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つくばの心理学 2021 12 However, this evidence is correlational which does not show causation. Therefore, Winson and I decided to use place cells to determine whether brain activity in sleep is related to memory consolidation. Animals were implanted with multiple electrodes in the hippocampus to record place cells. Upon recording from pairs of cells with very different place fields (i.e., on different arms of an 8-arm maze), one of the cells was exposed to its place field (very high firing rates) while the other cell was kept away from its place field (practically silent). The animals were then allowed to sleep for a couple of hours. To our amazement we observed what has been termed ‘neuronal reactivation’ in sleep. That is, the cells that were active during the awake state became active again in sleep. When we reversed the exposure (exposed second cell to its place field, keeping the first cell away from its place field) and let the animal sleep, the second cell became very active while the first returned to very low rates. This was perhaps the first concrete demonstration that neuronal activity in sleep may have to do with memory consolidation of daytime events. In subsequent experiments, we showed that various genes that are involved in synaptic plasticity and learning and memory also become reactivated in sleep as part of the molecular machinery for memory consolidation. One of the gene cascades involved in learning and memory is protein kinase A (PKA). In ongoing experiments in our laboratory, we are investigating whether PKA is one of the mechanisms involved in long-term fear memory consolidation, using fear conditioning as a paradigm. We are further analyzing whether sharp wave activity, seen in the EEG of the hippocampus during learning and again in sleep, may be the brain’s way of activating PKA activity for memory consolidation. Hippocampal Neuronal Functional Organization. A different set of experiments in our laboratory are aimed at determining neuronal functional organization in the hippocampus for spatial navigation. Although many of the components of the spatial navigation system have been well described (Nobel Prize in Physiology or Medicine 2014), how the spatial map is organized in the brain remains a mystery. We are currently using a combination of behavioral, anatomical, electrophysiological and molecular methods to determine how the spatial map is functionally organized in the hippocampus. We now have evidence of a clustered-type neuronal organization. These studies are currently ongoing in the lab and they hold the promise of providing a major clue of how the hippocampus is organized not only to encode for space but for any function that the hippocampus plays a role in.