A common view of the perirhinal cortex is that it belongs to the medial temporal lobe memory system and therefore participates in declarative memory. But the perirhinal cortex also seems to be a part of the ventral visual pathway — the 'what' pathway — indicating that it might also be involved in visual perception. Bussey et al. now add new fuel to this debate by showing that perirhinal lesions affect learning and performance of a visual discrimination task, and that this impairment is related to certain perceptual factors. These observations support a perceptual–mnemonic view of perirhinal function.

Rapid acquisition is a hallmark of declarative memory — information is acquired after one or a few trials. If the perirhinal cortex is part of this memory system, then its destruction should impair learning of rapidly acquired discrimination tasks. By contrast, if the perirhinal cortex has a role in perception, then increasing the perceptual difficulty of the task might affect learning in the absence of this cortical region. The authors tested these predictions in monkeys with perirhinal lesions, and found that learning was affected only when the task was made perceptually difficult. When the monkeys were presented with two greyscale photographs that were easy to discriminate and were rewarded for choosing one of them, they quickly learned to choose the correct picture. But if the discrimination was made more challenging by partially blending the photographs, then the lesioned monkeys showed impaired learning.

In a second experiment, Bussey et al. tested another prediction of the perceptual–mnemonic view: if the perirhinal cortex has a perceptual role, then increasing perceptual difficulty after the task has been acquired should impair performance in lesioned monkeys. So, the monkeys first learned the task with the easy-to-discriminate pictures and then the authors partially blended the pictures. This manipulation led to a marked drop in performance, satisfying the prediction.

A remarkable aspect of this study is that it was guided by theoretical work grounded on a rigorous connectionist model, the predictions of which were satisfied by the experiments. Although these results support a role for the perirhinal cortex in perception, this role is rather specific. Bussey et al. did not find any effect of the perirhinal lesions in learning or performing perceptually challenging discrimination tasks that involved differences in colour or size. The authors concluded that the perirhinal cortex is important for visual discriminations with a high degree of 'feature ambiguity' — a property that can emerge when features of an object are rewarded when they are part of one object, but not when they are part of another. But independently of this functional specificity, these data favour a perceptual–mnemonic function of the perirhinal cortex, as opposed to a strictly declarative role.