How memory works

Sleep's role in cognition

Older news items (pre-2010) brought over from the old website

A midday nap markedly boosts the brain's learning capacity

Following on from research showing that pulling an all-nighter decreases the ability to cram in new facts by nearly 40%, a study involving 39 young adults has found that those given a 90-minute nap in the early afternoon, after being subjected to a rigorous learning task, did markedly better at later round of learning exercises, compared to those who remained awake throughout the day. The former group actually improved in their capacity to learn, while the latter became worse at learning. The findings reinforce the hypothesis that sleep is needed to clear the brain's short-term memory storage and make room for new information. Moreover, this refreshing of memory capacity was related to Stage 2 non-REM sleep (an intermediate stage between deep sleep and the REM dream stage).

The preliminary findings were presented February 21, at the annual meeting of the American Association of the Advancement of Science (AAAS) in San Diego, Calif.

Helping memory consolidation while you sleep

The role of sleep in consolidating new learning is now well-established, but now a study intriguingly reveals that you can improve that learning by playing sounds associated with the learning while you are asleep. The study involved 12 volunteers learning to associate each of 50 images with a random location on a computer screen. Each object was paired with its associated sound. Some 45 minutes after they had successfully mastered this task, each participant lay down in a quiet, darkened room. Once deeply asleep, 25 of these sounds were played. Although none of the participants noticed these sounds, performance was subsequently more accurate for those objects whose sounds had been played during sleep. The findings reveal that memory consolidation can be directed to specific memories through use of such cues. Another recent study found smells could also be used in this way.

[1056] Rudoy, J. D., Voss J. L., Westerberg C. E., & Paller K. A.
(2009).  Strengthening Individual Memories by Reactivating Them During Sleep.
Science. 326(5956), 1079 - 1079.

Sleep helps reduce errors in memory

A study in which college students were shown lists of words and then, 12 hours later, asked to identify which words they had seen or heard earlier, found that those who trained at night and tested the following morning were less prone to falsely recognizing semantically similar words than those who trained in the morning and tested in the evening. It’s suspected that sleep may help strengthen the source of the memory, thus helping protect against false memories.

[254] Fenn, K. M., Gallo D. A., Margoliash D., Roediger H. L., & Nusbaum H. C.
(2009).  Reduced false memory after sleep.
Learning & Memory. 16(9), 509 - 513.

How sleep consolidates memory

A rat study provides clear evidence that "sharp wave ripples", brainwaves that occur in the hippocampus when it is "off-line", most often during stage four sleep, are responsible for consolidating memory and transferring the learned information from the hippocampus to the neocortex, where long-term memories are stored. The study found that when these waves were eliminated during sleep, the rats were less able to remember a spatial navigation task.

[1083] Girardeau, G., Benchenane K., Wiener S. I., Buzsaki G., & Zugaro M. B.
(2009).  Selective suppression of hippocampal ripples impairs spatial memory.
Nat Neurosci. 12(10), 1222 - 1223.

Memories practiced throughout the day, not just while sleeping

It is known that a certain amount of replaying of experiences occurs in the hippocampus immediately afterwards, but it has been thought that this is confined to the immediate past, while the replaying that occurs during sleep and is thought to be part of the memory consolidation process, ranges far more widely. Now a new rat study indicates that the replaying that occurs while the animal is awake is more extensive than thought, and more accurate than that which occurs during sleep. Data from the neurons indicated that the events being replayed (repeatedly) were from 20 to 30 minutes earlier, and involved different settings, indicating the replay wasn’t dependent on incoming sensory cues. It’s suggested that the less-accurate replays seen during sleep are more aimed at making connections, rather than consolidating the actual experience. The waking replays occurred during pauses in activity, perhaps suggesting the importance of making pauses for reflection during your day!

[933] Karlsson, M. P., & Frank L. M.
(2009).  Awake replay of remote experiences in the hippocampus.
Nature Neuroscience. 12(7), 913 - 918.

Creative problem solving enhanced by REM sleep

A study investigating the role of sleep in creative problem-solving has found that those who experienced REM sleep between two tests performed significantly better on the later test compared to those who simply had a quiet rest, or those who napped but had no REM sleep. The findings support the idea that REM sleep (when dreams occur) has a role in forming new associations. It’s suggested that the process may be facilitated by changes to neurotransmitter systems (cholinergic and noradrenergic) during REM sleep.

[1326] Cai, D. J., Mednick S. A., Harrison E. M., Kanady J. C., & Mednick S. C.
(2009).  REM, not incubation, improves creativity by priming associative networks.
Proceedings of the National Academy of Sciences. 106(25), 10130 - 10134.

Sleep may be important in regulating emotional responses

A study involving 44 college students who were asked to remember scenes with neutral or negative objects on a neutral background has found that those who trained and tested on the scenes in the evening remembered the negative scenes better than those who were trained and tested in the morning. However, neutral objects were not better remembered, and the backgrounds associated with negative objects were more poorly remembered by this group. The pattern persisted when the students were tested four months later. The findings suggest that the sleeping brain calculates what is most important about an experience and selects only what is adaptive for consolidation and long term storage.

Payne, J.D., Kensinger, E., Wamsley, E. & Stickgold, R. 2009. Sleep Promotes Lasting Changes in Memory for Emotional Scenes. Presented on June 11 at SLEEP 2009, the 23rd Annual Meeting of the Associated Professional Sleep Societies; Abstract ID: 1244.

