Copyright © 1999 Massachusetts Institute of Technology
During the first twenty-five centuries of studies of brain function, almost all investigators ignored or belittled the CEREBRAL CORTEX. One exception was the Alexandrian anatomist Erasistratus (fl. c. 290 B.C.E.), who on the basis of comparative studies attributed the greater intelligence of humans to their more numerous cortical convolutions. This view was ridiculed by Galen (129-199), the most influential of all classical biomedical scientists, whose sarcastic dismissal of any significant role for the cortex continued to be quoted into the eighteenth century. Another major exception was Thomas Willis (1621 - 1675), a founder of the Royal Society, and author of the first monograph on the brain. On the basis of his dissections, experiments on animals, and clinical studies of humans, he attributed memory and voluntary movement functions to the cortex. However, by far the dominant view on cortical function before the beginning of the nineteenth century was that the cortex was merely a protective rind (cortex means "rind" in Latin), a glandular structure (the early microscopists saw globules in cortex, probably artifacts), or a largely vascular structure made up of small blood vessels. The apparent insensitivity of the cortex to direct mechanical and chemical stimulation was used as an argument against the cortex having any important functions in sensation, mentation, or movement.
The systematic localization of different psychological functions in different regions of the cerebral cortex begins with Franz Joseph Gall (1758-1828) and his collaborator J. C. Spurzheim (1776 - 1832), the founders of phrenology. The central ideas of their phrenological system were that the brain was an elaborately wired machine for producing behavior, thought, and emotion, and that the cerebral cortex consisted of a set of organs with different functions. Postulating about thirty-five affective and intellectual faculties, they assumed that these were localized in specific cortical organs and that the size of each cortical organ was indicated by the prominence of the overlying skull, that is, by cranial bumps. Their primary method was to examine the skulls of a wide variety of people, from lunatics and criminals to the eminent and accomplished. Although the absurdity of their dependence on cranial morphology was quickly recognized in the scientific community, Gall's ideas about the cortex as a set of psychological organs stimulated investigation of the effects of cortical lesions in humans and animals and of structural variations across different cortical regions, and thus had a lasting influence on the development of modern neuroscience.
Examining a variety of animals, Pierre Flourens (1794-1867) found that different major brain regions had different functions; he implicated the cerebral hemispheres in willing, remembering, and perceiving, and the CEREBELLUM in movement. Within the cortex, however, he found no localization of function: only the size and not the site of the lesion mattered. Although these results appeared to refute the punctate localizations of Gall, they actually supported both the general idea of localization of function in the brain and the specific importance Gall had given to the cerebral hemispheres in cognition.
In 1861, Paul BROCA described several patients with longstanding difficulties in speaking, which he attributed to damage to their left frontal lobes. This was the first generally accepted evidence for localization of a specific psychological function in the cerebral cortex (and was viewed at the time as a vindication of Gall's ideas of localization). Soon after, Fritsch and Hitzig demonstrated specific movements from electrical stimulation of the cortex of a dog, and drew the inference that some psychological functions and perhaps all of them need circumscribed centers of the cortex. The next major development was Carl Wernicke's 1876 report of a second type of language difficulty, or APHASIA, namely, one in understanding language; he associated this type of aphasia with damage to the posterior cortex in the region where the occipital, temporal, and parietal areas meet. Furthermore, he extended the idea of specialized cortical areas by stressing the importance of the connections among different areas, particularly for higher mental functions.
The last years of the nineteenth century saw several acrimonious controversies about the location of the various cortical sensory areas, involving such figures as David Ferrier, E. A. Schafer, and Hermann Munk. These issues were resolved first in monkeys and then in humans, so that by the end of World War I (with its rich clinical material), the location and organization of the primary visual, auditory, somesthetic, and motor areas of the cortex had been defined. By this time, the cerebral cortex had been divided up into multiple regions on the basis of regional variations in its cellular or fiber structure. The more lasting of these cortical architectonic parcellations were those by Korbinian Brodmann and Constantin von Economo, who created the numbering and lettering schemes, respectively, that are still in use. Despite their new labels, however, the functions of vast regions of the cortex, other than the primary sensory and motor areas, remained mysterious. These regions were termed association cortex, initially because they were thought to be the site of associations among the sensory and motor areas. Under the influence of British association psychology (typified by John Stuart Mill and Alexander Bain), association cortex was believed to be the locus of the association of ideas, and after Pavlov, the locus of the linkage between conditioned stimuli and responses.
Parallel with the success of the localizers around the turn of the century, there was also a strong antilocalization tendency. Adherents of this view, such as C.E. Brown-Sequard, Friedrich Goltz, Camillo GOLGI, and Jacques Loeb, emphasized such phenomena as the variability and recovery of symptoms after brain damage. They stressed that higher cognitive functions, particularly INTELLIGENCE and MEMORY, could not be localized in specific regions of the cortex. Like Flourens, Goltz reported that it was the size and not the location of the lesion that determined the severity of its effects on such higher functions. This holistic view of brain function was reinforced by the rise of GESTALT PSYCHOLOGY.
The best-known investigator of the relative importance of the size and site of a cortical lesion was Karl S. LASHLEY, easily the foremost figure in the study of the brain in the 1940s and 50s. On the basis of a long series of experiments, particularly on rats in a complex maze, he proposed two principles of brain organization, "equipotentiality" and "mass action" (Lashley 1929). Equipotentiality was the apparent capacity of any intact part of a functional area to carry out, with or without reduction in efficiency, the functions lost by destruction of the whole. Lashley assumed equipotentiality to vary with different brain areas and with different functions and thought it might only hold for association cortex and for functions more complex than sensory or motor ones such as maze learning. Furthermore, equipotentiality was not absolute but subject to a law of mass action whereby the efficiency of a whole complex function might be reduced in proportion to the extent of brain injury within an equipotential area. He stressed that both principles were compatible with cortical localization of functions and himself reported several findings of specific cortical localizations.
