How sleep acts on the brain
"Consolidation" is a term that is bandied about a lot in recent memory research. Here's my take on what it means.
Initially, information is thought to be encoded as patterns of neural activity — cells "talking" to each other. Later, the information is coded in more persistent molecular or structural formats (e.g., the formation of new synapses). It has been assumed that once this occurs, the memory is "fixed" — a permanent, unchanging, representation.
With new techniques, it has indeed become possible to observe these changes (you can see videos here). Researchers found that the changes to a cell that occurred in response to an initial stimulation lasted some three to five minutes and disappeared within five to 10 minutes. If the cell was stimulated four times over the course of an hour, however, the synapse would actually split and new synapses would form, producing a (presumably) permanent change.
The hypothesis that new memories consolidate slowly over time was proposed 100 years ago, and continues to guide memory research. In modern consolidation theory, it is assumed that new memories are initially 'labile' and sensitive to disruption before undergoing a series of processes (e.g., glutamate release, protein synthesis, neural growth and rearrangement) that render the memory representations progressively more stable. It is these processes that are generally referred to as “consolidation”.
Recently, however, the idea has been gaining support that stable representations can revert to a labile state on reactivation.
In a way, this is not surprising. We already have ample evidence that retrieval is a dynamic process during which new information merges with and modifies the existing representation — memory is now seen as reconstructive, rather than a simple replaying of stored information
Researchers who have found evidence that supposedly stable representations have become labile again after reactivation, have called the process “reconsolidation”, and suggest that consolidation, rather than being a one-time event, occurs repeatedly every time the representation is activated.
This raises the question: does reconsolidation involve replacing the previously stable representation, or the establishment of a new representation, that coexists with the old?
Whether reconsolidation is the creating of a new representation, or the modifying of an old, is this something other than the reconstruction of memories as they are retrieved? In other words, is this recent research telling us something about consolidation (part of the encoding process), or something about reconstruction (part of the retrieval process)?
The principal player in memory consolidation research, in terms of brain regions, is the hippocampus. The hippocampus is involved in the recognition of place and the consolidation of contextual memories, and is part of a region called the medial temporal lobe (MTL), that also includes the perirhinal, parahippocampal,and entorhinal cortices. Lesions in the medial temporal lobe typically produce amnesia characterized by the disproportionate loss of recently acquired memories. This has been interpreted as evidence for a memory consolidation process.
Some research suggests that the hippocampus may participate only in consolidation processes lasting a few years. The entorhinal cortex, on the other hand, gives evidence of temporally graded changes extending up to 20 years, suggesting that it is this region that participates in memory consolidation over decades. The entorhinal cortex is damaged in the early stages of Alzheimer’s disease.
There is, however, some evidence that the hippocampus can be involved in older memories — perhaps when they are particularly vivid.
A recent idea that has been floated suggests that the entorhinal cortex, through which all information passes on its way to the hippocampus, handles “incremental learning” — learning that requires repeated experiences. “Episodic learning” — memories that are stored after only one occurrence — might be mainly stored in the hippocampus.
This may help explain the persistence of some vivid memories in the hippocampus. Memories of emotionally arousing events tend to be more vivid and to persist longer than do memories of neutral or trivial events, and are, moreover, more likely to require only a single experience.
Whether or not the hippocampus may retain some older memories, the evidence that some memories might be held in the hippocampus for several years, only to move on, as it were, to another region, is another challenge to a simple consolidation theory.
So where does all this leave us? What is consolidation? Do memories reach a fixed state?
My own feeling is that, no, memories don't reach this fabled "cast in stone" state. Memories are subject to change every time they are activated (such activation doesn't have to bring the memory to your conscious awareness). But consolidation traditionally (and logically) refers to encoding processes. It is reasonable, and useful, to distinguish between:
I think that "reconsolidation" is a retrieval process rather than part of the encoding processes, but of course, if you admit retrieval as involving a return to the active state and a modification of the original representation in line with new associations, then the differences between retrieval and encoding become less evident.
When you add to this the possibility that memories might "move" from one area of the brain to another after a certain period of time (although it is likely that the triggering factor is not time per se), then you cast into disarray the whole concept of memories becoming stable.
Perhaps our best approach is to see memory as a series of processes, and consolidation as an agreed-upon (and possibly arbitrary) subset of those processes.
