There has been some debate about where memories end up in our brains, whether in the hippocampus exclusively or distributed through much of the neocortex. The spectacular case of HM, who could not form new memories after his entire hippocampus was removed, (plus some other nearby structures), indicated that older memories were still accessible from elsewhere, while all new memories are formed and reside at least temporarily in the hippocampus. However some types of memories do not appear to get re-distributed from the hippocampus. HM was missing quite a bit of explicit (also called declarative) memory from prior to his surgery, for instance, including everything in the prior year, and decreasing amounts going backwards for eleven more years. Other types of memory, such a short-term, working, implicit, motor, and procedural memories remained functional, however, for new formation as well as retrieval.
It is known that sleep plays an important role in the "consolidation" and redistribution of explicit memories within the hippocampus and from there to the cortex. During sleep, significant memories are replayed, which strengthens their encoding and allows their replication to more stable storage in the cortex. Ironically, sleep prevents forgetting. The redistribution process can take months to years, accounting for the deficits experienced by HM. On the other hand, sleep impairs, while new novel experiences enhance, the consolidation of some memories within the hippocampus, probably by enhancing the salience of the entire sequence of experiences. Current work indicates that memories get to the cortex quite quickly, with a "parallel process" between both areas strengthening them over time.
A recent study looked at this memory consolidation process, and asked what happens to conflicting memories- which might prompt over-writing of an initial memory with a later, corrected one. Yes, this was another study done with rats and mazes, testing their ability to retain memories of locations over various time periods, and over intervening activities, such as sleep, after the target location was changed. The rats learned the locations of both targets quite quickly, and returned to those locations preferentially in future trials, a week later, no matter where the actual target was.
If the rats where allowed to sleep between the switched training sessions, they lost the first memory more than if they had been deprived of sleep and exposed to further novel events between training sessions. This led to a conclusion that the hippocampal encoding (but not the cortical) is enhanced by activity and novelty, rather than sleep. The next step in the experiment was to alter the memory type by allowing the rats to explore the training area extensively for a few days prior to the training. This allowed them to gain a fuller context for the experiences to come, context that is believed to be stored not only in the hippocampus, but also in the cortex, being part of the consolidated and distributed memory system. After this protocol, rats allowed to sleep significantly out-remembered the sleep deprived rats when tested, and performed particularly well if the experimenters threw in a cruel trial a day after training, where no target was present in the maze at all.
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| An example of one test of a rat trained the week before. The training runs were, first target top left, followed by sleep, and second, target at bottom right, followed by distraction and sleep deprivation. The rat clearly remembered the second training much better. This was true even if the training regimens were reversed, and the one followed by sleep occurred second. |
Lastly, the researchers studied molecular markers in their subject's brains, to see where cell and synapse growth was taking place in response to all these exciting events. For all conditions, the brains showed a great deal of neural activity and synaptic consolidation, i.e. expression of genes like cFos and Zif-268, right after training. However five hours later, things were a little different. Expression in the hippocampus was significantly down among animals who had gotten some sleep, but up if they were sleep deprived.
Conversely, marker expression in the cortex was the reverse- up in rats who had slept, down in those continually kept awake with more play and other novelties. This was particularly interesting since sleep alone drove a significant decline in cortical expression of these genes in control animals. That such brief training events can have effects on such gross brain areas through subsequent sleep, for hours and days, may argue more for the traumatic nature of the training, (done in water mazes, where the rats are desperately searching for a hidden platform), than normal learning in, say, a school environment.
Nevertheless, this kind of work shows what is going on in the field of memory research, as we try to figure out why, where, and how memories are distributed in the brain, which ones are kept, which ones erased, how they are schematized and compressed, and how they are retrieved again and altered during that retrieval. In this case, the researchers make the claim that their procedures have dissected a difference between cortical memory formation, which is enhanced by sleep and inhibited by intervening learning and activity, versus hippocampal memory formation, which experiences the reverse.
They did not have much to say in the end about conflicting memory formation, since the rats seemed to deal with this aspect just fine, (though less well after sleep). They remembered both maze solutions, even if one had been superceded by another for a few training runs. But the relational nature of cortical memory, which seems to grasp memories better if they are situated in a known matrix of prior experience, is interesting. And the speed of this cortical memory consolidation is also interesting- a matter of days, not the weeks or months that has been the model in the wake of HM.
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