Dissociations and associations between shape and category representations in the two visual pathways.

Accumulating evidence suggests that both visual pathways encode object structural properties (e.g., shape) as well as semantic representations (e.g., categories). In theory, the two codes could coexist in one representation reflecting both visual and semantic information. However, in practice, the recent literature is filling with reports that try to prove one by disproving the other. We present an event-related fMRI study that explicitly dissociates shape from category with a two-factorial design, allowing us to investigate the independent contribution of the two factors as well as their interactions. In 14 participants, we measured responses to 54 images comprising 6 semantic categories (minerals, animals, fruits/vegetables, musical instruments, sport items, tools) and 9 shape types. Our analyses included 15 regions of interest (ROIs) that altogether covered a large cortical area comprising both visual pathways. To investigate how information about object shape and category is distributed throughout the ventral and dorsal stream, dissimilarity matrices derived from similarity judgments acquired on both dimensions were compared with neural dissimilarity matrices derived from ROIs' multi-voxel activity patterns. Results revealed an independent contribution from each dimension in both visual pathways, with a transition from shape-preferring ROIs to category-preferring ROIs along the posterior-to-anterior anatomical axis. Interestingly, although both ventral and dorsal areas encoded object category, the nature of their representations largely differed; whereas the animate/inanimate division was primarily observed in ventral stream areas, dorsal stream areas distinguished objects based on their action-related properties. Furthermore, information content about shape evolved - from low-level (silhouette) shape to high-level (perceived) shape - from posterior to anterior areas, highlighting the same gradient along which category selectivity emerges. Our results show that representations of shape and category independently coexist and at the same time interact throughout the visual hierarchy, as such reconciling visual and semantic accounts of the functional organization in the visual system. Meeting abstract presented at VSS 2015.