| Research Abstract |
The purposes of this study are to investigate 1) the effects of translocation modes (active and passive) and cognitive requirement of spatial tasks on the activity of hippocampal place cells, and 2) the species differences (between monkeys and rats) of the activity of place cells. In the first fiscal year (1999), we recorded single neuron activity in the monkey hippocampus during the performance of place tasks in which the monkey was or was not required to recognize animal's own location in the environment, and, as a result, we clarified the existence of hippocampal neurons that showed location specific activity only when the monkey recognized the own location; the majority of these neurons lost location specificity in the task that did not require the monkey to recognize own location. In the second fiscal year (2000), we recorded hippocampal neuron activity during the performance of spatial navigation task in freely moving rats, and compared the activity with that during passive trans
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location restraining the rat in a small cage. In this comparison, we again demonstrated that the location specific activity observed in the freely moving condition disappeared when the rat was translocated passively. In the final fiscal year (2001), we developed an experimental system that made it possible to record simultaneously electroencephalograph (EEG) and single neuron activities in freely behaving monkeys, and performed a similar experiment to that of the above rat's study: we recorded EEG and neuron activities in the hippocampus during the performance of spatial navigation task, and compared the activities with those during passive translocation restraining the monkey in a chair. As a result, hippocampal EEG displayed rhythmic slow activity (RSA) at the initiation of locomotion during the free, active spatial navigation while RSA was not observed during the passive translocation. We have recorded activity from 5 hippocampal neurons, and we continues the recording to increase the number of neurons so as to identify response characteristics as neuronal population. In conclusion, we clarified that active locomotion of animals (both monkeys and rats) in the environment, recognizing spatial landmarks in the environment, is important for the representation of "place" in the hippocampus. Less
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