The role of the perirhinal cortex and hippocampus in learning, memory, and perception.

One traditional and long-held view of medial temporal lobe (MTL) function is that it contains a system of structures that are exclusively involved in memory, and that the extent of memory loss following MTL damage is simply related to the amount of MTL damage sustained. Indeed, human patients with extensive MTL damage are typically profoundly amnesic whereas patients with less extensive brain lesions centred upon the hippocampus typically exhibit only moderately severe anterograde amnesia. Accordingly, the latter observations have elevated the hippocampus to a particularly prominent position within the purported MTL memory system. This article reviews recent lesion studies in macaque monkeys in which the behavioural effects of more highly circumscribed lesions (than those observed to occur in human patients with MTL lesions) to different subregions of the MTL have been examined. These studies have reported new findings that contradict this concept of a MTL memory system. First, the MTL is not exclusively involved in mnemonic processes; some MTL structures, most notably the perirhinal cortex, also contribute to perception. Second, there are some forms of memory, including recognition memory, that are not always affected by selective hippocampal lesions. Third, the data support the idea that regional functional specializations exist within the MTL. For example, the macaque perirhinal cortex appears to be specialized for processing object identity whereas the hippocampus may be specialized for processing spatial and temporal relationships.