Consciousness Blog 18 December 2010


A number of recent studies support the idea that sleep has an essential role to play in the process of memory formation, and hence was one of the crucial evolutionary adaptations that led to the development of higher forms of consciousness.

The first study, by researchers at the University of York and Harvard Medical School and funded by the Economic and Social Research Council, suggests that sleep may help people both to remember a newly learned word and incorporate the new vocabulary into their stored lexicon.

Published in the Journal of Neuroscience, the study focused on two groups of subjects who were taught new words and then tested after a period of time for their retention of the new words in memory. The first group learn the words in the morning and were tested in the evening without intervening sleep, while the second group learnt the words in the evening and were tested in the morning after a period of sleep. The second group was significantly more successful at retaining the new memories.

An examination of the volunteers' brainwaves showed that deep sleep (slow-wave sleep) rather than rapid eye movement (REM) sleep or light sleep helped in strengthening the new memories; the researchers also observed 'spindles' of activity in the sleeping brain in which information is transferred from the hippocampus (known to be the region in which new long-term memories are formed) and the neo-cortex, where they are thought to be retained.

Co-author of the paper, Professor Gareth Gaskell, of the University of York's Department of Psychology, said: "We suspected from previous work that sleep had a role to play in the reorganisation of new memories, but this is the first time we've really been able to observe it in action, and understand the importance of spindle activity in the process."

Lead author, Dr Jakke Tamminen, said: "New memories are only really useful if you can connect them to information you already know. Imagine a game of chess, and being told that the rule governing the movement of a specific piece has just changed. That new information is only useful to you once you can modify your game strategy, the knowledge of how the other pieces move, and how to respond to your opponent's moves. Our study identifies the brain activity during sleep that organises new memories and makes those vital connections with existing knowledge."

The second study, Sleep’s Role in the Consolidation of Emotional Episodic Memories, published in Current Directions in Psychological Science, tested retention of episodic memories with emotional content as between two groups, one of which was sleep-deprived in the night following memory formation, while the other group slept normally. The tests were conducted on the following day, and then again six months later, with the same result: significantly better retention by the group that had slept normally. The researchers surmise that processing of the emotional content of the memories (with the involvement of the amygdala) was the primary agent of better retention. The FMRi scans showed retrieval activity modulated by the ventral medial prefrontal cortex (vMPFC) and the precuneus, and that activity between those two areas and the extended amygdala was greater among the normally rested group.

"Sleep is making memories stronger," says Jessica D. Payne of the University of Notre Dame, who co-wrote the review with Elizabeth A. Kensinger of Boston College. "It also seems to be doing something which I think is so much more interesting, and that is reorganizing and restructuring memories."

"In our fast-paced society, one of the first things to go is our sleep," Payne says. "I think that's based on a profound misunderstanding that the sleeping brain isn't doing anything." She believes that the reorganization of new memories during sleep (categorization and other mapping processes) is crucial to the brain's ability to be creative, and based on her results has taken to sleeping much more, herself!

The final study, Sleep Deprivation Facilitates Extinction of Implicit Fear Generalization and Physiological Response to Fear, published in . Biological Psychiatry, 68.11, again conducted on two groups of subjects, showed that deprival of sleep reduces the amount of fear associated with retention of newly-formed traumatic memories.

Dr. Kenichi Kuriyama, corresponding author, explained: "Sleep deprivation after exposure to a traumatic event, whether intentional or not, may help prevent PTSD (Post Traumatic Stree Disorder). Our findings may help to clarify the functional role of acute insomnia and to develop a prophylactic strategy of sleep restriction for prevention of PTSD."

"It would be nice if the benefits of sleep deprivation upon fear learning could be produced more easily for survivors of extreme stress," noted John Krystal, M.D., Editor of Biological Psychiatry and Professor and Chair of Psychiatry at Yale University. "New insights into the neurobiology of sleep dependent learning may make it possible for these people to take a medication that disrupts this process while leaving restorative elements of sleep intact."

This research confirms of course that it is the emotional content of episodic memory that makes it memorable and allows the brain to retain it successfully. The amygdala is part of the very ancient 'limbic' part of the brain, and it seems to make sense that remembering aspects of fearful episodes would be a good survival strategy; but this entailed the development of faster cortical circuitry, and the hypothesis, put forward by Edelman, is that the mechanism of sleep was required to allow the correction of mismatches between the fast cortex and the slower limbic systems.

Read more in Chapter Two of Agent Human by Michael Bell, The Evolution of Social Consciousness in Animals.


 

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