Predicting judged similarity of natural categories from their neural representations.

We report a combined behavioral and functional magnetic resonance imaging (fMRI) study of conceptual similarity among members of a natural category (mammals). The study examined the relationship between computed pairwise similarity of neural responses to viewed mammals (e.g. bear, camel, dolphin) and subjective pairwise similarity ratings of the same set of mammals, obtained from subjects after the scanning session. In each functional region of interest (fROI), measures of neural similarity were compared to behavioral ratings. fROIs were identified as clusters of voxels that discriminated intact versus scrambled images of mammals (no information about similarity was used to define fROIs). Neural similarity was well correlated with behavioral ratings in fROIs covering the lateral occipital complex in both hemispheres (with overlap of the fusiform and inferior temporal gyri on the right side). The latter fROIs showed greater hemodynamic response to intact versus scrambled images of mammals whereas the fROIs that failed to predict similarity showed the reverse pattern. The findings provide novel evidence that information about the fine structure of natural categories is coarsely coded in regions of the ventral visual pathway. Implications for the theory of inductive inference are discussed.