Sleep

How sleep acts on the brain

Sleep and cognition in children

A U.S. survey provides evidence that both children and adolescents tend to be getting less sleep than needed.

Depression, lower self-esteem, and lower grades, have all been found to be correlated with sleep deprivation in middle-school children.

Sleep disturbance in infants and young children has also been found to be associated with lower cognitive performance.

We all know that lack of sleep makes us more prone to attentional failures, more likely to make mistakes, makes new information harder to learn, old information harder to retrieve ... We all know that, right? And yet, so many of us still go to bed too late to get the sleep we need to function well. Of course, some of us go to sleep early enough, we just can’t get to sleep fast enough, or are prone to waking in the night. (Personally, I can count the times I’ve slept through the night without waking in the last fifteen years on my fingers).

I talk about the effect of sleep on memory elsewhere; I want to talk here about a sleep problem that we don’t tend to think about so much — the sleep deficit children are running.

A survey commissioned by the National Sleep Foundation found that 3-to-6-year-olds in the U.S. get about 10.4 hours sleep nightly, while experts recommend 11 to 13 hours. 1st graders to 5th graders who should be getting 10 to 11 hours are averaging just 9.5 hours.

And a study of middle-school children (11 to 14 year olds) found a direct correlation between sleep deprivation and depression, lower self-esteem, and lower grades. "The fewer hours of sleep that children got, the more depressed they were, the higher number of depressive symptoms [they had], and the lower their self-esteem and the lower their grades."

The second largest growth spurt occurs during these years (usually 10-14 for girls; 11-16 for boys), so this is a time when a lot of sleep is needed. But it’s also a time when children become more capable and more independent; when they’re likely to start taking on a lot more activities, work harder and longer, and are monitored less by their parents and caregivers. So ... it’s not surprising, when we stop and think about it, that a lot of these children are starting to pick up the bad habits of their parents — not getting enough sleep.

Which also points, in part, to the solution: if you’re a parent, remember that your children are, as always, modeling themselves on you. And sleep habits usually reflect a household pattern. If you’re a teacher, remember you need to educate the family, not just the child.

The National Institutes of Health (NIH) have identified adolescents and young adults (ages 12 to 25 years) as a population at high risk for problem sleepiness based on "evidence that the prevalence of problem sleepiness is high and increasing with particularly serious consequences."

Sleep disturbance in infants and young children has also been found to be associated with lower cognitive performance. Previous studies have looked at the severe end of the spectrum of sleep disorders — obstructive sleep apnea. More alarmingly, a new study of 205 5-year-old children found even mild sleep-disordered breathing symptoms (frequent snoring, loud or noisy breathing during sleep) were associated with poorer executive function and memory skills and lower general intelligence.

Before you panic, please note that some 30% of the participants had SBD symptoms, so it’s hardly uncommon (although there may have been a bias towards children with these symptoms; it does seem surprisingly high). You might also like to note that I personally had a blocked nose my entire childhood (always breathed through my mouth, and yes, of course I snored) and it didn’t stop me being top of the class, so ...

Nor is the research yet developed enough to know precisely what the connection is between SBD and cognitive impairment. However, it does seem that, if something can be done about the problem, it is probably worth doing (in my case, taking me off dairy would probably have fixed the problem! but of course noone had any idea of such factors back then).

Here’s a few links that may be of interest to parents and teachers:

ScienCentral article on the middle-school study:
http://sciencentral.com/articles/view.htm3?article_id=218392389

The NSF Sleep poll
http://www.sleepfoundation.org/article/press-release/national-sleep-foundation-2013-international-bedroom-poll 

a look at the school start times debate (I find this fairly amazing actually, because here in New Zealand, our children usually start school around 9am; the thought of kids starting school at 7.30 sends me into a spin!)
http://www.sleepfoundation.org/article/sleep-topics/school-start-time-and-sleep

The National Sleep Foundation also has a site for children who want to learn about sleep and healthy sleep habits: www.SleepforKids.org For children from 7 up; with educational games and activities, as well as a downloadable copy of NSF’s new Sleep Diary designed especially for children.

This article originally appeared in the November 2004 newsletter.

The role of consolidation in memory

"Consolidation" is a term that is bandied about a lot in recent memory research. Here's my take on what it means.

Becoming a memory

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.

Memory consolidation theory

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.

Memory as reconstruction

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

Reconsolidation of memories

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)?

Hippocampus involved in memory consolidation

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.

