Visual memory system and mechanisms of associative learning : Interaction between the temporal neocortex and the hippocampus

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Neuropsychological theories proposed a critical role of the interaction between the medial temporal lobe and neocortex in the formation of long-term memory for facts and events, which has often been tested by learning of a series of paired words or figures in humans. In this research project, I have been identifying neural mechanisms underlying the memory formation by single-unit recording with an animal model of visual pair-association task in monkeys.First, I tested a hypothesis that long-term representations of visual objects are acquired through learning in the neural network of the anterior inferotemporal (IT) cortex. I identified two mechanisms ; one is tuning and the other is association. In the IT cortex of monkeys performing the visual pair-association task, I found a group of neurons that manifested selective responses to both of the paired associates (pair-coding neuron). It provides strong evidence that single IT neurons acquire stimulus-selectivity through associative lear … More ning. Then I tested tuning mechanism by asking whether repeated exposures to a picture produce IT neurons that selectively respond to that particular form. Single-neuron activity was compared among geometrically similar patterns, only one of which had been exposed repeatedly to the monkey before the unit recording (learned pattern). I found that IT neurons responded best to the learned pattern. The reponse to a learned pattern is optimal, however, only locally, and there are presumably several local maxima in the whole form space of a single neuron. A pair-coding neuron would have at least two separate local maxima, correponding to each of the paired associates, in the form space.Second, I tested the role of the backward signal from medial temporal lobe to IT cortex. Ibotenic acid was injected unilaterally into the entorhinal and perirhinal cortex which provide massive backward projections ipsilaterally to IT cortex. After the lesion, the monkeys could learn a new set of paired associates. I found that (i) in spite of the lesion, the neurons responded selectively to the visual stimuli, which suggests preservation of the tuning mechanism, and (ii) the paired associates failed to elicit correlated responses in the cells tested with the lesion. I conclude that the limbic lesion disrupted the associative code of the IT neurons between the paired associates, without impairing the visual response to each stimulus. These results give concrete evidence for decomposition of the primate long-term memory system into different neural mechanisms. Less