atrophy (see, e.g., Bremner et al., 1995, 1997; Sheline et fashion may have functional impairment of the hippocampus
al., 1996, 1999). Such atrophy may arise as a consequence of and/or stria terminalis. Such functional impairment may arise as a
exposure to elevated levels of cortisol, as several authors have consequence of plastic changes in these regions as a function of
hypothesized (Gold, Goodwin, & Chrousos, 1988; McEwen, cumulative exposure to elevated glucocorticoids (McEwen, 1998).
1998). At the human level, age-related declines in hippocampal It has been known for some time that neurogenesis (the growth
volume have been related to elevated cortisol levels. The conse- of new neurons), primarily in the dentate region of the hippocam-
quences of such age-related changes have been examined in the pus, can occur in the postnatal period in rodents (see Gould &
cognitive domain, specifically in measures of declarative memory McEwen, 1993, for a review). However, it has only recently been
that are thought to be hippocampally mediated (Lupien et al., demonstrated that such plastic changes can occur in the adult
1998). However, there has been no study of which we are aware human hippocampus as well (Eriksson et al., 1998). The fact that
that has specifically related hippocampal morphometric differ- such neurogenesis can occur in adult humans raises the possibility
ences to context-dependent affective responding. On the basis that both salubrious as well as stressful conditions might influence
of the issues described above, we believe it is likely that this process and that these experience-induced hippocampal
glucocorticoid-induced changes in hippocampal structure will also changes, in turn, can have significant affective and cognitive
be accompanied by profound affective changes and will impair an consequences. Kempermann, Kuhn, and Gage (1997) have dem-
organism's ability to adaptively regulate emotion in a context- onstrated that exposure of adult mice to an enriched environment
appropriate fashion. The affective consequences of hippocampal produced a 15% increase in granule cell neurons in the dentate
dysfunction may be as, if not more, important to the adaptive gyrus of the hippocampus compared with littermates housed in
functioning of the organism as the cognitive changes that have standard cages. This basic phenomenon has been recently repli-
been featured so prominently in the human literature. One of the cated in rats (Nilsson, Perfilieva, Johansson, Orwar, & Eriksson,
important challenges in this area for the future will be to develop 1999) and extended by demonstrating that the rats raised in an
a better understanding of context from a human perspective given enriched environment who showed neurogenesis in the dentate
that most previous studies have been conducted at the animal level gyrus also exhibited improved performance in a spatial learning
and have relied on simple manipulations of housing conditions to test. Conversely, it has been shown that stress diminishes prolif-
alter context. eration of granule cell precursors in the dentate region of adult
marmoset monkeys (Gould, Tanapat, McEwen, Flugge, & Fuchs, emotion. In this experiment, participants were asked to suppress,
1998). maintain, or enhance their experiential responses to arousing neg-
Collectively, these findings highlight the plasticity of certain ative pictures. Reasoning that successful emotion regulation
regions of the brain that persists into adulthood and raise the should lead to more (in the enhance condition) or less (in the
possibility that interventions, even those occurring during adult- suppress condition) intense emotion, we used two well-validated
hood, not only can have effects on neuronal function but also can measures of emotional state as dependent variables: startle eye-
literally influence neurogenesis. The fact that the focus of this blink magnitude and corrugator region electromyogram activity
work is in the hippocampus indicates that a major substrate of (see Lang, 1995). To achieve the crucial experimental separation
context-dependent emotional responding is a key target for these of initial emotion and subsequent regulation, we collected data in
experientially induced changes. It is likely that other brain regions the presence of the emotional stimulus but prior to the regulation
as well exhibit plastic changes. Whether these involve neurogen- instruction, in addition to following the instruction. The instruction
esis or favor other mechanisms is a question that must be ad- that was presented 4 s following the onset of a negative picture
dressed in future work. For now, the extant findings provide a (total picture duration was 8 s) and simply instructed participants
rationale for examining the impact of therapeutic interventions, to either maintain, enhance, or suppress the emotion that they were
both pharmacological and behavioral, as well as naturally occur- experiencing. Replicating a vast literature, we indeed found that
ring environmental stressors and stress buffers, on neuronal struc- startle eyeblink magnitude and corrugator region activity were
ture and the central circuitry of emotion that might underlie increased during the negative pictures, relative to neutral pictures
changes in affective style. prior to any presentation of an instruction to regulate. This enabled
Finally, we turn our attention to another aspect of plasticity in us to verify that the stimuli we used were indeed producing the
affective responding--the group of phenomena we refer collec- intended emotion prior to the delivery of any instruction to regu-
tively to as affect regulation (see Gross, 1999, for a review). The late. We found that instructions to suppress a negative emotional
importance of emotion regulation has been recognized by psychol- response led to decreased eyeblink startle magnitude and corruga-
ogists and philosophers for many years, although as a construct, tor activity, whereas instructions to enhance similar responses led
emotion regulation remains vague. At the broadest level, it might to increases in both measures (compared within-subjects to the
be considered any process that maintains, accentuates, or attenu- maintain condition).
