m y oes the Brain Have So Many Visual Areas? Jon H. aas Department o f Psychology Vanderbilt University, Nashville Introduction The visual system of an advanced mammal, such as a macaque monkey, includes a number o f subcortical and cortical cente rs (see Kaas & Huerta, 1988 for review). The retin a projects to the suprachiasmatic nucleus, the pregen- iculate nucleus, th e lateral geniculat e nucleus, nuclei o f the accessory optic system, the pretectal nuclei, and the superior co lliculus. Mos t o f the very large pulvinar com- plex, which includes at least 7 or 8 nuclei, is also involved in vision. Parts o f the reticular nucleus, the basal ganglia, pons, and cerebellum are predominately visual in func- tion, and the parabigeminal nucleus and oculomotor nuclei of the brain stem are visual centers. Bu t much o f visual processing, especially processing related to con- scious aspects o f vision, occurs in visual cortex, where some 15- 30 visual areas partikipate. A n obvious question, given this multiplicity, is why so many areas? O f course an obvious question is not often new. Cowe y (198l), for example, titled a p aper ‘Why are there so many visual areas?” Kaas ( 198 2) subt itle d a pape r “Why do sensory systems have so many subdivisions?”, and Barlow (1 986 ) recently as ked “Why have multiple cortical areas?”. Cowey ( 19 81 ) noted that “the answer to the question . . is not clear” and “that is why it is worth discussing.” The gen eral consensus is that t he different visua l areas are specialized o mediate different aspects o f vision, but there are varyi ng opinions about the nature f these different aspects, and about what is gained by increasing the numbers o f visua l areas. The present paper considers what is a visual area, how species vary in numbers o f visual areas, and the possible advantages of having larger numbers o f visua l areas. 1 . WhatLaVLualArea? BrodmaM (1909) regarded cortic al areas as the “organs” of the brain, with specific functions or sets o f functions. Because o f their functional distinctiveness , he “organs” o f the brain were expected t o differ i n histologic al structure, and Brodmann used architect ural differenc es n the of humans and a wide-range o f other mammils to parcel cortex into areas of presumed functional significance. Eight decad es o f subsequent research support Brodmann’s assumption that cortex is sharply divided into a mosai c of functional ly distinct areas o r fields. However, the architectonic method, whe n u sed by itsel f, is of ten unreliable . Because the architectonic method and other ap- proaches all have the potential for error, cortic al areas are most reliably defined by multiple criteria. If cortical areas are functionallydis tinctsubdivisions o f the brain, areas are likely to differ along a number o f param eFers includin g histological structure, connections, and neuron properties (see Kaas, 1982). In addition, areas in sensory systems often syste matic ally represent a sensory surface. If a region Knas 121