Lashley's most famous (or infamous) result was that both principles held for the entire cerebral cortex of rats learning a complex maze. That is, performance in this maze was independent of the site of the cortical lesion and only dependent on its size. We now know that these mass action results were due to increasing encroachment on multiple areas critical for different components of maze learning with increasing size of lesion. In recent years, Lashley's specific ideas on equipotentiality and mass action (and many of his other contributions) are often forgotten, and he is inaccurately described as an extreme "antilocalizer," who thought the brain was like a bowl of jelly.
Starting in the 1930s, systematic evidence for the localization of various cognitive functions in regions of association cortex began to emerge, particularly from students and associates of Lashley. In an experiment still at the core of contemporary research on the frontal lobes, Carlyle Jacobsen showed that frontal cortex lesions impair the performance of delayed response tasks, in which the monkey must remember which of two cups a peanut was placed under, a deficit Jacobsen described as one of short-term memory. (This result, through no fault of Jacobsen's, led directly to the introduction of frontal lobotomy as a psychosurgical procedure in humans.) In another seminal experiment, K.-L. Chow, in 1950, showed that lesions of temporal cortex yield a deficit in pattern recognition, a finding that helped spark the study of extrastriate mechanisms in vision.
Up to the 1950s, the advances in understanding the functions of the cerebral cortex had relied almost entirely on the study of brain damage in humans and other primates. The introduction of evoked response and SINGLE-NEURON RECORDING techniques provided powerful new methods for studying localization of cortical function, methods soon revealing that much of association cortex was made up of areas devoted to processing specific aspects of a single sensory modality. Furthermore, these higher sensory areas were often involved in attentional and mnemonic functions as well as perceptual ones.
Most recently, the introduction of functional MAGNETIC RESONANCE IMAGING (fMRI) and POSITRON-EMISSION TOMOGRAPHY (PET) scanning have begun to radically enhance our understanding of the functional specialization of the cerebral cortex. As we begin to understand the parallel serial and hierarchical ways that the cortex processes, stores, and retrieves information, the phrase "localization of function" sounds increasingly archaic and simplistic.
Finger, S. (1994). Origins of Neuroscience. Oxford: Oxford University Press.
Gross, C. G. (1987). Early history of neuroscience. In G. Adelman, Ed., Encyclopedia of Neuroscience, vol. 2. Boston: Birkhauser, pp. 843-846.
Gross, C. G. (1997). From Imhotep to Hubel and Wiesel: the story of visual cortex. In J. H. Kaas, K. Rockland, and A. Peters, Eds., Cerebral Cortex. Vol. 12, Extrastriate Cortex in Primates. New York: Plenum Press.
Gross, C. G. (1998). Brain, Vision, Memory: Tales in the History of Neuroscience. Cambridge, MA: MIT Press.
Krech, D. (1963). Localization of function. In L. Postman, Ed., Psychology in the Making. New York: Knopf.
Lashley, K. S. (1929). Brain Mechanisms and Intelligence. Chicago: University of Chicago Press.
Boring, E. (1957). A History of Experimental Psychology. 2nd ed. New York: Appleton-Century-Crofts.
Brazier, M. (1988). A History of Neurophysiology in the 19th Century. New York: Raven Press.
Clarke, E., and K. Dewhurst. (1972). An Illustrated History of Brain Function. 2nd ed. San Francisco: Norman.
Clarke, E., and L. Jacyna. (1987). Nineteenth-Century Origins of Neuroscientific Concepts. Berkeley: University of California Press.
Clarke, E., and C. O'Malley. (1996). The Human Brain and Spinal Cord.: A Historical Study Illustrated by Writings from Antiquity to the Twentieth Century. San Francisco: Norman.
Corsi, P., Ed. (1991). The Enchanted Loom. Chapters on the History of Neuroscience. Oxford: Oxford University Press.
Fearing, F. (1970). A Study in the History of Physiological Psychology. Cambridge, MA: MIT Press.
Fulton, J. (1966). Selected Readings in the History of Physiology. 2nd ed. Springfield, IL: Thomas.
Harrington, A. (1987). Medicine, Mind and the Double Brain. Princeton: Princeton University Press.
Liddell, E. (1960). The Discovery of Reflexes. Oxford: Oxford University Press.
Meyer, A. (1971). Historical Aspects of Cerebral Anatomy. Oxford: Oxford University Press.
Neuburger, M. (1981). The Historical Development of Experimental Brain and Spinal Cord Physiology before Flourens. Baltimore: Johns Hopkins University Press.
Poytner, F. N. (1958). The History and Philosophy of Knowledge of the Brain and Its Functions. Oxford: Blackwell.
Polyak, H. (1957). The Vertebrate Visual System. Chicago: University of Chicago Press.
Shepherd, G. (1991). Foundations of the Neuron Doctrine. Oxford: Oxford University Press.
Singer, C. (1957). A Short History of Anatomy and Physiology from the Greeks to Harvey. New York: Dover.
Spillane, J. (1981). The Doctrine of the Nerves: Chapters in the History of Neurology. Oxford: Oxford University Press.
Young, R. (1970). Mind, Brain and Adaptation in the Nineteenth Century. Oxford: Oxford University Press.
Copyright © 1999 Massachusetts Institute of Technology