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Introduction RANDOLPH BLAKE AND ROBERT SHAPLEY This conference on the modularity of vision is devoted to contempo- rary developments surrounding one of the oldest problems in science and medicine: the localization of function in the brain. This volume contains a collection of transcribed presentations by scientists from a diverse array of disciplinesanatomy, neurophysiology, and psychology. Taken together, these presentations critically explore the idea that the processing of visual information is carried out in different visual areas of the brain. This idea has been fostered, in part, by exciting developments in the technology for studying the brain as well as by new ways of thinking about the brain as a sophisticated device for computing. HISTORICAL CONTEXT The idea that the brain can be subdivided into regions with specialized functions is an old one. As early as the second century ~D., the Egyptians theorized that the brain was divided into regions, each region having a particular functionthe emotions, for example, resided in the anterior ventricle and memories in the posterior ventricle. In the seventeenth century, the anatomist Thomas Willis (whose name is associated with the circle of Willis) realized that the brain tissue, not the fluid-filled spaces within the tissue, was the functional part of the brain. He localized memory in the convolutions of the cortex, imagination in the corpus callosum, and emotions in the base of the cerebrum. Of course, as we all know, Descartes went so far as to localize the soul in the pineal gland. All these early guesses at functional localization were based on philo- sophical speculation, not empirical proof. The nineteenth century anatomist 1
2 RANDOLPH BLARE AND ROBERT SHAP~F:Y Franz Joseph Gall provided the first empirically based theory of localiza- tion. Gall divided the brain into functional units based on what he believed was the brain's topography, as evidenced by the bumps on the outside of the head. His method, known as phrenology, postulated that the bumps on the exterior surface of the skull corresponded to regions of the brain that were particularly well developed. He assigned specific cognitive processes and even personality traits to each area. The scientific establishment ques- tioned Gall's methods, pointing out that the exterior of the skull is a poor representation of the structure of the brain within. Phrenology was wrong in its details but, as a method, it played a role in advancing the notion that mind is divisible into faculties spatially distributed throughout the brain. Gall's reputation as a distinguished anatomist meant that his beliefs were not to be dismissed lightly. Following Gall, the question of localization of function within the brain drew more and more attention. Gradually evidence accumulated in favor of the idea that the brain consists of an assembly of discrete, specialized machinery designed to accomplish particular tasks. nercentual. cognitive and motor. The evidence took several forms. ~ ~ , ~ Lesion experiments by the David Ferrier in England demonstrated specific behavioral deficits produced by carefully placed, localized lesions. At more or less the same time, anatomists were beginning to subdivide the brain into histologically distinct areas; probably the most famous of these brain cartographers was Korbinian Brodmann, whose areas are still part of the neuroanatomical landscape. Interesting evidence was also cropping up in the hospital clinics. Neurologists were beginning to describe syndromes wherein localized damage to the human brain produced rather specific functional losses. Perhaps the most famous case is the one described by Paul Broca, the neurologist who localized speech within the temporal lobe of the left hemisphere of the brain. --r By the early twentieth century, the question of broad functional seg- regation was settled: there was no doubt that the brain was divisible into discrete sensory and motor areas, and by the 1940s it was realized that there existed even finer subdivisions within these two broad functional categories. However, it wasn't until the late l95Os that we began to get a closer glimpse of the actual neural machinery housed within these specialized sensory ar- eas. The Nobel prizewinning work of lbrster Wiesel and David Hubel gave entirely new meaning to the notion of function specificity, particularly as it applied to the collation of neurons with similar receptive field properties. Their pioneering experiments set the stage for a more refined examination of visual information processing. From that work, we now know that in higher mammals Brodmann's areas 18 and 19 in fact consist of multiple representations of the visual field, varying in their size, internal organiza- tion, and pattern of connections. In very recent years physiologists have
INTRODUCTION 3 begun elucidating functional differences between these areas, and we're now seeing in the literature complex circuit diagrams. Some believe that the field has matured to the point at which it is reasonable to speculate intelligently about the specialized roles played by various extrastriate areas in visual perception and in visually guided behavior. And it is some of those speculations that we'll be hearing about today. But interestingly, there's more to the story than neuroanatomy and neurophysiology: there have also been important developments in the fields of computer science and psychophysics. Those developments likewise speak to the issue of the functional role of multiple visual maps in the brain. CURRENT DEVELOPMENTS This symposium on the modularity of vision is mainly about visual areas of the cerebral cortex and the role of the cerebral cortex in visual information processing. 1b those of us who work in what is called "early vision," that is, stages of visual processing up to and including the primary visual cortex or striate cortex, this symposium is mainly concerned with "late vision," that is, areas of the cerebral cortex that are involved in fairly high levels of visual image processing. The work is therefore very closely related to perception, and also to algorithms of high-level image processing by machines. One of the major issues in late visionor, indeed, in the entire field of vision research- is the nature of the processing of the visual signal by the brain: how is it done, where is it done, and so on. A more detailed question, one that is very much a current focus of research, is whether the visual system processes visual information in a serial or parallel manner. The idea of a strict serial or hierarchical mode of visual signal pro- cessing was most explicitly stated in the early papers of Hubel and Wiesel, though it has remained an idea of contemporary appeal since that time. The idea, in its simplest form, is that visual properties of neurons- the receptive fields are built up at each level by convergence of neural infor- mation from the previous stage of the visual pathway. The concept is that receptive fields of neurons in the primary striate cortex are composed of the sum of receptive fields in the lateral geniculate nucleus. The concept is also that the receptive fields of neurons in accessory visual areasthe extra striate cortex are built up by summing the activity of groups of neurons in V-1, the primary area, and so on in series from V-2 to V-3 and V-4. The name serial processing refers to a series of receptive field processing, one after the other. The concept of parallel processing, in contrast, is that there are sep- arate and independent lines of signal flow from the retina to the brain,
4 RANDOLPH BLAKE AND ROBERT SHIPLEY and from one area to another within the brain. The receptive field prop- erties of neurons in the cerebral cortex are determined not only by what stage or level of processing they are at, but also by which stream of signal how they are getting input from at the periphery. Perhaps the boldest expression of this hypothesis of parallel processing was presented initially by Jonathan Stone and Bogdan Dreyer in their work on the visual receptive field properties of neurons in primary visual cortex. The frontier of vision science represented here today has revealed that neither an extreme serial nor an extreme parallel view is sufficient to account for the way our brains are wired. There is evidence of segregation of parallel channels in the cortex segregated all the way from the eye to the cortex. There is also clear evidence for hierarchical serial processing within these parallel streams. The idea that there is strict serial or strict parallel processing seems to be ruled out: there is evidence for both. Another issue addressed in this symposium is the existence and the significance of multiple maps of the visual world inside our heads. Some rationale for the existence of these multiple maps comes from recent work on computational vision. Multiple maps of the outside world are found not only in vision; they are also found in the auditory system. They signify the importance of having multiple models of the world inside our head for subsequent processing in order to perform, in the case of vision, significant tasks about calculating the brightness and the color of objects and their shapes, for example. That the existence of spatial maps also shows us that, intrinsically, the analysis of visual space is done in parallel, by having available to the brain at any one time activity representing all visual space.
