Controversial issues in visual cortex mapping: Extrastriate cortex between areas V2 and MT in human and nonhuman primates

The visual cerebral cortex of primates includes a mosaic of anatomically and functionally distinct areas processing visual information. While there is universal agreement about the location, boundaries, and topographic organization of the areas at the earliest stages of visual processing in many primate species, i.e., the primary (V1), secondary (V2), and middle temporal (MT) visual areas, there is still ongoing debate regarding the exact parcella-tion of cortex located between areas V2 and MT. Several parcel-lation schemes have been proposed for extrastriate cortex even within the same species. With the exception of V1, V2, and MT, these schemes differ in areal borders, areal location, neighboring relations, number of areas, and nomenclature. As a result, most anatomical and physiological studies of these areas have been carried out following one or another scheme, in the absence of any general agreement. This situation is inevitably hampering our understanding of the function and evolution of these visual areas. The goal of this special issue is to provide a critical review and evaluation of the literature on the most controversial issues regarding the parcellation of extrastriate cortex, to identify the main reasons for the controversy, and to suggest critical future experimental approaches that could lead to a consensus about the anatomical and functional identity of these areas. Visual cortical areas have traditionally been defi ned on the basis of multiple converging criteria, including visuotopic organization, architectonics, pattern of corticocortical connections, and receptive fi eld properties. One major reason for the lack of consensus on the parcellation of extrastriate cortex is that for most areas these converging criteria have been obtained from different animals and even different primate species. This is because limitations inherent to the experimental approaches have made it impossible to characterize all of these aspects of cortical organization within the same animals. Moreover, due to technical limitations and time constraints, it has also been diffi cult to obtain extensive electrophysiological or anatomical mapping data from the same animal, especially in species with gyrencephalic brains, where most of these areas are located within diffi cult-to-access sulci. Therefore, the different cortical parcellation schemes that have been proposed over the years represent best attempts to summarize the sparse mapping data obtained from different animals, using a variety of different techniques, and often from different species. It is important to emphasize that most of the published data are consistent with several alternative parcellation schemes, albeit they have typically …