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Hierarchical organization

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370 Hierarchical organization and functional streams in the visual cortex David C. Van Essen and John H. R. Maunsell In the macaque monkey, a dozen distinct visual areas have been identified in the cerebral cortex. These areas can be arranged in a well-defined hierarchy on the basis of their pattern of interconnections. Physiological recordings suggest that there are at least two major functional streams in this hierarchy, one related to the analysis of motion and the other to the analysis o f form and color. Intheirpioneeringstudiesofthecat'svisual mation processing along the visual path- cortex, Hubel and Wiesel t6,17 obtained evi- way. They described cells in areas 17, 18 dence for many sequential stages of infor- and 19 whose receptive-field properties dif- V2 IIN5 ,Septembt¢r / :~.; fered greatly in their complexity and degree of selectivity. On the basis of these observa- tions, they proposed that the properties of cells at any given stage could be derived through appropriate inputs from cells at the immediately preceding stage, in an order proceeding from cells of the lateral genicu- late nucleus (LGN) to simple cells; com- plex cells, hypercomplex cells and, at the highest stage which they studied, 'higher- order hypercomplex cells'. Their scheme represented the simplest form of hierarchi- cal organization, with information being processed in exclusively serial fashion. As visual pathways were studied in great- er detail during the ensuing two decades, a variety of evidence was obtained that did not support a strictly serial ~heme of organ- ization. One of the first pieces of evidence was Hubel and Wiesers own finding ~7 that V4 IT P Fig. 1. Cortictd areas in the maa~ue monkey. Locations of areas are shown on a lateral view of the right hemi~here (inset) and on a two-directional unfolded map of the entire cerebral hem~phere. Visual areas are ~ and colored individually. A single shade is used for each of the other ~ sy~ (auditoty, somatosensory and motor), and individual areas are outlined but not: labelled On the cortical map. Abbreviations for visual areas: M T ( ~ temporal)u; MST (medial superior temporal)=7; VIP (ventral intraparietal)=7; VP (ventral posterior)S*; IT (inferotemporal)a. Evidence for the location and ~ of these are~ is discussed in Re# 2 8 and 35. Area POa is tentatively split imo two subdivisions on the basis of anatomical inhomogeneities ~. The prostriate area ( PS) , which is split by an artificial discontinuity, does not have known sensory functions in the macaque. ~) 1983. Elsevier Science Publishers B.V., Amsterdam 0378 - 5912t83/$01.00. For technical reasons we are unable to reproduce this figure in colour - see monthly issue for full colour reproduction.TINS - September 1 983 371 area 17 projects to several cortical areas (18, 19 and lateral suprasylvian cortex), thus demonstrating parallel outputs from a single area. Other important observations, discussed in a number of recent reviews 1°.22.3s.43,44, include the demonstra- tion of: (1) direct projections from the LGN to areas 18, 19 and lateral suprasylvian cor- tex as well as to area 17; (2) projections from areas 18 and 19 back to 17; (3) direct LGN inputs onto complex cells as well as simple cells; and (4) distinct retinal gang- lion cell classes, the X, Y and W cells, whose outputs remain partially segregated at cortical as well as subcortical levels. These various lines of evidence clearly suggest that information is to some degree processed in parallel fashion within the visual system. While a strictly sequential- processing scheme is thereby ruled out, it is equally clear that visual processing is not exclusively parallel, in so far as there does exist significant cross-talk between chan- nels. Thus, it is unfortunate that the ques- tion of cortical organization has sometimes been treated as a debate over parallel versus serial processing, as though these were the only alternatives that needed to be con- sidered 3. ~s47. It makes sense first to address the question of whether the visual system is in fact organized in hierarchical fashion. A hierarchy, by definition, is a system whose members can be unambiguously assigned to specific levels or ranks in rela- tion to one another. For example, it is obvious from what is known about retinal organization that retinal ganglion cells are at a more advanced level of processing than bipolar cells, which in turn are at a more advanced level than photoreceptors. Within each cell class there are distinct subclasses, such as the X, Y and W ganglion cells, which provide parallel outputs from a single hierarchical level. Although a pr/or/ there is no guarantee that higher levels of the system follow the same organizational principle. In particular, visual cortex might be organized as a complex network in which distinct hierarchical levels of pro- cessing simply do not exist. Examples of well-studied neural networks which appar- ently lack hierarchical organization include oscillatory circuits involved in rhythmic motor activities in invertebrates "~,a2. The structural framework of a system (for example whether or not it is hierarchi- cal) need not be the same at all levels of organization. For example, the aforemen- tioned oscillatory circuits, even if not hierarchical in their internal organization, may nonetheless be members of a larger hierarchical system whose other members also consist of multicellular ensembles rather than individual cells or cell classes. Along similar lines, it is possible that in the visual system different cortical ureas are related to one another in hierarchical fash- ion irrespective of whether the individual areas are internally organized as hierar- chies. This article is concerned mainly with the relationships among visual areas, rather than with their internal circuitry. Cortical visual areas During the past decade considerable progress has been made in identifying a large number of specific, well-defined sub- divisions of visually responsive cortex. Many of these visual areas differ greatly from the cytoarchitectonic subdivisions described by anatomists in the early 1900s. The currently recognized cortical areas have been identified on the basis of infor- mation relating to their connections,


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