Perirhinal and Anterolateral Entorhinal Cortex Activity Patterns Reflect Perceived Visual Similarity of Highly Similar Objects

Perirhinal Cortex (PRC) in the medial temporal lobe has been proposed to be an extension of the ventral visual stream (VVS; Murray, Bussey, Saksida, 2007). In support of this notion, evidence from numerous sources suggests that PRC supports discrimination of objects with high visual feature overlap. In the present fMRI study, we asked whether PRC activity patterns reflect the subjectively perceived visual similarity of objects, and whether these patterns are distinguishable at levels of similarity at which earlier VVS regions cannot distinguish between them. In addition, we investigated whether anterolateral entorhinal cortex (alERC), a region to which PRC projects, shows a similar response profile. We combined ultra-high resolution fMRI (isovoxel 1.7mm) in humans (N=23) with representational similarity analyses (RSA). Images of objects from multiple categories with differing degrees of visual similarity among exemplars were presented. We administered a variant of a 1-back task with catch trials that required identification of repetitions at the exemplar- and at the category-level. Participants also rated the perceived visual similarity in an inverse multi-dimensional scaling task (iMDS; Kriegeskorte and Mur, 2012). Behavioural results revealed sensitivity of performance on catch trials to variations in this similarity. RSA results of non-catch trials showed that patterns in early visual cortex, the lateral occipital region, PRC, and alERC (but not posteromedial ERC) correlated with participants’ perceived visual similarity of objects within categories, as expressed in the iMDS. Importantly, only PRC and alERC patterns exhibited such a relationship at the highest level of similarities. Furthermore, PRC activity patterns showed a relationship to perceived visual similarity that was uniquely related to participants’ own ratings. These findings suggest that the representational geometry of object representations in PRC and a downstream alERC region is tied to perceived similarity space, and that their fidelity is higher than in earlier VVS regions.