Sleep may help clear the brain for new learning

Although fruit flies may seem little like us, their response to sleep deprivation is similar, and so they are useful models for sleep effects on the human brain. In a recent study, flies genetically altered to make it easier to track individual synapses have revealed that during sleep the number of new synapses formed during earlier learning decreased. This decline didn’t happen if the flies were deprived of sleep. It’s theorized that this activity during sleep is a way of pruning the less relevant and important synapses (clearing away the junk, as it has been conceptualized). The study follows earlier fruit fly research showing that more learning resulted in longer sleep. It also supports recent rat research that found synaptic strength increases during the day, then weakens during sleep. The study also identified three genes essential to the links between learning and increased need for sleep, one of which is equivalent to a human gene known as serum response factor (SRF) and previously linked to brain plasticity.

[360] Donlea, J. M., Ramanan N., & Shaw P. J.
(2009).  Use-Dependent Plasticity in Clock Neurons Regulates Sleep Need in Drosophila.
Science. 324(5923), 105 - 108.

Sleep helps you learn complicated tasks & recover forgotten skills

A study involving 200 mostly female college students, who had little experience of video games. The students were taught to play a complicated, multisensory video game in which players must use both hands to deal with continually changing visual and auditory signals. Half were tested 12 hours after the training session, and the others 24 hours later. Some were given a night’s sleep before testing, others were tested the same day. Performance in the former dropped by half at testing, but when tested again the following morning, they showed a 10 percentage point improvement over their pre-test performance. For those given evening training, scores improved by about 7 percentage points, then went to 10 percentage points the next morning – which was maintained over the day. The findings indicate that although people may appear to forget much of their learning over the course of a day, a night’s sleep will restore it; moreover, sleep protected the memory from loss over the course of the next day. The findings confirm the role of sleep in consolidating memory for skills, and extends the research to complicated tasks.

[486] Brawn, T. P., Fenn K. M., Nusbaum H. C., & Margoliash D.
(2008).  Consolidation of sensorimotor learning during sleep.
Learning & Memory. 15(11), 815 - 819.

Sleep selectively preserves emotional memories

It’s now generally accepted that sleep plays an important role in consolidating procedural (skill) memories, but the position regarding other types of memory has been less clear.  A new study has found that sleep had an effect on emotional aspects of a memory. The study involved showing 88 students neutral scenes (such as a car parked on a street in front of shops) or negative scenes (a badly crashed car parked on a similar street). They were then tested for their memories of both the central objects in the pictures and the backgrounds in the scenes, either after 12 daytime hours, or 12 night-time hours, or 30 minutes after viewing the images, in either the morning or evening.  Those tested after 12 daytime hours largely forgot the entire negative scene, forgetting both the central objects and the backgrounds equally. But those tested after a night’s sleep remembered the emotional item (e.g., the smashed car) as well as those who were tested only 30 minutes later. Their memory of the neutral background was however, as bad as the daytime group. The findings are consistent with the view that the individual components of emotional memory become 'unbound' during sleep, enabling the brain to selectively preserve only that information it considers important.

[875] Payne, J. D., Stickgold R., Swanberg K., & Kensinger E. A.
(2008).  Sleep preferentially enhances memory for emotional components of scenes.
Psychological Science: A Journal of the American Psychological Society / APS. 19(8), 781 - 788.

Aging impairs the 'replay' of memories during sleep

During sleep, the hippocampus repeatedly "replays" brain activity from recent experiences, in a process believed to be important for memory consolidation. A new rat study has found reduced replay activity during sleep in old compared to young rats, and rats with the least replay activity performed the worst in tests of spatial memory. The best old rats were also the ones that showed the best sleep replay. Indeed, the animals who more faithfully replayed the sequence of neural activity recorded during their earlier learning experience were the ones who performed better on the spatial memory task, regardless of age. The replay activity occurs during slow-wave sleep.

[1319] Gerrard, J. L., Burke S. N., McNaughton B. L., & Barnes C. A.
(2008).  Sequence Reactivation in the Hippocampus Is Impaired in Aged Rats.
J. Neurosci.. 28(31), 7883 - 7890.

A nap can help you learn

A study of 33 younger adults (average are 23) has found that a 45 minute afternoon nap (containing only non-REM sleep) improved performance on 3 different declarative memory tasks, but only when the subjects had reached a certain level of performance during training.

[672] Tucker, M. A., & Fishbein W.
(2008).  Enhancement of declarative memory performance following a daytime nap is contingent on strength of initial task acquisition.
Sleep. 31(2), 197 - 203.

Brain connections strengthen during waking hours, weaken during sleep

New research provides support for a much-debated theory that we need sleep to give our synapses time to rest and recover. The human brain is said to expend up to 80% of its energy on synaptic activity, constantly adding and strengthening connections in response to stimulation. The researchers have theorized that we need an ‘off-line period’, when we are not exposed to the environment, to take synapses down. The rodent study has revealed by several measures that synapses — the all-important points of connection between neurons — are very active when the animal is awake and very quiet during sleep. The researchers feel that these findings support the idea that our brain circuits get progressively stronger during wakefulness and that sleep helps to recalibrate them to a sustainable baseline. This theory is of course opposite to the currently dominant hypothesis, that during sleep synapses are hard at work replaying the information acquired during the previous waking hours, consolidating that information by becoming even stronger.

[631] Vyazovskiy, V. V., Cirelli C., Pfister-Genskow M., Faraguna U., & Tononi G.
(2008).  Molecular and electrophysiological evidence for net synaptic potentiation in wake and depression in sleep.
Nat Neurosci. 11(2), 200 - 208.

