Neural Substrates Of Stimulus Recognition And Association Memory

National Institute Of Mental Health

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Abstract

The perirhinal cortex receives inputs from both unimodal sensory areas such as area TE (which plays a key role in color and form vision) and the caudal, granular insular cortex (which plays a key role in touch), as well as from multimodal regions including the cingulate cortex and orbital frontal cortex. Earlier work in this Section identified a role for perirhinal cortex in associating together the different sensory qualities of objects (e.g. tactual-visual associations). This work indicated that perirhinal cortex was important for object identification, but was equivocal regarding a role for perirhinal cortex in perception versus memory. More recently, our work showed that the perirhinal cortex does not, as conventionally thought, exclusively subserve declarative memory processes as part of the MTL, but rather functions in perceptual processes as well as in memory. We found that perceptual identification of individual objects depends on perirhinal cortex (Bussey et al., 2002, 2003). In other words, we have uncovered evidence that overturns the first proposition of the MTL memory system view, outlined above. [unreadable] [unreadable] The findings from recent studies were consistent with our predictions based on a hierarchical organization of representations in the ventral visual stream or object-analyzer pathway. One additional prediction of this hierarchical organization is that the perirhinal cortex should also be essential for associating together two different objects or images, especially if they occur together in a visual scene. To test this idea we presented subjects with overlapping pairs of objects (A vs B, B vs. C, C vs. A) in the context of a concurrent visual discrimination. In this task, the identity of the pair presented on any given trial indicates which object will be correct. (For example, if B is paired with A, then A is correct, but if B is paired with C, then B is correct.) To choose correctly, then, subjects need to associate the objects comprising a pair, i.e., to represent them together. Using this test design, we found that damage to perirhinal cortex impairs, and damage to the hippocampus facilitates, learning the concurrent discriminations. [unreadable] [unreadable] This project made progress in two main ways. First, we confirmed and extended our earlier work by showing that perirhinal cortex is essential not only for representing the conjunctions of features that compose individual objects, but also for representing the conjunctions of objects that compose visual scenes. Together with our earlier results, these findings argue against the first proposition of the prevailing MTL memory system view. Contrary to that view, the perirhinal cortex appears to play a role in representing information about objects for both mnemonic and perceptual purposes. Second, we found that damage to the perirhinal cortex and hippocampus have opposing effects on memory. Thus, the second proposition of the prevailing view that MTL components contribute roughly equally to declarative memory has also been contradicted. Instead, the results support the perceptual-mnemonic view, which holds that perirhinal cortex has a role in visual perception as well as memory, and that it is part of both the MTL and the ventral visual stream.