REMembering in Sleep

In his seminal book "The Interpretation of Dreams", Freud toyed with the meaning of dreams and because of his work, we will never look at cigars the same way (we could also thank William Clinton for that). Yet despite his influence, Freud largely missed the mark because he could not see inside the "black box" of the mind. But we can. This blog is about what happens when we dream.

Dreaming, in humans, largely occurs during two periods of sleep: during slow-wave sleep (SWS) and during rapid-eye movement (REM) sleep. These two periods of the sleep cycle are also present in rats, birds, and monkeys, but I am not going to try to convince you that they dream the same way we do. It is enough to know that what occurs in our brains during these two stages of sleep also occurs in theirs (both at the level of single-cell recordings and EEG). (However, if you have a dog, you might swear that he's chased a squirrel or two in dreamworld)

In order to understand what happens inside a rat's brain during sleep, it is necessary to first understand what happens when they're awake, specifically when they are learning. In a brain region known as the hippocampus (part of Sagan's reptilian brain), unique cells called "place-cells" respond to unique spatial positions, much like how road signs indicate your position in a city.

To understand this better, imagine a linear track in which a rat can navigate. While the rat is navigating, unique place-cells will respond at specific positions on the track. Place-cell A might respond best when the rat is at the left-most edge of the track and not respond at any other position on that track. Place-cell B, on the other hand, might respond to the right-most edge of the track, but will fail to respond to other positions. Other place-cells would respond to regions in the middle of the track and so forth; in the end, a complete "map" of the rat's running could be reconstructed from all place-cell-responses. If the rat were running left-to-right on the track, place-cell A would respond first, followed by other place-cells (in the middle of the track), and finally end with place-cell B. We will identify these particular sequences of place-cell-responses as "temporal-sequences".

Temporal sequences are not intrinsically generated and require training to learn and memorize them. That is, before entering the linear-track, place-cell A would not even have a position-preference. However, after running the linear-track, place-cell A would NOW respond only to the left-most edge of the track, and this position-specificity can be retained for days. In the end, the temporal-pattern of place-cell-responses acts as a unique memory trace.

Now, back to the original idea. During slow-wave sleep (SWS) and REM sleep, these memory traces (ie: temporal sequences) are replayed in the hippocampus (as the rat lays motionless). Even more amazing, temporal sequences of cell-activity are also replayed in other brain regions such as the visual cortex and the prefrontal cortex (responsible for converting memories from the short-term to the very-long-term). The point is that sleep-related replay is observed in various and distinct brain regions, and the playback is coordinated between these regions.

A Hebbian paradigm of learning endorses a use-it-or-lose-it model of long-term learning and sleep-related replay has been suggested as a method for "using it". Neurologically, sleep-related replay is not very different from mental-rehearsal when awake. These "activities" have been suggested to facilitate the conversion of short-lived memories to long-lived ones.

If you think humans are exceptional to rats in this, you'd be wrong. Sleep-related memory-replay has also been observed in humans (epilepsy patients with deep-brain electrodes), and even more, memory-recall is better for subjects allowed to experience SWS or REM sleep than subjects who are prevented.

So, sleep well readers. You might have learned something.