Sleep reinforces the temporal sequence in memory

Following on from research showing long-term memory is consolidated during sleep through the replaying of recently encoded experiences, a study has found that the particular order in which they were experienced is also strengthened, probably by a replay of the experiences in "forward" direction. The study involved students being asked to learn triplets of words presented one after the other. Those whose recall of the order of the words was tested after sleep showed better recall, but only when they were asked to reproduce the learned words in forward direction.

[368] Drosopoulos, S., Windau E., Wagner U., & Born J.
(2007).  Sleep Enforces the Temporal Order in Memory.
PLoS ONE. 2(4), e376 - e376.

Sleep protects against interference

A study involving 48 people (aged 18—30) found that those who learned 20 pairs of words at 9pm and were tested at 9am the following morning, after a night’s sleep, performed better than those who learned them at 9am and were tested at 9pm of the same day. Moreover, for those who were given a second list of word pairs to remember just before testing, where the first word in each pair was the same as on the earlier list, the advantage of sleep was dramatically better. For those who experienced the interference manipulation, those in the sleep group recalled 12% more word pairs than the wake group, but with interference, the recall rate was 44% higher for the sleep group.

The findings were presented by Dr Jeffrey Ellenbogen at the American Academy of Neurology’s 59th Annual Meeting in Boston, April 28 – May 5, 2007.

Sleeping helps us put facts together

And in yet another sleep study, researchers found evidence that sleep also helps us see the big picture. The study involved 56 students who were shown oval images of colorful abstract patterns nicknamed "Fabergé eggs." Participants were first shown a combination of five pairs of the eggs, all of which were given ratings. The students were given 30 minutes to learn which shape rated higher and so should be chosen over another shape. They were not told the hidden connection that linked all five pairs together. They were then tested either after 20 minutes, after 12 hours, or after 24 hours. Half of those in the 12-hour group slept before the test, the other half did not. The 20-minute group performed the worst, showing no evidence of seeing the pattern. Those who had longer before being tested were much more likely to show signs of inferential judgment (75% vs 52%), and for the most distant (and difficult) inferential judgment, the students who had had periods of sleep in between learning and testing significantly outperformed those who hadn’t slept (93% vs 69%). The researchers are interested in exploring whether meditation can provide a similar benefit.

[749] Ellenbogen, J. M., Hu P. T., Payne J. D., Titone D., & Walker M. P.
(2007).  Human relational memory requires time and sleep.
Proceedings of the National Academy of Sciences. 104(18), 7723 - 7728.

More on how memories are consolidated during sleep

A new study sheds more light on how memory is consolidated during sleep. Using a new technique, the research confirms that new information is transferred between the hippocampus and the cerebral cortex, and, unexpectedly, provides evidence suggesting that the cerebral cortex actively controls this transfer.

[834] Hahn, T. T. G., Sakmann B., & Mehta M. R.
(2006).  Phase-locking of hippocampal interneurons' membrane potential to neocortical up-down states.
Nat Neurosci. 9(11), 1359 - 1361.

Still more on how memories are consolidated during sleep

In research following up an earlier study in which rats were shown to form complex memories for sequences of events experienced while they were awake, and that these memories were replayed while they slept, it has been shown that these replayed memories do contain the visual images that were present during the running experience. By showing that the brain is replaying memory events in the visual cortex and in the hippocampus at the same time, the finding suggests that this process may contribute to or reflect the result of the memory consolidation process.

[317] Ji, D., & Wilson M. A.
(2007).  Coordinated memory replay in the visual cortex and hippocampus during sleep.
Nat Neurosci. 10(1), 100 - 107.

Brainwave oscillations responsible for memory benefits of sleep?

Passing a mild electrical current through the brain while students were asleep improved their ability to remember words on waking up. 13 medical students were given 46 pairs of words to learn. Before sleeping, they remembered an average 37.42 words; after sleep, those not given the stimulation remembered an average of 39.5, while those given the stimulation remembered an average of 41.27. The memory enhancement only occurred at a certain frequency and during a particular part of the sleep cycle, confirming the idea that slow oscillations of electrical activity are responsible for the memory consolidation effects of sleep. The benefit also only applied to fact learning; skill learning was not affected.

[238] Marshall, L., Helgadottir H., Molle M., & Born J.
(2006).  Boosting slow oscillations during sleep potentiates memory.
Nature. 444(7119), 610 - 613.,,1940475,00.html

More support that sleep helps consolidate learning

An experiment involving fruitflies has found that those in a social environment with at least 30 other flies slept four times as long during their daytime naps as flies in isolation. There was no difference in night-time sleep. The length of the nap increased with the size of the group they socialized with. Confirming that this effect was due to an increase in social interactions, rather than, for example, physical exhaustion from flying around more, flies deprived of their sight and sense of smell (meaning they could still fly around but could not socialize) showed no difference in daytime sleep patterns. Of 49 genes known to be involved in learning and memory, switching off seventeen (all related to long-term memory) made the flies sleep equally long regardless of whether they were social or not.

[894] Ganguly-Fitzgerald, I., Donlea J., & Shaw P. J.
(2006).  Waking Experience Affects Sleep Need in Drosophila.
Science. 313(5794), 1775 - 1781.