ates emotional responses. Regulatory processes can be automatic In this study (Jackson et al., 2000), we also found that ability to
or effortful, and they can be both statelike and traitlike. The suppress negative emotion was inversely correlated with ability to
clinical importance of emotion regulation can be seen by review- enhance negative emotion, suggesting that individual differences
ing the diagnostic criteria for Axis I and II psychopathology in the in the ability to regulate emotion as assessed in this paradigm vary
Diagnostic and Statistical Manual (4th ed.; American Psychiatric with the valence and direction of the requested regulation instruc-
Association, 1994), many of which include disturbances in regu- tion. Of great interest to us is whether the active effort to suppress
latory processes. Because this literature has been recently reviewed negative emotion is associated with a phasic increase in the recip-
by Gross (1999), only a few issues directly pertinent to our earlier rocal coupling between the medial PFC and the amygdala resulting
discussion are addressed in this article. First, though this has not in prefrontal inhibition of amygdala activation as described above.
been emphasized in recent commentaries, it is difficult to rigor- This is a fully tractable problem and can be addressed by exploit-
ously separate the processes involved in the production of an ing the time resolution that is now possible to achieve using
emotion per se from those that might be involved in the regulation event-related fMRI (Menon & Kim, 1999). Whether long-term
of that emotion. Given the fact of parallel processing in the brain, traitlike and more automatic forms of emotion regulation are
it is likely that these two components of emotional responding associated with actual structural changes in the brain is a question
unfold as at least partially overlapping processes. Moreover, be- that has not yet been addressed in current research.
cause different systems reflect different aspects of emotion and
these occur at varying time courses, it is likely that regulatory S u m m a r y , Future Trends, and C o n c l u s i o n s
processes may influence some systems more than others and also
that such influences may exhibit temporal asynchrony. All of these This article was intended to showcase some modem develop-
ments in affective neuroscience and to illustrate how this approach
facts collectively pose a formidable challenge to the investigator
who wishes to obtain separate measures of emotion and regulation might facilitate a more mechanistic understanding of the underly-
and to determine the influence of the latter on the former. Further- ing subcomponents of emotion and affective style. Two major
more, there are likely to be important differences among individ- neural territories--the PFC and the amygdala--in the circuitry of
uals in different aspects of emotional regulation. In fact, such emotion were featured. Both the dorsolateral and ventromedial
differences are likely to form key components of what we refer to sectors of the PFC have been associated with different aspects of
emotion. Asymmetries in the PFC have been linked to approach
as affective style.
In our earlier discussion of affective style, we introduced the and withdrawal systems, with sectors of the left PFC more asso-
ciated with the approach system and certain forms of positive
idea of examining individual differences in the capacity to turn off
negative emotions once they are turned on and the associated
differences in the chronometry of negative affective responding. 5 Whether the mechanisms underlying voluntary emotion regulation are
We (Jackson, Malmstadt, Larson, & Davidson, 2000) sought to similar to or different from those that underlie automatic emotion regula-
develop a paradigm where we could crisply distinguish between tion has not been rigorously addressed. Also unknown is whether individ-
emotion and regulation and to separate these processes in time. To ual differences in the voluntary regulation of emotion predict individual
do this, we chose to examine voluntary regulation5 of negative differences in more automatic forms of emotion regulation.
