Hierarchical processing of simple shapes and natural movies in rat visual cortex

Recent studies have revealed a surprising degree of functional specialization in rodent visual cortex. However, these studies fall short of establishing a functional hierarchy. We designed a study in rats that targets two hallmarks of the hierarchical object vision pathway in primates: higher tolerance for image transformations and selectivity for behaviorally relevant dimensions. We targeted five visual areas from primary visual cortex (V1) over areas LM, LI, LL, up to lateral occipito-temporal cortex (TO).We examined the responses of single neurons in these regions to six simple shapes used previously to probe monkey anterior infero-temporal cortex. These shapes were slowly jittering around a particular position of the stimulus display during 4s per presentation. After delineating the receptive field (RF) of each neuron, we presented the six shapes around one (Exp. A; N=299 cells) or two (Exp. B; N=258 cells) positions within the RF. First, we quantified the selectivity of populations of neurons in each visual area using all data from Exp. A plus the most responsive position of Exp. B. Overall discrimination performance was highly significant in all visual areas, although it decreased from V1 to higher visual areas. Neighboring areas were correlated with respect to which shape pairs were easiest to discriminate. This correlation suggests that the representation of shape transforms gradually across areas. In addition, we found evidence for an increase in position tolerance along the five areas. In TO, the preference for different shapes at one position was most closely related to the shape preference at another position. Finally, we found strong correlations between TO selectivity and behavioral performance of rats in a discrimination task. These findings demonstrate that the functional specialization in lateral rodent visual cortex reflects a processing hierarchy resulting in the emergence of tolerance and complex selectivity.status: publishe