The Afferent, Intrinsic, and Efferent Connections of Primary Visual Cortex in Primates

Because of its distinctive architecture, connections, and functions, primary visual cortex, area 17 or V1 of primates, can be easily identified in most mammals (Kaas, 1987). V1 (also referred to as striate cortex) is particularly distinctive in primates, and, as a result, it was the first cortical area identified histologically (see Gennari, 1782, in Fulton, 1937). V1 of most, if not all, primates has a number of conspicuous features that distinguish this structure from its homologue in other mammals. Unlike carnivores, such as cats and ferrets, almost all of the visual input relayed from the lateral geniculate nucleus (LGN) of primates terminates in V1 (Benevento and Standage, 1982; Bullier and Kennedy, 1983; see Henry, 1991, for review), and lesions of V1 produce a severe deficit known as cortical blindness (e.g., Cowey and Stoerig, 1989). In addition, visual cortex of all primates is activated by physiologically and morphologically distinguishable streams, or channels, of inputs that are relayed from the retina to V1 in a manner unique to primates (Kaas and Huerta, 1988; Casagrande and Norton, 1991). Furthermore, the intrinsic connections of V1 in primates exhibit both vertical (laminar) and areal (modular) distinctions that appear designed to create new output channels from input channels via features of internal circuitry. Finally, the output streams project to visual areas that seem to be organized in a manner unique to primates. In particular, the major cortical target of V1, the second visual area, V2, is composed of three morphologically distinct modules that are differentially activated from V1, and at least one other major target of V1, the middle temporal visual area or MT, appears to be a unique specialization of primates (Kaas and Preuss, 1993). These common features of visual cortex in primates are of particular interest because these specializations relate to vision in humans as well as other primates. In this review, we focus on common features that have been described for V1 across a variety of primate species, and therefore are most likely to be present in most or all primates. In addition, we describe differences in V1 organization across primate groups, since these differences may relate to functional specializations and adaptations in the greatly varied primate order. Features that vary across taxa, when related to behavioral niches, may provide clues as to the significance of variations. Finally, this review briefly compares V1 in primates with V1 in some nonprimates to emphasize the distinctiveness of V1 in primates.