Memory more complex than we thought

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:

  • the initial encoding, the "working memory" state, when new information is held precariously in shifting patterns of neural activity,
  • the later encoding processes, when the information is consolidated into a more permanent form with the growth of new connections between nerve cells,
  • the (possibly much) later retrieval processes, when the information is retrieved in, most probably, a new context, and is activated anew

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.

References: 
  • Frankland, P.W., O'Brien, C., Ohno, M., Kirkwood, A. & Silva, A.J. 2001. -CaMKII-dependent plasticity in the cortex is required for permanent memory. Nature, 411, 309-313.
  • Gluck, M.A., Meeter, M. & Myers, C.E. 2003. Computational models of the hippocampal region: linking incremental learning and episodic memory. Trends in Cognitive Sciences, 7 (6), 269-276.
  • Haist, F., Gore, J.B. & Mao, H. 2001. Consolidation of human memory over decades revealed by functional magnetic resonance imaging. Nature neuroscience, 4 (11), 1139-1145.
  • Kang, H., Sun, L.D., Atkins, C.M., Soderling, T.R., Wilson, M.A. & Tonegawa, S. (2001). An Important Role of Neural Activity-Dependent CaMKIV Signaling in the Consolidation of Long-Term Memory. Cell, 106, 771-783.
  • Lopez, J.C. 2000. Shaky memories in indelible ink. Nature Reviews Neuroscience, 1, 6-7.
  • Miller, R.R. & Matzel, L.D. 2000. Memory involves far more than 'consolidation'. Nature Reviews Neuroscience, 1, 214-216.
  • Slotnick, S.D., Moo, L.R., Kraut, M.A., Lesser, R.P. & Hart, J. Jr. 2002. Interactions between thalamic and cortical rhythms during semantic memory recall in human. Proc. Natl. Acad. Sci. U.S.A., 99, 6440-6443.
  • Spinney, L. 2002. Memory debate focuses on hippocampal role. BioMedNet News, 18 March 2002.
  • Wirth, S., Yanike, M., Frank, L.M., Smith, A.C., Brown, E.N. & Suzuki, W.A. 2003. Single Neurons in the Monkey Hippocampus and Learning of New Associations. Science, 300, 1578-1581.
  • Zeineh, M.M., Engel, S.A., Thompson, P.M. & Bookheimer, S.Y. 2003. Dynamics of the Hippocampus During Encoding and Retrieval of Face-Name Pairs, Science, 299, 577-580.

Subliminal & sleep learning

Subliminal learning achieved notoriety back in 1957, when James Vicary claimed moviegoers could be induced to buy popcorn and Coca-Cola through the use of messages that flashed on the screen too quickly to be seen. The claim was later shown to be false, but though the idea that people can be brainwashed by the use of such techniques has been disproven (there was quite a bit of hysteria about the notion at the time), that doesn’t mean the idea of subliminal learning is crazy.

Ten years ago, researchers demonstrated that subliminal messages do indeed affect human cognition — and showed the limits of that influence [1]. The study demonstrated that, to have an effect on a person’s decision, the subliminal message had to be received very very soon before that decision (a tenth of a second or less), and the person had to be forced to make the decision very quickly. Moreover, there was no memory trace detectable, indicating no permanent record was stored in memory.

But even such brief, low-level learning seems to require some level of attention. A study [2] found that subliminal learning doesn’t occur if the subliminal stimuli are presented during what has been termed an "attentional blink" You may recall when I’ve discussed multi-tasking, I’ve said that we can’t do two things at the same time — that tasks have to "queue" for attention. When a bottleneck occurs in the system, this attentional "blink" occurs.

But low-level sensory processing, which is an automatic process, isn’t affected by the attentional blink, so the finding that subliminal learning is affected by the blink indicates that subliminal stimuli require some high-level cognitive processing.

This finding has been confirmed by other studies. One such study [3] also has implications for reading. Participants in the study were shown either words or pronounceable nonwords and asked to perform either a lexical task (to identify whether the word they saw was a real word or a nonsense word) or a pronunciation task on the words. Unbeknownst to the participants however, they had been first presented with a subliminal word that either matched or didn't match the target word. People performed the tasks faster when the subliminal word was identical to the target word. However (and this is the interesting bit), the researchers then applied a magnetic pulse (transcranial magnetic stimulation) to the key brain regions of the brain before presenting the subliminal information. By applying TMS to one brain area or the other, they found they could selectively disrupt the subliminal effect for either the lexical or pronunciation task. In other words, it seems that, even when the stimulus is subliminal, the brain takes into account the conscious task instructions. Our expectations shape our processing of subliminal stimuli.