Human study supports value of daytime napping for learning

REM sleep, when most dreaming occurs, has been shown in a number of studies to be important in consolidating procedural (skill) learning, while non-REM (slow-wave) sleep seems to be more important for declarative (knowledge-based) learning. However, because normal sleep contains both REM and non-REM cycles, research hasn’t been able to clearly distinguish the effects. Now a new study using brief daytime napping confirms the role of non-REM sleep for declarative learning. Volunteers who memorized pairs of words and practiced tracing images in a mirror test scored 15% better in the word test if they had been allowed a nap in the six hour period before being tested. However, they did no better at the action test.

[414] Tucker, M. A., Hirota Y., Wamsley E. J., Lau H., Chaklader A., & Fishbein W.
(2006).  A daytime nap containing solely non-REM sleep enhances declarative but not procedural memory.
Neurobiology of Learning and Memory. 86(2), 241 - 247.

Sleep makes memories resistant to interference

It’s pretty clear now that sleep consolidates procedural (skill) learning, but the question of whether or not it helps other types of memory is still very much a matter of debate. However, a new study has found a marked effect of sleep on our ability to remember information. The study involved 60 healthy college-aged adults, who were asked them to memorize 20 pairs of random words. Half were given the words at 9am and tested at 9pm, and the other half were given the words at 9pm and tested at 9am. While the sleepers did perform better (94% recall compared to 82%), it was the introduction of another factor that made the benefits of sleep undeniable. Participants who were given a new set of words to learn just 12 minutes before testing revealed a dramatic difference — sleepers recalled 76% of the original words compared to 32% of the sleepless.

[974] Ellenbogen, J. M., Hulbert J. C., Stickgold R., Dinges D. F., & Thompson-Schill S. L.
(2006).  Interfering with Theories of Sleep and Memory: Sleep, Declarative Memory, and Associative Interference.
Current Biology. 16(13), 1290 - 1294.

Asleep or awake we retain memory

We’ve learned that skill memory is reinforced during sleep, but now new imaging technology reveals that this kind of reinforcement occurs while we’re awake too — even while we’re learning something new.

[475] Peigneux, P., Orban P., Balteau E., Degueldre C., Luxen A., Laureys S., et al.
(2006).  Offline Persistence of Memory-Related Cerebral Activity during Active Wakefulness.
PLoS Biol. 4(4), e100 - e100.

How sleep improves memory

While previous research has been conflicting, it does now seem clear that sleep consolidates learning of motor skills in particular. A new imaging study involving 12 young adults taught a sequence of skilled finger movements has found a dramatic shift in activity pattern when doing the task in those who were allowed to sleep during the 12 hour period before testing. Increased activity was found in the right primary motor cortex, medial prefrontal lobe, hippocampus and left cerebellum — this is assumed to support faster and more accurate motor output. Decreased activity was found in the parietal cortices, the left insular cortex, temporal pole and fronto-polar region — these are assumed to reflect less anxiety and a reduced need for conscious spatial monitoring. It’s suggested that this is one reason why infants need so much sleep — motor skill learning is a high priority at this age. The findings may also have implications for stroke patients and others who have suffered brain injuries.

[670] Walker, M. P., Stickgold R., Alsop D., Gaab N., & Schlaug G.
(2005).  Sleep-dependent motor memory plasticity in the human brain.
Neuroscience. 133(4), 911 - 917.

More evidence that learning is consolidated during sleep

A new study provides more evidence for the role of sleep in the consolidation of long-term memories. In the study, volunteers learned the layout of a virtual town, and were then tested by having to quickly find routes to various locations in the town. Those so trained showed greater activity in their hippocampus and an adjacent learning-related region (compared to those not trained) as they took the route tests, with greater activity correlated with better performance. They also showed greater hippocampal brain activity during sleep. Most importantly, the higher the gain in post-sleep performance on the tests, the higher had been their NREM brain activity during sleep. No such correlation was found in REM brain activity. The findings support the view that spatial memory traces are processed during NREM sleep in humans.

[1182] Aerts, J., Luxen A., Maquet P., Peigneux P., Laureys S., Fuchs S., et al.
(2004).  Are spatial memories strengthened in the human hippocampus during slow wave sleep?.
Neuron. 44(3), 535 - 545.

Mentally, sleep may be as active a state as waking state

Why do we sleep? A question we keep asking. Recent research leads us another step in the road. The study has identified a number of genes upregulated specifically during sleep – at least as many as are turned on while we are awake. These "sleep genes" largely fall into four categories: genes involved in synaptic plasticity (supporting the view that sleep aids memory consolidation); genes underlying translation (supporting observations that protein synthesis increases during sleep); genes regulating membrane and vesicle trafficking; and genes for synthesizing cholesterol (which may be crucial for synapse formation and maintenance, which could, in turn, enhance neural plasticity (the brain's ability to change and learn)). The study also found, to the researchers’ surprise, that the cerebellum showed largely the same pattern of gene-expression during sleep as the cortex.

[1021] Cirelli, C., Gutierrez C. M., & Tononi G.
(2004).  Extensive and divergent effects of sleep and wakefulness on brain gene expression.
Neuron. 41(1), 35 - 43.

