The Functional Organization of the Ventral Visual Pathway in Humans

N Kanwisher

McGovern Institute for Brain Research, Massachusetts Institute of Technology, Cambridge, United States

Over the last fifteen years, functional imaging studies in humans have provided a richly detailed view of the functional organization of the ventral visual pathway in humans. In this talk I will take stock of what we have learned so far, and attempt to identify the most important unanswered questions. In particular, functional imaging has powerfully complemented prior behavioral and neuropsychological work in enabling us to discover the major components of the processing machinery that holds our representation of the visual world. The most robust findings are a set of brain regions that respond selectively to faces, places, bodies, and objects. Each of these regions is found in approximately the same location in virtually every normal subject, thus constituting part of the fundamental architecture of the human visual mind and brain. Beyond this widely-replicated set of results, though, lie numerous controversies and unanswered questions. First, does the representation of a given object occupy much of the expanse of the ventral pathway (the “distributed” view), or are some objects primarily represented in a small number of focal regions? Here I will argue that although pattern analyses do show that many category-selective regions hold some information about nonpreferred stimuli, the important question is which of this information is used, that is, which plays a causal role in perception – a question that is hard to address with neuroimaging but that can be tackled with TMS, electrical stimulation, and patient studies. Second, how does the functional organization of the ventral pathway arise in development, and why do the functionally specific regions land where they do in the brain? Here I will argue that in contrast to widespread claims, much of the organization of the ventral pathway (including the FFA) is nearly adultlike by late childhood. These results underscore is the importance of looking at much younger children or even infants, something that is nearly impossible with fMRI in humans. Further, the deepest questions about the development of the ventral pathway concern the role of experience, and the question of whether an early-developing functional or structural organization instructs the later development of category-selective cortical regions – questions that are currently wide open. Third, we have not made enough progress on the central problem of characterizing the representations and computations that exist in each of these regions, a question that may require the temporal and neuron-level precision available only in animal models. Fourth, what is the connectivity of each of these regions to each other and the rest of the brain? Although clues are emerging from diffusion and resting functional studies, neither method is perfect, leaving this fundamental question largely unanswered. Perhaps the biggest open question concerning the functional organization of the ventral visual pathway is whether functionally distinctive regions are best thought of as discrete processors, or whether it makes more sense to consider the whole ventral pathway as a single processor in which each of these regions simply constitutes a local peak in the functional response. To the extent that the different regions have distinctive connectivity and cytoarchitecture, that would support the interpretation of these regions as distinct entities. On the latter view, the question would still remain of why that landscape would contain the particular replicable configuration it does, and what if any are the dimensions represented by axes of this broader 'map'.

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