Studies on neural basis of sleep-related memory processes by means of the functional MRI

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Neural activity during memory processes was investigated by means of the functional magnetic resonance imaging (fMRI). Event-related fMRI data was acquired while volunteers saw and memorized visually presented words. Recognition tests for memorized words (old words) visually displayed with new words were carried out after two different study-test delay intervals (0 h and 24 h). Volunteers had a 7 h sleep during 24 h period after encoding. Neural activity elicited by old words was classified according to whether each word was later remembered or not. Differences in neural activity elicited by old words that were later remembered and later forgotten, "subsequent memory effects", were analyzed. This procedure was applied to both study-test delays. Subsequent memory effects associated with two study-test delays were compared. Neural activity during encoding associated with later remembered words for both study-test delays was found in hippocampus and left dorsal inferior frontal gyrus. In several regions, including left ventral inferior frontal gyrus and left superior frontal gyrus, neural activity during encoding was greater for 24 h study-test delay than 0 h study-test delay. These results suggested that the neural activity in the regions exhibiting greater activations in 24 h study-test delay predicts later remembered or forgotten, and that these regions may play an important role in sleep related memory consolidation processes. Significant hippocampal activities for remembered words were found during the recognition test phase 24 h after the encoding phase. The magnitude of these activities were related to sleep structure between encoding and recognition. These results suggested that the memory consolidation processes might occur at least in part during sleep and that neural activity in recognition test could estimate that structure.