It has been known for some time that specific patterns of nerve firing in "place cells" of the rat hippocampus occur during learning a visual maze and that these patterns are replayed during sleep, apparently as a part of memory consolidation. Wilson's laboratory at M.I.T. (reporting in Nature Neuroscience) have now studied multicell spiking patterns in both the visual cortex and hippocampus during slow-wave sleep in rats. As Nicholas Wade notes in the NYTimes, the recordings capture dialogue between the hippocampus, where initial memories of the day's events are formed, and the neocortex, the sheet of neurons on the outer surface of the brain that mediates conscious thought and contains long-term memories.
Ji and Wilson found that spiking patterns not only in the visual cortex but also in the hippocampus were organized into frames, defined as periods of stepwise increase in neuronal population activity. The multicell firing sequences evoked by awake experience were replayed during these frames in both regions. Furthermore, replay events in the sensory cortex and hippocampus were coordinated to reflect the same experience. These results imply simultaneous reactivation of coherent memory traces in the cortex and hippocampus during sleep that may contribute to or reflect the result of the memory consolidation process. Because the fast rewinds in the neocortex tended to occur fractionally sooner than their counterparts in the hippocampus, Wilson thinks the dialogue is probably being initiated by the neocortex, and reflects a querying of the hippocampus's raw memory data.
Wade's review quotes comments from Wilson:
“The neocortex is essentially asking the hippocampus to replay events that contain a certain image, place or sound...The neocortex is trying to make sense of what is going on in the hippocampus and to build models of the world, to understand how and why things happen...These models are presumably used to direct behavior...They are able to generate expectations about the world and plausibly fill in blanks in memory.
Though the neocortex learns from the hippocampus, the raw memory traces, from childhood onward, are not transferred and are probably retained in the hippocampus... If so, the forgetfulness of age would arise because of problems in accessing the hippocampus, not because the data has vanished.
The subject matter of the neocortex-hippocampus dialogue in rats seems mostly to concern recent events. This is consistent with what people report when awoken from nondreaming sleep — usually small snatches of information about recent events. Dr. Wilson also said that the new findings, by showing activity in the visual neocortex, confirmed that rats had humanlike dreams with visual imagery, a possibility some researchers had doubted."