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Sleep, especially slow-wave sleep, is necessary for the consolidation of memory (see, e.g., Journal Watch Psychiatry Feb 12 2007). This animal study illuminates the neurobiology of the process. Researchers implanted electrodes in the skulls of rats trained to run a particular maze. On 2 to 3 consecutive days, the researchers studied depolarization patterns in hippocampal and visual-cortex cells during maze running and during sleep both immediately before and after maze running.
During slow-wave sleep, neurons in the neocortex demonstrated phasic depolarized and hyperpolarized states called up-down states (UDS), and hippocampal neurons displayed UDS in phase with cortical UDS (the hippocampus, which does not have intrinsic UDS, was thus entrained by the neocortex). In a detailed examination of maze running, specific visual-cortex and hippocampal cells were found to be activated sequentially, reflecting sequential activation by unique visual cues. During slow-wave sleep after maze running, neurons in the hippocampus and the visual cortex were activated in the same sequence as during maze running. Cortical UDS slightly preceded hippocampal activation. The replay was faster than the original activation, occurred within the first hour of sleep, and did not occur during sleep immediately preceding maze running.
Ji D and Wilson MA. Coordinated memory replay in the visual cortex and hippocampus during sleep. Nat Neurosci 2007 Jan; 10:100-7.
Comment
This elegant version of neuronal mind reading suggests that short-term visual memory stored temporarily in the hippocampus during learning is transferred to the visual cortex during slow-wave sleep in a back-and-forth process cued by the neocortex. The short-term information in the hippocampus might be erased after it is stored more permanently in the cortex. The lack of coordinated UDS during sleep that occurs some time after running a maze that has already been learned indicates that cortico-hippocampal cross-talk involves consolidation of short-term memory rather than reflecting already-learned information. It makes sense that replay during sleep would be faster, as it does not have to occur in “real time.”