More on what goes on during sleep

Brain activity patterns vary during sleep, with particular distinction being made between “REM” sleep and “deep” sleep. Both these phases of sleep have been associated with memory processing. The chemical composition of the brain also varies a great deal in the sleep and wakefulness cycle. New research from Germany now report that some of these differences are crucial in memory formation during sleep. In particular, the level of acetylcholine (a neurotransmitter) is high during wakefulness and REM sleep but drops to the minimum in deep sleep. In an experiment that involved subjects performing two memory tasks – learning 40 pairs of semantically related words, and learning to trace figures seen in a mirror – before sleeping for four hours, it was found that those who were given a cholinesterase inhibitor, (cholinesterase being an enzyme that breaks down acetylcholine), performed significantly less well in the wordlist task on wakening. The mirror-tracing task didn't seem to be affected. This supports the idea that a low level of acetylcholine is necessary for strengthening explicit memory during deep sleep, and fits in with a proposed two-stage model of long-term memory formation, in which the cortex transfers newly acquired experiential data to the hippocampus for processing and temporary storage (a process requiring high levels of acetylcholine), and then, during sleep, the processed memory traces in the hippocampus are relayed back to the cortex for long-term storage. This feedback process is blocked by acetylcholine and, thus, only happens in sleep when the acetylcholine level drops to the minimum.
The research may also have important implications for treating memory loss associated with Alzheimer's disease, as cholinesterase inhibitors are widely used in such treatment. Because of common side-effects of the drug, patients are usually told to take it at night, which may well weaken the drug’s effectiveness.

[999] Gais, S., & Born J.
(2004).  Low acetylcholine during slow-wave sleep is critical for declarative memory consolidation.
Proceedings of the National Academy of Sciences of the United States of America. 101(7), 2140 - 2144.

Now definite? Memories are consolidated during sleep

Researchers of a new study claim that their research finally settles the question of whether or not sleep consolidates new memories. The study involved detailed recording of specific learning- and memory- related areas (hippocampus and forebrain) in the brains of rats. The rats were exposed to four kinds of novel objects. Analysis of brain signals before, during, and after this experience, revealed "reverberations" of distinctive brain wave patterns across all the areas being monitored for up to 48 hours after the novel experience. This pattern was much more prevalent in slow-wave sleep than in REM sleep. Previous studies by the same researchers have found that the activation of genes that affect memory consolidation occurs during REM sleep, not slow-wave sleep. It is proposed that both stages of sleep are important for memory consolidation. Previous studies have tended to focus solely on the hippocampus, and have observed brain activity for a much shorter period.

[793] Ribeiro, S., Gervasoni D., Soares E. S., Zhou Y., Lin S-C., Pantoja J., et al.
(2004).  Long-lasting novelty-induced neuronal reverberation during slow-wave sleep in multiple forebrain areas.
PLoS Biology. 2(1), E24 - E24.
Full text available at

Sleep helps insight

A new German study provides evidence for what we all suspected — “sleeping on” a problem can really work. In the study, participants were given a mathematical puzzle to solve; a puzzle which could be solved by trial-by-trial learning, or almost immediately if participants grasped the hidden rule. After training in the trial-by-trial learning, some of the participants were allowed to sleep through the night, while others were prevented from sleeping. When they returned to the problem eight hours later, those that had slept were twice as likely to realize the rule. Another group that trained in the morning, and were then tested later that day, were also slower at finding the rule, suggesting that the slowness was not solely due to fatigue. Sleep did not, however, help participants who had not had the initial training. It is suggested that sleep can act to restructure new memory representations.

[1382] Wagner, U., Gais S., Haider H., Verleger R., & Born J.
(2004).  Sleep inspires insight.
Nature. 427(6972), 352 - 355.

Stages of memory clarified in sleep studies

Two new studies add to our understanding of the effects of sleep on memory. Both studies involved young adults and procedural (skill) learning, and found temporary declines in performance in particular contexts (a brief description of these studies is given here). 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 surprising aspect to this is the time it appears to take for memories to initially stabilize. The studies also confirm the role of sleep in the consolidation process.

[1027] Fenn, K. M., Nusbaum H. C., & Margoliash D.
(2003).  Consolidation during sleep of perceptual learning of spoken language.
Nature. 425(6958), 614 - 616.

[518] Walker, M. P., Brakefield T., Allan Hobson J., & Stickgold R.
(2003).  Dissociable stages of human memory consolidation and reconsolidation.
Nature. 425(6958), 616 - 620.,9865,1059138,00.html

More support for the theory that sleep is necessary to consolidate memories

A study used fear conditioning in mice to investigate the effect of sleep deprivation on memory. The mice were given a mild electric shock either in a distinctive setting, or subsequent to a tone. Those who experienced the tone continued to freeze when they heard the tone on the following day, whether or not they had been deprived of sleep. Those who associated the environment with the shock, however, were less likely to freeze after sleep deprivation. Mice who had been deprived of sleep during the five hours following training, spent just 4% of their time frozen when returned to the ‘shock environment’ the following day, compared to 15% among mice who were allowed to sleep during this period. The five hours following training was a critical period – those who were deprived of sleep in the 5-10 hours after training showed no sign of memory impairment. The fact that the context association was affected but not the tone cue, suggests that sleep is affecting processes in the hippocampus (important in context memory but not memory for specific facts or events).

[625] Graves, L. A.
(2003).  Sleep Deprivation Selectively Impairs Memory Consolidation for Contextual Fear Conditioning.
Learning & Memory. 10(3), 168 - 176.

Another step in understanding how sleep affects memory

The value of sleep for memory takes a further step in being understood in new rodent research, which found that, as the rodents slept, the thalamus at the base of their brains originated bursts of electrical activity (“sleep spindles”), which were then detected in the somatosensory neocortex. Some 50 msec later, the hippocampus responded with a pulse of electricity (a “ripple”). "This neocortical-hippocampal dialogue may provide a selection mechanism for the time-compressed replay of information learned during the day." It’s suggested that the ripple is the hippocampus sending back neat, compact waves of memory to the neocortex where they are filed away for future reference. Most of this activity took place during slow wave sleep, the stage which makes up the majority of the sleep cycle.

