One may inquire into what precisely is meant by cerebral localization. Does it refer to the physiologic function of a circumscribed group of neurons in the cerebral cortex, indicated clinically by a loss of that function when the neurons in question are destroyed? This is the way in which neurologists have assigned functions to particular areas of the cerebral cortex. However, from what we know of the rich connectivity of all parts of the specialized cortical centers, one must assume that this is only partly the case. Most who ponder this subject believe that the organization of cerebral function is based on discrete networks of closely interconnected afferent and efferent neurons in several regions of the brain. These ensembles must be linked by both regional and more widespread systems of fibers. This is especially apparent in the discussion of the anatomy of complex cognitive properties such as intelligence, as described in Chap. 20. Thus, many basic functions are anchored in one cortical region and a lesion there causes loss of a particular ability. But it is apparent from physiologic studies such as functional imaging and electromagnetic stimulation that widely distributed networks are engaged, which nonetheless encompasses the region that can be ablated and eliminate the function in question.
These aspects of cerebral localization—brought out so clearly in the writings of Wernicke, Dejerine, and and the Russian school of physiologists and psychologists and extended by Geschwind (1965). In keeping with the model of interconnected networks, they viewed function not as the direct property of a particular, highly specialized region of the cerebrum but as the product of complex, diffusely distributed activity by which sensory stimuli are analyzed and integrated at various levels of the nervous system and then united, through a system of temporarily acquired connections, into a working mosaic adapted to accomplish a particular task. To some extent, this model has been corroborated by functional imaging studies, which show increased metabolic activity in several cortical regions during almost every form of human behavior, including willed motor acts, language tasks, and those coinciding with perceptive and apperceptive sensory experiences. Within such a functional system, the initial and final points (the task and the effect) remain unchanged, but the intermediate links (the means of performance of a given task) may be modified within wide limits and will never be exactly the same on two consecutive occasions. Thus, when a certain act is called for by a spoken command, the dominant temporal lobe must receive the message and transmit it to the premotor areas. Or it may be initiated by the intention of the individual, in which case the first measurable cerebral activity (a “readiness potential”) occurs anterior to the premotor cortex. The motor cortex is also always under the dynamic control of the proprioceptive, visual, extrapyramidal, and vestibular systems. Thus, a lesion that affects any one of several elements in the act may cause loss of a skilled ability.
Another theoretical scheme of cerebral function identifies cortices of similar overall structure and divides the cerebral mantle into three longitudinally oriented zones, the triune brain articulated by Paul MacLean. A central vegetative neuronal system (allocortex and hypothalamus) provides the mechanisms for all internal functions, the milieu intérieur of Bernard and Cannon. An outer zone, comprising the sensorimotor and association cortices and their projections, provides the mechanisms for perceiving the external world and interacting with it, and a region between them (limbic–paralimbic cortices) that provides the bridges that permit the adaptation of the organism’s needs to the external environment. This ostensibly metaphoric concept of central nervous system function, first proposed by Broca, was elaborated by Yakovlev and has been adopted more recently by Benson and by Mesulam (1998).
Such a model retains to a large degree the cytoarchitectural similarities among areas that serve similar functions (i.e., the scheme of Brodmann discussed further on) and also respects the sequence of brain maturation (myelination) of connecting pathways proposed by Flechsig (see Fig. 27-3). In this way, localization may be viewed as the product of genetic patterns of structure, which mature during development, and their synaptic formations, which permit the development of complex circuits during lifelong learning and experience.