Another study [4] suggests that motivation is important, and also, perhaps, that some stimuli are more suitable than others. The study found that thirsty people could be encouraged to drink more, and also pay more for their drink, after being exposed to subliminal smiling faces. Subliminal frowning faces had the opposite effect. However, how much, and whether, the faces had an effect on drinking, depended on the person’s thirst. Those who weren’t thirsty weren’t affected at all. Smiles and frowns are of course stimuli to which we are very responsive.

So clearly, although it is possible to be unconsciously affected by stimuli that can’t be consciously detected, the effect is both small and fleeting. However, that doesn’t mean more long-term effects can’t be experienced as a result of information we’re not conscious of.

Psychologists make a distinction between explicit memory and implicit memory. Explicit memory is what you’re using when you remember or recognize something — it’s what we tend to think of as "memory". Implicit memory, on the other hand, is a concept that reflects the fact that sometimes people act in ways that are clearly affected by earlier experiences they have had, even though they are not consciously recalling such experiences.

Another study [5] that used erotic images (because, like smiling and frowning faces, these are particularly potent stimuli, making it easy to see a response) found that when your eyes are presented with erotic images in a way that keeps you from becoming aware of them, your brain can still detect them — evidenced by the way people respond to the images according to their gender and sexual orientation.

The study is more evidence that the brain processes more visual information than we are conscious of — which is an important part in the process of determining what we’ll pay attention to. But the researchers believe that the information is probably destroyed at an early stage of processing — in other words, as with subliminal stimuli, there is probably no permanent record of the experience.

Which leads me to sleep learning. This was a big idea when I was young, in the science fiction I read — the idea that you could easily master new languages by being instructed while you were asleep.

Well, the question of whether learning can take place during sleep (and I’m not talking about the consolidation of learning that’s occurred earlier) is one that has been looked at in animal studies. It has been shown that simple forms of learning are indeed possible during sleep. However, the way in which associations are formed is clearly altered even for simple learning, and complex forms of learning do not appear to be possible.[6]

As far as humans are concerned, the evidence is that any learning during sleep must occur during the lightest stage of sleep, when you still have some awareness of the world around you, and that what you are learning must be already familiar (presented previously while you were awake and paying attention) and not requiring any understanding.

All the evidence suggests that, although information can be processed without conscious awareness, there are severe limitations on that information. If you want to "know" something in the proper meaning of the word — be able to recall it, think about it — you need to actively engage with the information. No free lunches, I’m afraid!

But that doesn’t mean unconscious influences don’t have important implications for learning and memory. A paper provided online in the Scientific American Mind Matters blog describes how a single, 15-minute intervention erased almost half the racial achievement gap between African American and white students. And this is entirely consistent with a number of studies showing how our cognitive performance is affected by what we think of ourselves (which is affected by what others think of us).

This article first appeared in the Memory Key Newsletter for March 2007

References: 
  1. Greenwald, A.G., Draine, S.C. & Abrams, R.L. 1996. Three Cognitive Markers of Unconscious Semantic Activation. Science, 273 (5282), 1699-1702.
  2. Seitz, A. et al. 2005. Requirement for High Level Processing in Subliminal Learning. Current Biology, 15, R753-R755, September 20, 2005.
  3. Nakamura, K. et al. 2006. Task-Guided Selection of the Dual Neural Pathways for Reading. Neuron, 52, 557-564.
  4. Winkielman, P. 2005. Paper presented at the American Psychological Society annual convention in Los Angeles, May 26-29. Press release
  5. Jiang, Y. et al. 2006. A gender- and sexual orientation-dependent spatial attentional effect of invisible images. PNAS, 103 (45), 17048-17052.
  6. Coenen, A.M. & Drinkenburg, W.H. 2002. Animal models for information processing during sleep. International Journal of Psychophysiology, 46(3), 163-175.

Biological clocks and memory

I’ve always been interested in the body’s clocks — and one of the most interesting things is that it is clocks, in the plural. It appears the main clock is located in a part of the brain structure called the hypothalamus (a very important structure in the brain, although not one of much importance to learning and memory). The part of the hypothalamus that regulates time is called the suprachiasmatic nuclei. These cells contain genes that switch on, off, and on again over a 24-hour period, and send electrical pulses and hormones through the body. This is the body’s master clock.