[907] 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(5625), 1578 - 1581.

Napping reverses information overload

Evidence is mounting that sleep helps information processing and learning. A new study has showed that subjects performing a visual task (reporting the horizontal or vertical orientation of three diagonal bars against a background of horizontal bars in the corner of a computer screen) got worse over the course of four daily practice sessions. However, allowing subjects a 30-minute nap after the second session prevented any further deterioration, and a 1-hour nap actually boosted performance in the third and fourth sessions back to morning levels. It appears that the fatigue is limited to the brain visual system circuits involved in the task. When the image was switched to a different right corner of the computer screen on the fourth practice session, subjects performed about as well as they did in the first session -- or after a short nap. Recordings of brain activity reveal that the 1-hour naps contained more than four times as much deep, or slow wave sleep and rapid eye movement (REM) sleep than the half-hour naps.

[758] Mednick, S. C., Nakayama K., Cantero J. L., Atienza M., Levin A. A., Pathak N., et al.
(2002).  The restorative effect of naps on perceptual deterioration.
Nat Neurosci. 5(7), 677 - 681.

Improving motor skills through sleep

People taught a simple motor sequence (to type a sequence of keys on a computer keyboard as quickly and accurately as possible) practised it for 12 minutes and were then re-tested 12 hours later. Those who practised in the morning and tested later that same day improved their performance by about 2%. Those trained in the evening and re-tested after a good night's sleep, however, improved by about 20%. The amount of improvement was directly correlated with the amount of Stage 2 (a stage of non-rapid eye movement or NREM) sleep experienced, particularly late in the night. "This is the part of a good night's sleep that many people will cut short by getting up early in the morning."

[767] Laureys, S., Peigneux P., Perrin F., & Maquet P.
(2002).  Sleep and motor skill learning.
Neuron. 35(1), 5 - 7.

Controversy over sleep's role in memory

Does sleep play a role in memory or not? Two new research papers reach opposite conclusions. One is from Robert Stickgold, who has published several papers supporting the role of sleep in memory consolidation. But the other is a new review of REM sleep studies concluding that REM (rapid eye movement) sleep, or dreaming, plays little role in memory formation, chiefly on the basis that depriving animals and humans of REM sleep by awakening them or by drug treatments does not impair their ability to form long-term memories. In addition, the time spent in REM sleep does not correlate with learning ability across humans, nor is there a positive relation between amount or intensity of REM sleep and learning ability across species.

[987] Stickgold, R., Hobson J. A., Fosse R., & Fosse M.
(2001).  Sleep, Learning, and Dreams: Off-line Memory Reprocessing.
Science. 294(5544), 1052 - 1057.

[1388] Siegel, J. M.
(2001).  The REM sleep-memory consolidation hypothesis.
Science (New York, N.Y.). 294(5544), 1058 - 1063.

New motor skills consolidated during sleep

An imaging study that sheds light on the gain in performance observed during the day after learning a new task. Following training in a motor skill, certain brain areas appear to be reactived during REM sleep, resulting in an optimization of the network that subtends the subject's visuo–motor response.

[775] van der Linden, M., Cleeremans A., Smith C., Maquet P., Laureys S., Peigneux P., et al.
(2001).  Experience-dependent changes in cerebral functional connectivity during human rapid eye movement sleep.
Neuroscience. 105(3), 521 - 525.

Deep "slow wave" sleep necessary to consolidate memories

Sleep is necessary to consolidate memories. Remembering a new task is more difficult if you don't sleep within 30 hours of learning the task. "Catch-up" sleep on subsequent nights doesn't make up for losing that first night's sleep. Moreover, it appears that the deep "slow wave" sleep that occurs in the first half of the night is the type of sleep necessary to consolidate memories. Other types of memory however, may require "REM" sleep (that occurs while you are dreaming).

Stickgold, R., James, L. & Hobson, J.A. 2000. Visual discrimination learning requires sleep after training. Nature Neuroscience,3, 1237-1238.

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Brain flexibility predicts learning speed

June, 2011

New analytic techniques reveal that functional brain networks are more fluid than we thought.

A new perspective on learning comes from a study in which 18 volunteers had to push a series of buttons as fast as possible, developing their skill over three sessions. New analytical techniques were then used to see which regions of the brain were active at the same time. The analysis revealed that those who learned new sequences more quickly in later sessions were those whose brains had displayed more 'flexibility' in the earlier sessions — that is, different areas of the brain linked with different regions at different times.

At this stage, we don’t know how stable an individual’s flexibility is. It may be that individuals vary significantly over the course of time, and if so, this information could be of use in predicting the best time to learn.

But the main point is that the functional modules, the brain networks that are involved in specific tasks, are more fluid than we thought. This finding is in keeping, of course, with the many demonstrations of damage to one region being compensated by new involvement of another region.


[2212] Bassett, D. S., Wymbs N. F., Porter M. A., Mucha P. J., Carlson J. M., & Grafton S. T.
(2011).  Dynamic reconfiguration of human brain networks during learning.
Proceedings of the National Academy of Sciences. 108(18), 7641 - 7646.


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New insight into insight, and the role of the amygdala in memory

April, 2011

A new study suggests that one-off learning (that needs no repetition) occurs because the amygdala, center of emotion in the brain, judges the information valuable.

