| Research Abstract |
This project had three aims. First, implementing echo planar magnetic resonance imaging (EPI) technology. Though 1-shot EPI method has an advantage that enables image acquisition in less than 0.1s, this advantage was not fully appreciated at the time of the proposal of this project in 1994, because most researchers followed block designs that were originally developed for PET experiments. We developed a new method of "event-related fMRI", which enables us to utilize time resolution of EPI and to detect transient cognitive neuronal activity. Second, measuring human cerebral cortical activity in cognitive task. By using the event-related fMRI in Wisconsin Card Sorting Task (WCST), we detected neuronal activity that was related to cognitive-shift processes in posterior parts of the inferior frontal sulcus. The same area of the inferior frontal sulcus was activated not only in WSCT but also in tasks which require cognitive inhibition processes, e.g. in the Go/NoGo task. Interestingly, we a
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lso found the same area to be activated in a working memory task. Third, neural circuits were analyzed in the frontal, temporal, and limbic cortices in monkeys. Regarding the temporo-limbic interaction, we analyzed the role of backward projections from area 36 to area TE. We made unilateral lesion of area 36 with ibotenic acid, and then tested the neuronal responses in area TB. We found that backward signal from area 36 to area TE is indispensable to create and maintain the associative memory in area TE neurons. Regarding the front-temporal interaction, we developed a novel monkey preparation called "partial-split-brain" preparation, in which posterior part of the corpus callosum and the anterior commissure were cut and the anterior part of the corpus callosum was left intact. By using this partial-split-brain preparation, we demonstrated that, although visual long-term memory is stored in the temporal cortex, memory retrieval is under the executive control of the prefrontal cortex. Then we used single-unit recording method in this partial-split-brain preparation, and directly identified the top-down signal from the prefrontal cortex to the inferotemporal cortex. The top-down signal was found to encode semantic meaning of visual objects rather than stimulus identity itself. Less
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