But it is not the only clock in the body. Each organ in the body uses the time signal from the master clock to set its own clock. As a consequence, different systems in the body operate on different schedules. Thus blood pressure peaks at one particular time of the day, and levels of the stress hormone cortisol rise and fall in accordance with the clock that governs this.

The effect of this is that certain physical disorders are more likely to occur at particular times, and, more significantly, that certain medications may be far more effective at certain times.

What does all this have to do with learning and memory?

Well, not a whole lot of research has been done on the effects of time of day on cognitive performance, but what has been done is reasonably consistent. It seems clear that, for many people (but not all), there are significant time of day effects. The most reliable is that, in general, teenagers and young adults perform best (mentally) in the afternoon, while older adults (seniors) perform best in the morning.

Having said that, let’s qualify it a little.

Let’s start with a table. Now, this represents the findings of one study [4], so let’s not get carried away with the illusion of precision cast by actual numbers. Nevertheless, it is interesting. These percentages represent the preferences reported by the young and old participants in the study. These preferences correlated with improved performance on a memory test.

  Young Old
Definite morning 0% 34%
Moderate morning 8% 49%
No preference 57% 10%
Moderate evening 29% 6%
Definite evening 6% 1%

Now the first thing to note is how marked the differences are between young and old. Of particular interest is how many of the younger adults had no preference. Compare this with that of older adults. The second finding of particular note is how pronounced the preference for the morning is in older adults — 83% preferred morning. And, most interesting of all, is a finding from another study by the same researchers [5]: when tested at their preferred time, older adults performed comparably to younger adults on a memory task. Younger adults, by contrast, seem able to perform well at all times.

There is also some evidence [3] that the deleterious effect of interference (the intrusion of irrelevant words, objects, events) is worse for older adults at those times of day when their performance is poorer. Older adults are more vulnerable to interference than younger adults.

The findings for teenagers and young adults may also apply to children. One study [2] found that below-grade-level students who received reading instruction in the afternoon improved their performance more than those students who received instruction in the morning.

But it must always be remembered that this general principle that morning is better for the aged, and afternoon better for the young, does not apply to each and every individual. As the table tells us, time of day affects some people more than others, and time preference is an individual matter, not entirely predicted by age. This is underscored by a study [1] that found improved performance when students were taught at times that matched their preferences. There was also some evidence that, for some students at least, achievement was greater when they were taught during their teacher's ideal time of day.

None of this is an argument that you should resign yourself to learning only at your preferred time of day! But you could use the information to modify your strategies. For example, by scheduling difficult work for your optimal time (assuming you have an optimal time, and are not one of those fortunate people who have no strong preference). You can also try and counteract the effect by, for example, drinking coffee during your nonoptimal time of day (this was found to be effective in one study with older adults [6]).

References: 
  1. Ammons, T.L., Booker, J.L. & Killmon, C.P. 1995. The effects of time of day on student attention and achievement. (ERIC Document Reproduction Service No. ED 384 592)
  2. Barron, B., Henderson, M. & Spurgeon, R. 1994. Effects of time of day instruction on reading achievement of below grade readers. Reading Improvement, 31(1), 56–60.
  3. Hasher, L., Chung, C., May, C.P. & Foong, N. 2002. Age, Time of Testing, and Proactive Interference. Canadian Journal of Experimental Psychology, 56, 200-207.
  4. Intons-Peterson, M.J., Rocchi, P., West, T., McLellan, K. and Hackney, A. 1998. Aging, optimal testing times, and negative priming.Journal of Experimental Psychology: Learning, Memory, and Cognition, 24(2), 362-376.
  5. Intons-Peterson, M.J., Rocchi, P., West, T., McLellan, K. and Hackney, A. 1999. Age, testing at preferred or nonpreferred times (testing optimality), and false memory. Journal of Experimental Psychology: Learning, Memory, and Cognition, 25(1), 23-40.
  6. Ryan, L., Hatfield, C. & Hofstetter, M. 2002. Caffeine Reduces Time-of-Day Effects on Memory Performance in Older Adults. Psychological Science, 13 (1), 68-71.
  7. West, R., Murphy, K.J., Armilio, M.L., Craik, F.I.M. & Stuss, D.T. 2002. Effects of Time of Day on Age Differences in Working Memory. Journals of Gerontology Series B, 57 (1), P3-P10

The role of sleep in memory

Why do we need sleep?