Most memory research has concerned itself with learning over time, but many memories, of course, become fixed in our mind after only one experience. The mechanism by which we acquire knowledge from single events is not well understood, but a new study sheds some light on it.

The study involved participants being presented with images degraded almost beyond recognition. After a few moments, the original image was revealed, generating an “aha!” type moment. Insight is an experience that is frequently remembered well after a single occurrence. Participants repeated the exercise with dozens of different images.

Memory for these images was tested a week later, when participants were again shown the degraded images, and asked to recall details of the actual image.

Around half the images were remembered. But what’s intriguing is that the initial learning experience took place in a brain scanner, and to the researchers’ surprise, one of the highly active areas during the moment of insight was the amygdala. Moreover, high activity in the amygdala predicted that those images would be remembered a week later.

It seems the more we learn about the amygdala, the further its involvement extends. In this case, it’s suggested that the amygdala signals to other parts of the brain that an event is significant. In other words, it gives a value judgment, decreeing whether an event is worthy of being remembered. Presumably the greater the value, the more effort the brain puts into consolidating the information.

It is not thought, from the images used, that those associated with high activity in the amygdala were more ‘emotional’ than the other images.





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Working memory has more layers than thought

April, 2011

A new study provides further support for a three-tier model of working memory, where the core only holds one item, the next layer holds up to three, and further items can be passively held ready.

Readers of my books and articles will know that working memory is something I get quite excited about. It’s hard to understate the importance of working memory in our lives. Now a new study tells us that working memory is in fact made up of three areas: a core focusing on one active item, a surrounding area holding at least three more active items (called the outer store), and a wider region containing passive items that have been tagged for later retrieval. Moreover, the core region (the “focus of attention”) has three roles (one more than thought) — it not only directs attention to an item and retrieves it, but it also updates it later, if required.

In two experiments, 49 participants were presented with up to four types of colored shapes on a computer screen, with particular types (eg a red square) confined to a particular column. Each colored shape was displayed in sequence at the beginning with a number from 1 to 4, and then instances of the shapes appeared sequentially one by one. The participants’ task was to keep a count of each shape. Different sequences involved only one shape, or two, three, or four shapes. Participants controlled how quickly the shapes appeared.

Unsurprisingly, participants were slower and less accurate as the set size (number of shape types) increased. There was a significant jump in response time when the set-size increased from one to two, and a steady increase in RT and decline in accuracy as set-size increased from 2 to 4. Responses were also notably slower when the stimulus changed and they had to change their focus from one type of shape to another (this is called the switch cost). Moreover, this switch cost increased linearly with set-size, at a rate of about 240ms/item.

Without getting into all the ins and outs of this experiment and the ones leading up to it, what the findings all point to is a picture of working memory in which:

  • the focus contains only one item,
  • the area outside the focus contains up to three items,
  • this outer store has to be searched before the item can be retrieved,
  • more recent items in the outer store are not found any more quickly than older items in the outer store,
  • focus-switch costs increase as a direct function of the number of items in the outer store,
  • there is (as earlier theorized) a third level of working memory, containing passive items, that is quite separate from the two areas of active storage,
  • that the number of passive items does not influence either response time or accuracy for recalling active items.

It is still unclear whether the passive third layer is really a part of working memory, or part of long-term memory.

The findings do point to the need to use active loads rather than passive ones, when conducting experiments that manipulate cognitive load (for example, requiring subjects to frequently update items in working memory, rather than simply hold some items in memory while carrying out another task).



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Sleep reorganizes your memories

December, 2010

New studies show how sleep sculpts your memories, emphasizing what’s important and connecting it to other memories in your brain.

The role of sleep in consolidating memory is now well-established, but recent research suggests that sleep also reorganizes memories, picking out the emotional details and reconfiguring the memories to help you produce new and creative ideas. In an experiment in which participants were shown scenes of negative or neutral objects at either 9am or 9pm and tested 12 hours later, those tested on the same day tended to forget the negative scenes entirely, while those who had a night’s sleep tended to remember the negative objects but not their neutral backgrounds.

Follow-up experiments showed the same selective consolidation of emotional elements to a lesser degree after a 90-minute daytime nap, and to a greater degree after a 24-hour or even several-month delay (as long as sleep directly followed encoding).

These findings suggest that processes that occur during sleep increase the likelihood that our emotional responses to experiences will become central to our memories of them. Moreover, additional nights of sleep may continue to modify the memory.

In a different approach, another recent study has found that when volunteers were taught new words in the evening, then tested immediately, before spending the night in the sleep lab and being retested in the morning, they could remember more words in the morning than they did immediately after learning them, and they could recognize them faster. In comparison, a control group who were trained in the morning and re-tested in the evening showed no such improvement on the second test.

Deep sleep (slow-wave sleep) rather than rapid eye movement (REM) sleep or light sleep appeared to be the important phase for strengthening the new memories. Moreover, those who experienced more sleep spindles overnight were more successful in connecting the new words to the rest of the words in their mental lexicon, suggesting that the new words were communicated from the hippocampus to the neocortex during sleep. Sleep spindles are brief but intense bursts of brain activity that reflect information transfer between the hippocampus and the neocortex.

The findings confirm the role of sleep in reorganizing new memories, and demonstrate the importance of spindle activity in the process.