affect and other regions in the right PFC more associated with changes in the circuitry of emotion. Though there are some data on
negative affect and withdrawal. Studies of patients with discrete the impact of antidepressant medication on brain function (see,
PFC lesions, as well as studies using measures of regional brain e.g., Bench, Frackowiak, & Dolan, 1995) and one study on the
activation in neurologically intact normal subjects and patients effects of cognitive therapy on regional glucose metabolism in
with psychiatric disorders, have provided evidence to support these patients with obsessive-compulsive disorder (Baxter et al., 1992),
proposals. The PFC is likely to play a role in affective working there are no data on plastic changes in the brain as a consequence
memory, a process critical to anticipating future affective out- of practicing methods designed to increase positive affect, such as
comes (see, e.g., Watanabe, 1996). The amygdala is clearly im- meditation. The Dalai Lama himself has called attention to this
portant for several aspects of emotional processing, though there possibility in his recent best seller The Art of Happiness (the Dalai
still remain many questions about its contributions to human Lama & Cutler, 1998). In this book, he discussed the implications
emotion and affective style. It does appear to be the case that the of research on neural plasticity for increasing levels of happiness
amygdala is activated by stimuli that elicit certain forms of neg- and explained:
ative affect, particularly fear. It is also the case that individual
The systematic training of the mind--the cultivation of happiness, the
differences in amygdala activation, both at baseline and in re-
genuine inner transformation by deliberately selecting and focusing
sponse to negative affect-arousing stimuli, account for significant
on positive mental states and challenging negative mental states--is
variance in measures of emotion-related cognitive function (e.g., possible because of the very structure and function of the brain . . . .
rapidity of aversive learning, recall of negative affective stimuli) But the wiring in our brains is not static, not irrevocably fixed. Our
and in self-report measures of negative dispositional mood. How- brains are also adaptable. (pp. 44-45)
ever, studies using discrete excitotoxic lesions of the amygdala in
nonhuman primates that preserve fibers of passage suggest that the Although the decade of the brain is just now ending, it is at the
amygdala is not required for the expression of both behavioral and close of this decade that psychologists finally have both the theory
biological components of an anxious temperament. Other evidence and the methods to begin addressing some of the most important
suggests that the PFC may be a more critical site for the expression questions about human brain function that have for so long eluded
of these individual differences. The final two sections above ad- rigorous study. Understanding how feeling is instantiated in the
dressed issues related to plasticity in the central circuitry of emo- brain and using the brain's architecture to meaningfully parse the
tion and consequently, in affective style. Evidence at the animal domain of emotion is now possible through the modem methods of
level was reviewed to illustrate the profound impact of environ- affective neuroscience. Research on plasticity has revealed new
mental events in shaping the neural circuitry of emotion. Data are information about and realistic hope for ways to shape the circuitry
now available that reveal changes down to the level of gene of emotion to promote increased well-being and positive affect.
expression as a direct consequence of environmental manipula- The downstream consequences of these patterns of neural activity
tions. Moreover, in addition to the critical role of the early envi- for endocrine, autonomic and immune function can be studied to
ronment, new findings reveal neurogenesis in the human hip- provide clues to the mechanistic understanding of how emotion
pocampus in adults, indicating that plasticity continues unabated might affect physical health and disease. Delineating the interac-
throughout the life course. The hippocampus and other intercon- tion between mind and body through the brain is now more
nected structures were highlighted for their contribution to tractable than ever before and it is our view that the domain of
context-dependent affective responding. Stressors produce eleva- emotion--affective neuroscience--will be where new insights and
tions in cortisol, which in turn result in hippocampal cell death improved understanding are most visible in this new century.
and/or decreases in hippocampal neurogenesis. This chain of
events provides one mechanism whereby negative life events can
result in abnormalities in context-dependent emotional responses.
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Individual differences in anterior brain asymmetry and fundamental Accepted January 7, 2000 •
Präfrontaler Kortex
Affekt (Psychologie)
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