A lot of theories have been thrown up over the years as to what we need sleep for (to keep us wandering out of our caves and being eaten by sabertooth tigers, is one of the more entertaining possibilities), but noone has yet been able to point to a specific function of the sleep state that would explain why we have it and why we need so much of it.

One of the things we do know is that young birds and mammals need as much as three times the amount of sleep as adult birds and mammals. It has been suspected that neuronal connections are remodeled during sleep, and this has recently been supported in a study using cats (Cats who were allowed to sleep for six hours after their vision was blocked in one eye for six hours, developed twice as many new or modified brain connections as those cats who were kept awake in a dark room for the six hours after the period of visual deprivation).

Certainly a number of studies have shown that animals and humans deprived of sleep do not perform well on memory tasks, and research has suggested that there may be a relationship between excessive daytime sleepiness (EDS) and cognitive deficits. A recent study has found that for seniors at least, EDS is an important risk factor for cognitive impairment.

The effect of sleep on memory and learning

Some memory tasks are more affected be sleep deprivation than others. A recent study, for example, found that recognition memory for faces was unaffected by people being deprived of sleep for 35 hours. However, while the sleep-deprived people remembered that the faces were familiar, they did have much more difficulty remembering in which of two sets of photos the faces had appeared. In other words, their memory for the context of the faces was significantly worse. (The selective effect of sleep on contextual memory is also supported in a recent mouse study – see below)

While large doses of caffeine reduced the feelings of sleepiness and improved the ability of the sleep-deprived subjects to remember which set the face had appeared in, the level of recall was still significantly below the level of the non-sleep-deprived subjects. (For you coffee addicts, no, the caffeine didn’t help the people who were not sleep-deprived).

Interestingly, sleep deprivation increased the subjects’ belief that they were right, especially when they were wrong. In this case, whether or not they had had caffeine made no difference.

In another series of experiments, the brains of sleep-deprived and rested participants were scanned while the participants performed complex cognitive tasks. In the first experiment, the task was an arithmetic task involving working memory. Sleep-deprived participants performed worse on this task, and the fMRI scan confirmed less activity in the prefrontal cortex for these participants. In the second experiment, the task involved verbal learning. Again, those sleep-deprived performed worse, but in this case, only a little, and the prefrontal areas of the brain remained active, while parietal lobe activity actually increased. However, activity in the left temporal lobe (a language-processing area) decreased. In the third study, participants were given a "divided-attention" task, in which they completed both an arithmetic and a verbal-learning task. Again, sleep-deprived participants showed poorer performance, depressed brain activation in the left temporal region and heightened activation in prefrontal and parietal regions. There was also increased activation in areas of the brain that are involved in sustained attention and error monitoring.

These results indicate that sleep deprivation affects different cognitive tasks in different ways, and also that parts of the brain are able to at least partially compensate for the effects of sleep deprivation.

Sleep deprivation mimics aging?

A report in the medical journal The Lancet, said that cutting back from the standard eight down to four hours of sleep each night produced striking changes in glucose tolerance and endocrine function that mimicked many of the hallmarks of aging. Dr Eve Van Cauter, professor of medicine at the University of Chicago and director of the study, said, "We suspect that chronic sleep loss may not only hasten the onset but could also increase the severity of age-related ailments such as diabetes, hypertension, obesity and memory loss."

Should we draw any conclusion from the finding that sleep deprivation increased the subjects’ belief that they were right, especially when they were wrong, and the finding that chronic sleep deprivation may mimic the hallmarks of aging? No, let us merely note that many people become more certain of their own opinions as they mature into wisdom.

Is sleep necessary to consolidate memories?

This is the big question, still being argued by the researchers. The weight of the evidence, however, seems to be coming down on the answer, yes, sleep is necessary to consolidate memories — although maybe for only some types of memory. Most of the research favoring sleep’s importance in consolidation has used procedural / skill memory — sequences of actions.

From this research, it does seem that it is the act of sleep itself, not simply the passage of time, that is critical to convert new memories into long-term memory codes.

Some of the debate in this area concerns the stage of sleep that may be necessary. The contenders are the deep "slow wave" sleep that occurs in the first half of the night, and "REM" (rapid eye movement) sleep (that occurs while you are dreaming). Experiments that have found sleep necessary for consolidation tend to support slow-wave sleep as the important part of the cycle, however REM sleep may be important for other types of memory processing.

Sleep studies cast light on the memory cycle

Two new studies provide support both for the theory that sleep is important for the consolidation of procedural memories, and the new theory of what I have termed the "memory life-cycle".

