Copyright © 1999 Massachusetts Institute of Technology
Our knowledge about the organization of the CEREBRAL CORTEX is derived, in part, from the search for a therapeutic intervention for a particular disease -- epilepsy. Wilder Penfield (1891-1976), stimulated by his postgraduate work with Otfrid Foerster, a pioneer in the development of modern neurosurgical procedures to relieve seizures in epileptic patients, began a prolonged scientific study of the surgical treatment of epilepsy at McGill University in 1928. By 1934, Penfield had founded Montreal Neurological Institute (MNI), which he served as director until his retirement in 1960. Penfield was soon joined by others, including Herbert Jasper, who introduced the EEG to the operating room, and by D. O. HEBB and Brenda Milner, who introduced the idea of systematic neuropsychological assessment of surgical patients. The foundation and the establishment of the endowment for the MNI, or "Neuro," which has become an international center for training, research, and treatment related to the brain and diseases of the nervous system, may be Penfield's most lasting legacy. The idea of a neurological hospital, integrated with a multidisciplinary brain research complex, providing a center where a mutidisciplinary team of both scientists and physicians might study the brain, has served as a model for the establishment of similar units throughout the world.
By the mid-1930s, Penfield and his colleagues were employing electrical stimulation and systematic mapping techniques adapted from physiological work with animals. These procedures were employed to aid in excising those regions of brain tissue that served as the focus of epileptic activity in patients whose seizures were not adequately controlled by available drugs. Electrical brain stimulation provided the "gold standard" by which the functional properties of brain regions might be determined. This Montreal Procedure was used both to localize the epileptogenic tissue itself and to minimize surgical damage by first mapping critical motor, somatosensory, language-related brain tissue by applying brief, low-voltage electrical current through thin wire electrodes to sites on the cortical surface of the brains of fully conscious human patients. It was then noted which parts of the body moved, or what bodily sensations were reported in response to each stimulus. By the late 1930s, Penfield and his coworkers had created the first systematic maps of both the human primary motor and somatosensory cortex. Their data indicated there was a point-to-point relation between parts of the body and these neocortical regions (i.e., that motor and somatosensory cortex were both somatotopically organized) and that these distributions of the body surface were distorted, leading to the construction of his famous sensory and motor homunculistylized cartoons of the body surface with the relative prominence of different body parts reflecting the extent of their representation in the cortex. A sensorimotor integrative conception of brain organization was also promoted by his finding that 25 percent of the stimulation points yielding sensory experiences were located in precentral motor cortical regions. Subsequent investigation of nonhuman subjects led to identification of analogous maps or representations of visual and auditory external worlds and, together with Penfield's own mapping work, helped shape our view of cortical organization for decades. While his original work suggested that there were several somatosensory cortical representations of the external environment, it was not until the late 1970s that more refined anatomical and physiological techniques revealed dozens of maps in each modality, rather than just one or two.
Careful study of hundreds of patients by Penfield and his coworkers (and more recently by George Ojemann and his colleagues at the University of Washington) also provided clear evidence of cerebral asymmetry or HEMISPHERIC SPECIALIZATION. For example, the pooled data from many patients yielded the first direct confirmation of conclusions inferred from previous postmortem correlations, by establishing a map of language-related zones of the left hemisphere that included not only the traditional areas of Paul BROCA and Carl Wernicke, but also the supplementary speech zone. Stimulation in these regions of the left hemisphere usually arrested (or initiated) speech during the stimulation period or produced other forms of language interference such as misnaming and impaired word repetition, whereas stimulation of the right hemisphere seldom did. Penfield's research also furnished other evidence that did not support traditional localizationist models of language. For example, stimulation of anterior and posterior speech zones had remarkably similar effects on speech function, and the extent of these cortical language zones varied considerably among patients.
Accounts of apparent awaking of long-lost childhood and other memories by temporal lobe epileptics during electrical stimulation of the region were recorded by Penfield in the 1930s. This data, together with evidence provided in the neuropsychological studies of such patients after surgery by Brenda Milner and others, made it clear to Penfield that the medial temporal region, including the HIPPOCAMPUS, was of special importance in respect to human MEMORY (and emotion).
Penfield's early observations on seizures arising from deep midline portions of the brain also had an important impact on the development of ideas about the neural substrate of CONSCIOUSNESS. In 1938 he proposed a "centrencephalic" system that stressed the role of the upper brain stem in the integration of higher functions. In arguing that consciousness is more closely related to the brainstem than the cortex, he foreshadowed Moruzzi and Magouns's (1949) conception about the role of the midbrain reticular formation. "Consciousness," he later wrote, "exists only in association with the passage of impulses through ever-changing circuits between the brainstem and cortex. One can not say that consciousness is here or there. But certainly without centrencephalic integration, it is nonexistent." Penfield's lifelong search for a better understanding of the functional organization of the brain and its disorders during epileptic seizures is symbolized by this hypothesis of the central integrating mechanism. Never localized in any specific area of gray matter, but "in wider-ranging mechanisms," it represented a conceptual bridge he envisaged between brain and mind (cf. MIND-BODY PROBLEM).
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Penfield, W. (1958). The Excitable Cortex in Conscious Man. Springfield, IL: Charles C. Thomas.
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Copyright © 1999 Massachusetts Institute of Technology