For more, see the research reports
A study involving epilepsy patients who had electrodes implanted into their brain has revealed that memory consolidation during sleep doesn’t simply involve reactivation of the new memories.
Participants were given pictures to memorize, before taking an afternoon nap. Surprisingly, brainwave activity showed that both the pictures participants later remembered and those they later forgot, were reactivated during sleep. What was crucial was not the reactivation of the picture-specific gamma band activity, but its conjunction with “ripples” (extremely rapid fluctuations in activity) in the hippocampus. Only when the reactivation occurred at the same time as the ripples in the hippocampus did participants remember the picture.
What determined whether this happened? The evidence suggests that longer (and thus deeper) processing of the picture is needed, not simply a quick superficial look.
This phenomenon only occurred during nonREM sleep, not during wakefulness (the circumstances of sleep meant little time was spent in REM sleep).
The findings confirm earlier research with rodents.
Paper available at https://www.nature.com/articles/s41467-018-06553-y
A laboratory study has found that sleeping after watching a trauma event reduced emotional distress and memories related to traumatic events. The laboratory study involved 65 women being shown a neutral and a traumatic video. Typically, recurring memories of certain images haunted the test subjects for a few days (these were recorded in detail in a diary). Some participants slept in the lab for a night after the video, while the other group remained awake.
Those who slept after the film had fewer and less distressing recurring emotional memories than those who were awake. This effect was particularly evident after several days.
One of the reasons for this benefit is thought to be that the memory consolidation processes that happen during sleep help contextualize the memories. This is interesting in view of the recent theory that PTSD is associated with a deficit in contextual processing.
However, I'd note that there is conflicting evidence about the effects of sleep on negative memories (for example, see http://www.memory-key.com/research/news/sleep-preserves-your-feelings-about-traumatic-events).
Sleep, as I have said on many occasions, helps your brain consolidate new memories. I have reported before on a number of studies showing how sleep helps the learning of various types of new information. Most of those studies have looked at procedural learning (learning new skills), or verbal learning. A new study adds to these by looking at face-name associations.
The small study, involving 14 young adults, found that that they were significantly better at remembering faces and names if they were given an opportunity to have a full night's sleep hours after seeing those faces and names for the first time.
Participants were shown 20 photos of faces with corresponding names and asked to memorize them. After a twelve-hour period, they were then shown the photos again with either a correct or incorrect name. They were also asked to rate their confidence in their answer. Each participant completed the test twice — once with an interval of sleep in between and once with a period of regular, waking day activities in between.
After a night's sleep, participants correctly matched 12% more of the faces and names, and were much more confident of their answers.
Of course, this is not a huge difference, given the small number of face-name pairs, and the sample is small. I would have also liked to see further testing 12 hours later, so that we could compare the effects of a day followed by a night, versus a night followed by a day (this would have required more stimuli and more participants, of course).
So, not madly exciting, but taken in context of other research, it adds to the growing evidence that sleep helps you consolidate new learning of all kinds.
This sounds like pseudoscience, but it appears in Journal of Neuroscience, so … Weirdly, a rat study has found that sleeping on the side (the most common posture for humans and other animals) is the best position for efficiently removing waste from the brain.
Brain waste includes amyloid-beta and tau proteins, whose build-up is a critical factor in the development of Alzheimer's disease.
The study used imaging of the glymphatic pathway, which clears waste products from the brain by filtering cerebrospinal fluid through the brain and exchanging it with interstitial fluid. The process is most efficient during sleep, and its efficiency is affected by the level of consciousness. The researchers compared glymphatic transport during sleep when anesthetized rodents’ brains were in three positions—lateral (side), prone (down), and supine (up).
Of course, these findings need to be confirmed in humans (which might be tricky!), but there is, after all, no harm in changing your sleep position, if you don't already sleep on your side (though I concede it can be a difficult thing to change).
Apart from providing a practical tip for fighting age-related cognitive decline and dementia, the finding also supports the idea that one of the purposes of sleep is to ‘clean up’ the mess that accumulates while we are awake.
The finding is also consistent with increasing evidence that sleep disturbances are a factor in the development and progression of dementia.
We know sleep helps consolidate memories. Now a new study sheds light on how your sleeping brain decides what’s worth keeping. The study found that when the information that makes up a memory has a high value—associated with, for example, making more money—the memory is more likely to be rehearsed and consolidated during sleep.
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