In the first study, 100 young adults (18 to 27) learned several different finger-tapping sequences. It was found that participants remembered the sequence even if they learned a second sequence 6 hours later, and performance on both sequences improved slightly after a night's sleep. However, if, on day 2, people who had learned one sequence were briefly retested on it and then trained on a new sequence, their performance on the first sequence plummeted on day 3. If the first sequence wasn't retested before learning the new sequence, they performed both sequences accurately on day 3.

In another study, 84 college students were trained to identify a series of similar-sounding words produced by a synthetic-speech machine. Participants who underwent training in the morning performed well in subsequent tests that morning, but tests later in the day showed that their word-recognition skill had declined. However, after a full night's sleep, they performed at their original levels. Participants trained in the evening performed just as well 24 hours later as people trained in the morning did. Since they went to bed shortly after training, those in the evening group didn't exhibit the temporary performance declines observed in the morning group.

On the basis of these studies, researchers identified three stages of memory processing: the first stage of memory — its stabilization — seems to take around six hours. During this period, the memory appears particularly vulnerable to being “lost”. The second stage of memory processing — consolidation — occurs during sleep. The third and final stage is the recall phase, when the memory is once again ready to be accessed and re-edited. (see my article on consolidation for more explanation of the processes of consolidation and re-consolidation)

The researchers made a useful analogy with creating a word-processing document on the computer. The first stage is when you hit “Save” and the computer files the document in your hard drive. On the computer, this takes seconds. The second stage is comparable to someone coming and tidying up your word document — reorganizing it and tightening it up.

The most surprising aspect of this research is the time it appears to take for memories to initially stabilize — seconds for the computer saving the document, but up to six hours for us!

See news reports on sleep's role in memory

See news reports on the effects of sleep deprivation

Added January 2012: a downloadable pdf with all articles and news reports pertaining to sleep, circadian rhythms, and meditation

References: 
  1. Drummond, S.P.A., Brown, G.G., Stricker, J.L., Buxton, R.B., Wong, E.C. & Gillin, J.C. 1999. Sleep deprivation-induced reduction in cortical functional response to serial subtraction. NeuroReport, 10 (18), 3745-3748.
  2. Drummond, S.P.A., Brown, G.G., Gillin, J.C., Stricker, J.L., Wong, E.C. & Buxton, R.B. 2000. Altered brain response to verbal learning following sleep deprivation. Nature, 403 (6770),655-7.
  3. Drummond, S.P.A., Gillin, J.C. & Brown, G.G. 2001. Increased cerebral response during a divided attention task following sleep deprivation. Journal of Sleep Research, 10 (2), 85-92.
  4. Fenn, K.M., Nusbaum, H.C. & Margoliash, D. 2003. Consolidation during sleep of perceptual learning of spoken language. Nature, 425, 614-616.
  5. Frank, M.G., Issa, N.P. & Stryker, M.P. 2001. Sleep Enhances Plasticity in the Developing Visual Cortex. Neuron, 30, 275-287.
  6. Graves, L.A., Heller, E.A., Pack, A.I. & Abel, T. 2003. Sleep Deprivation Selectively Impairs Memory Consolidation for Contextual Fear Conditioning. Learning & Memory, 10, 168-176.
  7. Harrison, Y. & Horne, J.A. 2000. Sleep loss and temporal memory. The Quarterly Journal of Experimental Psychology, 53A (1), 271-279. Research report
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Reactivate if you want to remember

action replay box

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.

Cognitive decline in old age related to poorer sleep

A new study confirms the role slow-wave sleep plays in consolidating memories, and reveals that one reason for older adults’ memory problems may be the quality of their sleep.

Recent research has suggested that sleep problems might be a risk factor in developing Alzheimer’s, and in mild cognitive impairment. A new study adds to this gathering evidence by connecting reduced slow-wave sleep in older adults to brain atrophy and poorer learning.

The study involved 18 healthy young adults (mostly in their 20s) and 15 healthy older adults (mostly in their 70s). Participants learned 120 word- nonsense word pairs and were tested for recognition before going to bed. Their brain activity was recorded while they slept. Brain activity was also measured in the morning, when they were tested again on the word pairs.

As has been found previously, older adults showed markedly less slow-wave activity (both over the whole brain and specifically in the prefrontal cortex) than the younger adults. Again, as in previous studies, the biggest difference between young and older adults in terms of gray matter volume was found in the medial prefrontal cortex (mPFC). Moreover, significant differences were also found in the insula and posterior cingulate cortex. These regions, like the mPFC, have also been associated with the generation of slow waves.

