Vision Sciences Society Meeting edition:8 location:Naples, FL, U.S.A. date:9-14 May 2008
Studies using functional magnetic resonance imaging (fMRI) have revealed cortical regions with strong selectivity for the category objects belong to, especially for faces, buildings, and headless bodies or body parts. We investigated the role of perceived shape for this neural selectivity for highly familiar object categories. We scanned nine human volunteers while they viewed images from six conditions, with two sub-categories from each of the aforementioned categories: old african faces, non-african baby faces, rural buildings, skyscrapers, hands, and headless torsos. Twenty other volunteers rated pairs of images in terms of perceived shape. Analyses of the pattern of selectivity in ventral visual cortex with multivariate techniques revealed a fine-grained organization of ventral visual cortex that extended beyond the typical three object categories. More specifically, we have found small differences in the activation pattern for old faces versus baby faces, and very clear differences between hands and torsos. Several aspects of this fine-grained organization were not consistent with a role of perceived shape. The clearest dissociation was that the activation pattern of torsos was somewhere in the middle between faces and hands while the perceived shape of torsos was more similar to the skyscrapers than to the faces and the hands. As a consequence, a semantic distinction between "living/body-related" and "non-living/artefact" is very clear in the brain but not in ratings of perceived shape. These results contrast with findings that object selectivity for unfamiliar, artificial objects reflects differences in perceived shape (Op de Beeck et al., 2007, Society for Neuroscience meeting). In conclusion, we have found a detailed and multi-level category selectivity for familiar, meaningful objects that is not fully explained by differences in perceived shape.