Whereas certain regions of the cerebrum are committed to special perceptual, motor, sensory, mnemonic, and linguistic activities, the underlying intricacy of the anatomy and psychophysical mechanisms in each region are just beginning to be envisioned. The lateral geniculate-occipital organization in relation to vision and recognition of form, stemming from the work of Hubel and Wiesel, may be taken as an example. In area 17, the polar region of the occipital lobe, there are discrete, highly specialized groups of neurons, each of which is activated in a small area of lamina 4 in response to spots of light or lines transmitted via particular cells in the lateral geniculate bodies; other groups of adjacent cortical neurons are essential for the perception of color. Lying between the main unimodal receptive areas for vision, audition, and somesthetic perception are zones of integration called heteromodal cortices. Here neurons respond to more than one sensory modality or neurons responsive to one sense are interspersed with neurons responsive to another.
The integration of cortical with subcortical structures is reflected in volitional or commanded movements. A simple movement of the hand, for example, requires activation of the premotor cortex (also called accessory motor cortex), which projects to the striatum and cerebellum and back to the motor cortex via a complex thalamic circuitry before the direct and indirect corticospinal pathways can activate certain combinations of spinal motor neurons, as described in Chaps. 3 and 4.
Interregional connections of the cerebrum are required for all natural sensorimotor functions; moreover, as indicated above, their destruction disinhibits or “releases” other areas. Denny-Brown referred to the latter as cortical tropisms. Thus, destruction of the premotor areas, leaving the precentral and parietal lobes intact, results in release of sensorimotor automatisms such as groping, grasping, and sucking. Parietal lesions result in complex avoidance movements to contactual stimuli. Temporal lesions lead to a visually activated reaction to every observed object and its oral exploration, and limbic emotional and sexual mechanisms are rendered hyperactive.
Another group of disorders known as disconnection syndromes depend not merely on involvement of certain cortical regions but more specifically on the interruption of interand intrahemispheric fiber tracts. Extensive white matter lesions may virtually isolate certain cortical zones and result in a functional state that is the equivalent of destruction of the overlying cortical region. Some of these disconnections are indicated schematically in Fig. 21-6; the usually involved fiber systems include the corpus callosum, anterior commissure, uncinate temporofrontal fasciculus, occipitoand temporoparietal tracts. An example is the isolation of the perisylvian language areas from the rest of the cortex, as occurs with anoxic–ischemic infarction of border zones between major cerebral arteries (see “Disconnection Syndromes” further on).
The frontal lobes lie anterior to the central or rolandic sulcus and superior to the sylvian fissure (Fig. 21-1). They are larger in humans (30 percent of the cerebrum) than in any other primate (9 percent in the macaque). Several systems of neurons are located here, and they subserve different functions. Brodmann areas 4, 6, 8, and 44 relate specifically to motor activities. The primary motor cortex, that is, area 4, is directly connected with somatosensory neurons of the anterior part of the postcentral gyrus as well as with other parietal areas, thalamic and red nuclei, and the reticular formation of the brainstem. The supplementary motor cortex, a portion of area 6, shares most of these connections. As pointed out in earlier chapters, all motor activity requires sensory guidance, and this comes from the somesthetic, visual, and auditory cortices and from the cerebellum via the ventral tier of thalamic nuclei.