When mPFC volume was taken into account, age no longer significantly predicted the extent of the decline in slow-wave activity — in other words, the decline in slow-wave activity appears to be due to the brain atrophy in the medial prefrontal cortex. Atrophy in other regions of the brain (precuneus, hippocampus, temporal lobe) was not associated with the decline in slow-wave activity when age was considered.

Older adults did significantly worse on the delayed recognition test than young adults. Performance on the immediate test did not predict performance on the delayed test. Moreover, the highest performers on the immediate test among the older adults performed at the same level as the lowest young adult performers — nevertheless, these older adults did worse the following day.

Slow-wave activity during sleep was significantly associated with performance on the next day’s test. Moreover, when slow-wave activity was taken into account, neither age nor mPFC atrophy significantly predicted test performance.

In other words, age relates to shrinkage of the prefrontal cortex, this shrinkage relates to a decline in slow-wave activity during sleep, and this decline in slow-wave sleep relates to poorer cognitive performance.

The findings confirm the importance of slow-wave brainwaves for memory consolidation.

All of this suggests that poorer sleep quality contributes significantly to age-related cognitive decline, and that efforts should be made to improve quality of sleep rather than just assuming lighter, more disturbed sleep is ‘natural’ in old age!

Reviewing alcohol's effects on normal sleep

A review on the immediate effects of alcohol on sleep has found that alcohol shortens the time it takes to fall asleep, increases deep sleep, and reduces REM sleep.

Because sleep is so important for memory and learning (and gathering evidence suggests sleep problems may play a significant role in age-related cognitive impairment), I thought I’d make quick note of a recent review bringing together all research on the immediate effects of alcohol on the sleep of healthy individuals.

The review found that alcohol in any amount reduces the time it takes to fall asleep, while greater amounts produce increasing amounts of deep sleep in the first half of the night. However, sleep is more disrupted in the second half. While increased deep sleep is generally good, there are two down sides here: first, it’s paired with sleep disruption in the second half of the night; second, those predisposed to problems such as sleepwalking or sleep apnea may be more vulnerable to them. (A comment from the researchers that makes me wonder if the relationship between deep sleep and slow-wave activity is more complicated than I realized.)

Additionally, at high doses of alcohol, REM sleep is significantly reduced in the first half, and overall. This may impair attention, memory, and motor skills. Moreover, at all doses, the first REM period is significantly delayed, producing less restful sleep.

The researchers conclude that, while alcohol may give the illusion of improving sleep, it is not in fact doing so.

Reference: 

[3269] Ebrahim, I. O., Shapiro C. M., Williams A. J., & Fenwick P. B. (2013).  Alcohol and Sleep I: Effects on Normal Sleep. Alcoholism: Clinical and Experimental Research. n/a - n/a.

Rest briefly after learning

A small study with older adults provides support for the idea that learning is helped if you follow it with a few minutes ‘wakeful rest’.

Back in 2010, I briefly reported on a study suggesting that a few minutes of ‘quiet time’ could help you consolidate new information. A new study provides more support for this idea.

In the first experiment, 14 older adults (aged 61-81) were told a short story, with instructions to remember as many details as possible. Immediately afterward, they were asked to describe what happened in the story. Ten minutes then elapsed, during which they either rested quietly (with eyes closed in a darkened room), or played a spot-the-difference game on the computer (comparing pairs of pictures). This task was chosen because it was non-verbal and sufficiently different from the story task to not directly compete for cognitive resources.

This first learning phase was followed by five minutes of playing the spot-the-difference game (for all participants) and then a second learning phase, in which the process was repeated with a second story, and participants experienced the other activity during the delay period (e.g., rest if they had previously played the game).

Some 30 minutes after the first story presentation (15 minutes after the second), participants were unexpectedly asked to once again recall as many details as they could from the stories. A further recall test was also given one week later.

Recall on the first delayed test (at the end of both learning phases) was significantly better for stories that had been followed by wakeful resting rather than a game. While recall declined at the same rate for both story conditions, the benefits of wakeful resting were maintained at the test one week later.

In a second experiment, the researchers looked at whether these benefits would still occur if there was no repetition (i.e., no delayed recall test at the time, only at a week). Nineteen older adults (61-87) participated.

As expected, in the absence of the short-delay retrieval test, recall at a week was slightly diminished. Nevertheless, recall for stories that had been followed by rest was still significantly better than recall for stories followed by the game.

