It is worthwhile to point out that these broadened concepts of cerebral function, which apply to all mental activities, contradicts both the historical notion that there is a functional equivalence of all cerebral regions and also the more recently developed one that assumes strict localization of any given activity.
From these remarks, it follows that subdivision of the cerebrum into frontal, temporal, parietal, and occipital lobes is somewhat of an abstraction in terms of landmarks and cerebral function. Some of these delineations were made long before our first glimmer of knowledge about the function of the cerebrum. Even when neurohistologists began parceling the neocortex, they found that their areas did not fall neatly within zones bounded by sulci and fissures. Therefore, when the terms frontal, parietal, temporal, and occipital are used, it is largely to provide the clinician with familiar and manageable anatomic landmarks for localization (Fig. 21-1).
The current method of study of cortical activity is by functional imaging techniques (positron emission tomography [PET] and functional magnetic resonance imaging [fMRI]). Invariably, an ensemble of areas, a “network” of the variety described earlier, is activated to perform even seemingly simple tasks such as recalling a name, visualizing or identifying an object, or carrying out a commanded task. The fact that multiple areas of the cortex are entrained may seem at odds with the classic view of lesional neurology, but as already stated, the discrepancy is one of epistemology in that normal function does not equate with abnormal function as exposed by a focal lesion. A lesion in the cerebrum merely exposes the site at which damage results in the greatest loss of that particular function but does not reveal the much wider area that is essential for the full normal operation of that function. Imaging studies similarly demonstrate that certain regions of the cortex are necessary to fully conduct particular behaviors, but they are not sufficient for their enactment.
Pertinent to this subject are a number of morphologic and physiologic observations. Along strictly histologic lines, Brodmann distinguished 47 different areas of cerebral cortex (Fig. 21-2), and von Economo identified more than twice that number. Although this parceling was severely criticized by Bailey and von Bonin (and the data upon which Brodmann based his system were never published), it is still used by physiologists and clinicians, who find that the Brodmann areas do indeed approximate certain functional zones of the cerebral cortex (Fig. 21-3). Also, the cortex has been shown to differ in its various parts by virtue of connections with other areas of the cortex and with the thalamic nuclei and other lower centers. Hence, one must regard the cortex as a heterogeneous array of many anatomic systems, each with highly organized intercortical and diencephalic connections.
The sheer size of the cortex is remarkable. Unfolded, it has a surface extent of about 4,000 cm2, about the size of a full sheet of newsprint (right and left pages). Contained in the cortex are many billions of neurons (estimated at 10 to 30 billion) and five times this number of supporting glial cells. The intercellular synaptic connections number in the trillions. Because nerve cells look alike and presumably function alike, the remarkable diversity in human intelligence, store of knowledge, and behavior must depend on the potential for almost infinite variations in neuronal interconnectivity.