NEURAL SUBSTRATES OF STIMULUS RECOGNITION AND ASSOCIATION MEMORY

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Memory loss typically accompanies strokes of the anterior communicating artery, viral infections of the brain, normal aging, and is a central feature of certain diseases such as Alzheimer's and AIDs dementia complex as well. To understand the mechanisms underlying storage of information in the brain we have examined the effects of selective cerebral ablations and disconnections on different types of memory in macaque monkeys. The storage of stimulus memories in monkeys is achieved in large part by a strip of cortex lying at the base of the medial temporal lobe. This region is comprised of the entorhinal cortex and perirhinal cortex, abbreviated here as "rhinal cortex". The rhinal cortex appears to be critical for accurate stimulus identification, stimulus recognition, and stimulus-stimulus association. Further, the rhinal cortex receives information from all the modality-specific neocortical regions, after processing through a series of cortical fields, and mediates stimulus recognition in at least the visual and somatic sensory modalities (and perhaps other sensory modalities as well). The rhinal cortex appears to be responsible for stimulus-stimulus associative memory both within sensory modalities (i.e. intramodal associations) and across sensory modalities (i.e. crossmodal associations). Moreover, the rhinal cortex can mediate these types of information storage in the absence of the medial temporal lobe limbic structures, the amygdala and hippocampus. It is proposed that the rhinal cortex is the kernel of a memory system specialized for storing knowledge about objects, and that this system is analogous to a semantic memory system in humans. Stimulus memories can be linked to affective states and reward through connections of the rhinal cortex with the amygdala. Stimulus memories can be linked with locations and motor acts through connections of the rhinal cortex with the hippocampal formation.