Damage to the cortices anterior to areas 6 and 8, that is, areas 9, 10, 45, and 46, the prefrontal cortex, and also the anterior cingulate gyri, has less easily defined effects on motor behavior. The prefrontal cortex is heteromodal and has strong reciprocal connections with the visual, auditory, and somatosensory cortices. Of these, the visuomotor relationships are the most powerful. These frontal areas as well as the supplementary motor areas are involved in the planning and initiation of sequences of movement, as indicated in Chap. 4. In the monkey, for example, when a visual signal evokes movement, some of the prefrontal neurons become active immediately preceding the motor response; other prefrontal neurons are activated if the response is to be delayed. With prefrontal lesions on one side or the other, a series of motor abnormalities occur, for example, slight grasping and groping responses, a tendency to imitate the examiner’s gestures and to compulsively manipulate objects that are in front of the patient (imitation and utilization behavior described by Lhermitte [1983]), reduced and delayed motor and mental activity (abulia), motor perseveration or impersistence (with left and right hemispheric lesions, respectively), and paratonic rigidity on passive manipulation of the limbs (oppositional resistance, or gegenhalten).
Incontinence is another manifestation of frontal lobe disease. Rightor left-sided lesions involving the posterior part of the superior frontal gyrus, the anterior cingulate gyrus, and the intervening white matter result in a loss of control of micturition and defecation (Andrew and Nathan). There is no warning of fullness of the bladder or of the imminence of urination or bowel evacuation, and the patient is surprised at suddenly being wet or soiled. Less-complete forms of the syndrome are associated with frequency and urgency of urination during waking hours. The patient is embarrassed unless an element of indifference is added when the more anterior (nonmotor) parts of the frontal lobes are the sites of disease.
In the spheres of speech and language, a number of abnormalities other than Broca aphasia appear in conjunction with disease of the frontal lobes: laconic speech, lack of spontaneity of speech, telegrammatic speech (agrammatism), loss of fluency, perseveration of speech, a tendency to whisper instead of speaking aloud, and dysarthria. These are more prominent with left-sided lesions and are fully described in Chap. 22.
In general, when one speaks of cognitive and behavioral aspects of frontal lobe function, reference is made to the more anterior (prefrontal) parts rather than the motor and linguistic parts. These most recently developed parts of the human brain, called the “organ of civilization” by Halstead and repeated by Luria, have the most elusive functions.