Neural Substrates Of Stimulus Recognition And Association Memory

National Institute Of Mental Health

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Abstract

Research in the last year addressed the ability to access and report the contents of memory, an ability we term memory awareness. Humans demonstrate memory awareness by collecting information when necessary and acting immediately when informed. In addition to humans, apes, and Old World monkeys show an awareness of their memories by searching for information when they have lack a specific memory or knowledge. In this reporting period, we studied the memory of five capuchin monkeys, a New World monkey. First these monkeys watched an experimenter put food into one of four opaque tubes (called baiting) and, in a control condition, the experimenter put food into the tube outside the monkeys sight. After making sure that the monkeys attended to the experimenters actions, only about half the monkeys looked at the tubes more often when they had not seen the tubes baited. When we increased the effort required to look at the tubes, none of the monkeys looked at the tubes any more often when they had not seen them baited compared to when they had seen the tubes baited. This finding suggests that the memory system in capuchin monkeys is, relative to humans, apes and Old World monkeys, less capable of assessing its own memories. In another aspect of this project, we found, contrary to the prevailing doctrine about the MTL memory system, that the perirhinal cortex does not function exclusively in declarative memory, but rather functions in perceptual processes as well as in memory. We found that perceptual identification of individual objects depends on the perirhinal cortex. The proponents of the prevailing doctrine of the neuropsychology of memory missed the crucial role that the perirhinal cortex plays in visual perception because this contribution can only be seen when subjects must discriminate complex combinations of visual features. Simple stimuli fail to make large demands on the system, and therefore the role of the perirhinal cortex cannot be detected. Thus the perirhinal cortex not only contributes to the storage of these representations in memory, but it also plays a key role in the perception of objects. In addition, each of the five parts of the MTL, namely the perirhinal cortex, the entorhinal cortex, the parahippocampal cortex, the hippocampus and the amygdala, has a specialized function, and many have subdivisions with yet more specialized functions. Normal performance may require all parts of the MTL for some tasks, but our more refined experimental analysis has revealed some of their specialized contributions. Contrary to the prevailing doctrine in the neuropsychology of memory, we found that the perirhinal cortex functions very differently from the hippocampus. The hippocampus, unlike the perirhinal cortex, plays little role, if any, in perception of high-order combinations of visual features and, hence, contributes relatively little to the perception and identification of objects. Instead, the hippocampus contributes mostly to the memory of places and path integration, i.e., maintaining a record of the places explored and exploited to gain resources. Ongoing studies are examining the neurotransmitters involved in visual perceptual and mnemonic processing within the perirhinal cortex.