It’s worth noting that, in a post-session interview, only 3 participants (of the 33 total) reported thinking about the story during the period of wakeful rest. One participant fell asleep. Twelve participants reported thinking about the stories at least once during the week, but there was no difference between these participants’ scores and those who didn’t think about them.

These findings support the idea that a quiet period of reflection after new learning helps the memories be consolidated. While the absence of interfering information may underlie this, the researchers did select the game specifically to interfere as little as possible with the story task. Moreover, the use of the same task as a ‘filler’ between the two learning phases was also designed to equalize any interference it might engender.

The weight of the evidence, therefore, is that ten minutes of wakeful resting aided memory by providing the mental space in which to consolidate the memory. Moreover, the fact that so few participants actively thought about the stories during that rest indicates that such consolidation is automatic and doesn’t require deliberate rehearsal.

The study did, of course, only involve older adults. I hope we will see a larger study with a wider participant pool.

Sleep learning making a comeback?

Two new studies provide support for the judicious use of sleep learning — as a means of reactivating learning that occurred during the day.

Back when I was young, sleep learning was a popular idea. The idea was that a tape would play while you were asleep, and learning would seep into your brain effortlessly. It was particularly advocated for language learning. Subsequent research, unfortunately, rejected the idea, and gradually it has faded (although not completely). Now a new study may presage a come-back.

In the study, 16 young adults (mean age 21) learned how to ‘play’ two artificially-generated tunes by pressing four keys in time with repeating 12-item sequences of moving circles — the idea being to mimic the sort of sensorimotor integration that occurs when musicians learn to play music. They then took a 90-minute nap. During slow-wave sleep, one of the tunes was repeatedly played to them (20 times over four minutes). After the nap, participants were tested on their ability to play the tunes.

A separate group of 16 students experienced the same events, but without the playing of the tune during sleep. A third group stayed awake, during which 90-minute period they played a demanding working memory task. White noise was played in the background, and the melody was covertly embedded into it.

Consistent with the idea that sleep is particularly helpful for sensorimotor integration, and that reinstating information during sleep produces reactivation of those memories, the sequence ‘practiced’ during slow-wave sleep was remembered better than the unpracticed one. Moreover, the amount of improvement was positively correlated with the proportion of time spent in slow-wave sleep.

Among those who didn’t hear any sounds during sleep, improvement likewise correlated with the proportion of time spent in slow-wave sleep. The level of improvement for this group was intermediate to that of the practiced and unpracticed tunes in the sleep-learning group.

The findings add to growing evidence of the role of slow-wave sleep in memory consolidation. Whether the benefits for this very specific skill extend to other domains (such as language learning) remains to be seen.

However, another recent study carried out a similar procedure with object-location associations. Fifty everyday objects were associated with particular locations on a computer screen, and presented at the same time with characteristic sounds (e.g., a cat with a meow and a kettle with a whistle). The associations were learned to criterion, before participants slept for 2 hours in a MR scanner. During slow-wave sleep, auditory cues related to half the learned associations were played, as well as ‘control’ sounds that had not been played previously. Participants were tested after a short break and a shower.

A difference in brain activity was found for associated sounds and control sounds — associated sounds produced increased activation in the right parahippocampal cortex — demonstrating that even in deep sleep some sort of differential processing was going on. This region overlapped with the area involved in retrieval of the associations during the earlier, end-of-training test. Moreover, when the associated sounds were played during sleep, parahippocampal connectivity with the visual-processing regions increased.

All of this suggests that, indeed, memories are being reactivated during slow-wave sleep.

Additionally, brain activity in certain regions at the time of reactivation (mediotemporal lobe, thalamus, and cerebellum) was associated with better performance on the delayed test. That is, those who had greater activity in these regions when the associated sounds were played during slow-wave sleep remembered the associations best.

The researchers suggest that successful reactivation of memories depends on responses in the thalamus, which if activated feeds forward into the mediotemporal lobe, reinstating the memories and starting the consolidation process. The role of the cerebellum may have to do with the procedural skill component.

The findings are consistent with other research.

All of this is very exciting, but of course this is not a strategy for learning without effort! You still have to do your conscious, attentive learning. But these findings suggest that we can increase our chances of consolidating the material by replaying it during sleep. Of course, there are two practical problems with this: the material needs an auditory component, and you somehow have to replay it at the right time in your sleep cycle.

Reference: 
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