Clarifying homologies in the mammalian cerebral cortex: the case of the third visual area (V3).

1. Experiments in mammalian models are the main source of information on the neural architecture underlying human visual perception, establishing scientific boundaries for the interpretation of experiments using non-invasive techniques in humans and for the realistic modelling of visual processes. Thus, it is important to define the homology between visual areas in different species. 2. To date, relatively few visual areas can be defined with certainty across mammalian Orders. Here, we review the evidence pointing to the fact that the third visual area (V3; or area 19) is a crucial node of a system involved in shape recognition that exists in most, if not all, eutherian mammals. 3. The size and shape of area V3 are variable, even between species that belong to the same Order. Although some features of the visuotopic organization of V3 are constant (including the relative location of the representations of the upper and lower quadrant and correspondence between the anterior border and the representation of the vertical meridian of the visual field), others are variable between species and even individuals. A complex pattern of representation, involving topological discontinuities, can exist. 4. In addition to its location in relation to the first (V1) and second (V2) visual areas, the identification of V3 homologues can be aided by certain other features, including low myelination, weak cytochrome oxidase reactivity, response properties that are indicative in the processing of stimulus shape, relationship to clusters of neurons forming interhemispheric connections and projections from the koniocellular (W-cell-like) components of the lateral geniculate nucleus. 5. Recent research in primates has clarified the organization of the V3 homologue in members of this Order. Regions of cortex that were formerly thought to belong to V3 (including a densely myelinated region near the dorsal midline) are better considered as part of a separate dorsomedial area, involved in motion analysis and visuomotor integration. The redefined V3, which includes the 'ventral posterior area' and parts of the dorsolateral complex proposed by earlier studies, is very similar to V3 (area 19) of other species in terms of structure and function.