Amazing stuff! More on the enigma of longer term memory formation!
"The eyes may be the window to the soul, but the pupil is key to understanding how, and when, the brain forms strong, long-lasting memories ...
By studying mice equipped with brain electrodes and tiny eye-tracking cameras, the researchers determined that new memories are being replayed and consolidated when the pupil is contracted during a substage of non-REM sleep.
When the pupil is dilated, the process repeats for older memories. The brain’s ability to separate these two substages of sleep with a previously unknown micro-structure is what prevents “catastrophic forgetting” in which the consolidation of one memory wipes out another one. ...
The recordings showed that the temporal structure of sleeping mice is more varied, and more akin to the sleep stages in humans, than previously thought.
By interrupting the mice’s sleep at different moments and later testing how well they recalled their learned tasks, the researchers were able to parse the processes. When a mouse enters a substage of non-REM sleep, its pupil shrinks, and it’s here the recently learned tasks – i.e., the new memories – are being reactivated and consolidated while previous knowledge is not. Conversely, older memories are replayed and integrated when the pupil is dilated. ..."
From the abstract:
"Recently acquired memories are reactivated in the hippocampus during sleep, an initial step for their consolidation. This process is concomitant with the hippocampal reactivation of previous memories posing the problem of how to prevent interference between older and recent, initially labile, memory traces. Theoretical work has suggested that consolidating multiple memories while minimizing interference can be achieved by randomly interleaving their reactivation.
An alternative is that a temporal microstructure of sleep can promote the reactivation of different types of memories during specific substates.
Here, to test these two hypotheses, we developed a method to simultaneously record large hippocampal ensembles and monitor sleep dynamics through pupillometry in naturally sleeping mice. Oscillatory pupil fluctuations revealed a previously unknown microstructure of non-REM sleep-associated memory processes. We found that memory replay of recent experiences dominated in sharp-wave ripples during contracted pupil substates of non-REM sleep, whereas replay of previous memories preferentially occurred during dilated pupil substates. Selective closed-loop disruption of sharp-wave ripples during contracted pupil non-REM sleep impaired the recall of recent memories, whereas the same manipulation during dilated pupil substates had no behavioural effect. Stronger extrinsic excitatory inputs characterized the contracted pupil substate, whereas higher recruitment of local inhibition was prominent during dilated pupil substates. Thus, the microstructure of non-REM sleep organizes memory replay, with previous versus new memories being temporally segregated in different substates and supported by local and input-driven mechanisms, respectively. Our results suggest that the brain can multiplex distinct cognitive processes during sleep to facilitate continuous learning without interference."
Pupil size in sleep reveals how memories are sorted and preserved (original news release)
Sleep microstructure organizes memory replay (no public access)
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