Taken together, these studies point to sleep being more important to memory than has been thought. The past decade has seen a wealth of studies establishing the role of sleep in consolidating procedural (skill) memory, but these findings demonstrate a deeper, wider, and more ongoing process. The findings also emphasize the malleability of memory, and the extent to which they are constructed (not copied) and reconstructed.



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Distinguishing between working memory and long-term memory

November, 2010

A study with four brain-damaged people challenges the idea that the hippocampus is the hub of spatial and relational processing for short-term as well as long-term memory.

Because people with damage to their hippocampus are sometimes impaired at remembering spatial information even over extremely short periods of time, it has been thought that the hippocampus is crucial for spatial information irrespective of whether the task is a working memory or a long-term memory task. This is in contrast to other types of information. In general, the hippocampus (and related structures in the mediotemporal lobe) is assumed to be involved in long-term memory, not working memory.

However, a new study involving four patients with damage to their mediotemporal lobes, has found that they were perfectly capable of remembering for one second the relative positions of three or fewer objects on a table — but incapable of remembering more. That is, as soon as the limits of working memory were reached, their performance collapsed. It appears, therefore, that there is, indeed, a fundamental distinction between working memory and long-term memory across the board, including the area of spatial information and spatial-objection relations.

The findings also underscore how little working memory is really capable of on its own (although absolutely vital for what it does!) — in real life, long-term memory and working memory work in tandem.



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Individual differences in ability to gauge your own accuracy

October, 2010

Differences in the size and connectivity of a region in the prefrontal cortex underlie how accurate people are in judging their own performance.

Metamemory or metacognition — your ability to monitor your own cognitive processes — is central to efficient and effective learning. Research has also shown that, although we customarily have more faith in person’s judgment the more confident they are in it, a person’s accuracy and their confidence in their accuracy are two quite separate things (which is not to say it’s not a useful heuristic; just that it’s far from infallible). A new study involving 32 participants has looked at individual differences in judging personal accuracy when assessing a geometric image, comparing these differences to differences in the brain.

The perceptual test used simple stimuli, and each one was customized to the individual's level of skill in order to achieve a score of 71%. In keeping with previous research, there was considerable variation in the participants’ accuracy in assessing their own responses. But the intriguing result was that these differences were reflected in differences in the volume of gray matter in the right anterior prefrontal cortex. Moreover, those who were better at judging their own performance not only had more neurons in that region, but also tended to have denser connections between the region and the white matter connected to it. The anterior prefrontal cortex is associated with various executive functions, and seems to be more developed in humans compared to other animals.

The finding should not be taken to indicate a genetic basis for metacognitive ability. The finding implies nothing about whether the physical differences are innate or achieved by training and experience. However it seems likely that, like most skills and abilities, a lot of it is training.



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Link between brain acid and cognition offers hope for an effective ‘smart’ pill

September, 2010

Experiments with mice have found that inhibiting the production of kynurenic acid in the brain has dramatic benefits for cognitive performance.

Commercial use is a long way off, but research with mice offers hope for a ‘smart drug’ that doesn’t have the sort of nasty side-effects that, for example, amphetamines have. The mice, genetically engineered to produce dramatically less (70%) kynurenic acid, had markedly better cognitive abilities. The acid, unusually, is produced not in neurons but in glia, and abnormally high levels are produced in the brains of people with disorders such as schizophrenia, Alzheimer's and Huntington's. More acid is also typically produced as we get older.

The acid is produced in our brains after we’ve eaten food containing the amino acid tryptophan, which helps us produce serotonin (turkey is a food well-known for its high tryptophan levels). But serotonin helps us feel good (low serotonin levels are linked to depression), so the trick is to block the production of kynurenic acid without reducing the levels of serotonin. The next step is therefore to find a chemical that blocks production of the acid in the glia, and can safely be used in humans. Although no human tests have yet been performed, several major pharmaceutical companies are believed to be following up on this research.




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Tools that assess bias in standardized tests are flawed

August, 2010

Study shows the tools used to assess whether mental ability tests are biased couldn’t find bias when it was deliberately inserted, casting the fairness of common tests into doubt.

Manipulation of nearly 16 million individual samples of scores and more than 8 trillion individual scores on commonly used tests, including civil service and other pre-employment exams and university entrance exams, has revealed that the tools used to check tests of "general mental ability" for bias overwhelmingly and repeatedly missed the bias inserted in the data. In other words, we’ve been testing potential test bias with a biased procedure.


[1668] Aguinis, H., Culpepper S. A., & Pierce C. A.
(2010).  Revival of Test Bias Research in Preemployment Testing.
Journal of Applied Psychology. 95(4), 648 - 680.




Connection between navigation, object location, & autobiographical memory

January, 2010

The existence of specialized neurons involved in spatial memory has now been found in humans, and appear to also help with object location and autobiographical memory.

Rodent studies have demonstrated the existence of specialized neurons involved in spatial memory. These ‘grid cells’ represent where an animal is located within its environment, firing in patterns that show up as geometrically regular, triangular grids when plotted on a map of a navigated surface. Now for the first time, evidence for these cells has been found in humans. Moreover, those with the clearest signs of grid cells performed best in a virtual reality spatial memory task, suggesting that the grid cells help us to remember the locations of objects. These cells, located particularly in the entorhinal cortex, are also critical for autobiographical memory, and are amongst the first to be affected by Alzheimer's disease, perhaps explaining why getting lost is one of the most common early symptoms.


[378] Doeller, C. F., Barry C., & Burgess N.
(2010).  Evidence for grid cells in a human memory network.
Nature. 463(7281), 657 - 661.




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