Neocortical Functions

As the name suggests, the neocortex is, phylogenetically, the most recent part of the cerebrum to be explored and shows great development in primates and especially man. The neocortex is the organ par excellence subserving those functions we consider uniquely human, or highly developed only in man, such as abstract reasoning, aesthetic values, judgment, and language. There is disagreement among investigators as to the exact anatomic limits of the neocortex. In general, however, the term refers to the dorsolateral portion of the cerebral cortex. This is the surface of the cerebral hemispheres which is visible without sectioning the exposed brain as viewed from the top and sides (Fig. 7).

Traditionally, the neocortex is divided into two functional parts: primary projection areas and association areas. The fibers of the primary projection areas connect the cortex with deeper structures and mediate sensory and motor functions. Neurons of the primary motor system take their origin largely from the precentral gyrus and course downward to synapse eventually with lower motor neurons in the brain and spinal cord. The latter terminate in skeletal muscles. Fibers of the primary sensory projection system take their origin in the various sensory receptors of the body. These course upward and eventually terminate, after synapse in the thalamus, in the cortical areas sub-serving the various sense modalities. Those concerned with general sensation, such as touch, pressure, and heat, end chiefly in the postcentral gyrus (Fig. 7). Recent evidence suggests that the specificity of the precentral gyrus and postcentral gyrus as motor and sensory areas, respectively, is not as great as it was once thought and that considerable overlap exists (47).

The remainder of the neocortex consists of association areas. These are the areas concerned with higher mental activity, as language and reasoning. Traditionally these areas are conceived of as mediating between sensory and motor areas in a kind of transcortical reflex. That is, the sensory projection areas organize the input and impinge upon the fibers of association areas where the messages are elaborated. From here the messages are passed on to motor projection areas (output). However, experimental evidence to support this conception has been generally lacking. On the basis of recent anatomic and neuropsychologic data, Pribram (47, 48) suggests an alternative to the transcortical model of neocortical organization. He uses the term extrinsic sectors to refer to those neocortical areas whose fibers enter or leave the cerebral hemispheres. These correspond roughly to the primary projection areas. The fibers of the intrinsic sectors, corresponding roughly to association areas, remain within the cerebrum. In Pribram's model, the principal interaction of extrinsic and intrinsic systems occurs at the thalamic level. That is, the specific contribution of intrinsic neocortex to the final output of the extrinsic system is mediated by the convergence of influences from both intrinsic and extrinsic systems by subcortical mechanisms. The intrinsic system may influence also the input of the extrinsic systems by regulation of peripheral sensory mechanisms (47).

The localization of function within intrinsic neocortex has not been so successful as localization within the extrinsic system, e.g., motor and sensory areas. On the basis of behavioral experiments with primates, Pribram suggests a simple division of the intrinsic system into two sectors: an anterior sector which corresponds to the frontal lobes and a posterior sector which corresponds to the parietal, temporal, and preoccipital regions.

Lesions of the posterior intrinsic sector disrupt the animal's ability to perform discriminations involving one or another sensory modality. For example, lesions of the inferotemporal area of the posterior intrinsic system disrupt the animal's ability to learn a task involving a visual discrimination. In contrast, lesions of the frontal intrinsic sector differentially impair the class of behaviors which involve a time delay, such as the delayed-alternation task described earlier.

Other functions of the neocortex were alluded to earlier. Lesions involving parts of the inferior frontal convolution on the left side are associated with expressive aphasia (In 50 per cent of left-handed individuals the speech areas are in the right hemisphere). Lesions of the superior temporal gyrus on the left side produce receptive aphasia. Alternatively, lesions of the parietal and temporal lobes on the right side commonly produce defects in spatial and temporal orientation.

This lateralization of speech functions to the left hemisphere and spatial and temporal functions to the right are striking exceptions to functional symmetry in the body. In other instances of anatomically identical paired structures arranged symmetrically about the body, such as the adrenals, kidneys, and ovaries, function is also identical. However, the neocortex is an instance of functional asymmetry in the presence of anatomic symmetry.

Recent refinements in technics for the experimental analysis of behavior (see Section III) and the development of newer means of manipulating brain function, as surgical lesions and chemical stimulation, promise further knowledge of the neocortex. These technics have also yielded much information about the paleocortex and limbic system.