consolidation

Why learning gets harder as we get older

February, 2013

A mouse study shows that weakening unwanted or out-of-date connections is as important as making new connections, and that neurological changes as we age reduces our ability to weaken old connections.

A new study adds more support to the idea that the increasing difficulty in learning new information and skills that most of us experience as we age is not down to any difficulty in acquiring new information, but rests on the interference from all the old information.

Memory is about strengthening some connections and weakening others. A vital player in this process of synaptic plasticity is the NMDA receptor in the hippocampus. This glutamate receptor has two subunits (NR2A and NR2B), whose ratio changes as the brain develops. Children have higher ratios of NR2B, which lengthens the time neurons talk to each other, enabling them to make stronger connections, thus optimizing learning. After puberty, the ratio shifts, so there is more NR2A.

Of course, there are many other changes in the aging brain, so it’s been difficult to disentangle the effects of this changing ratio from other changes. This new study genetically modified mice to have more NR2A and less NR2B (reflecting the ratio typical of older humans), thus avoiding the other confounds.

To the researchers’ surprise, the mice were found to be still good at making strong connections (‘long-term potentiation’ - LTP), but instead had an impaired ability to weaken existing connections (‘long-term depression’ - LTD). This produces too much noise (bear in mind that each neuron averages 3,000 potential points of contact (i.e., synapses), and you will see the importance of turning down the noise!).

Interestingly, LTD responses were only abolished within a particular frequency range (3-5 Hz), and didn’t affect 1Hz-induced LTD (or 100Hz-induced LTP). Moreover, while the mice showed impaired long-term learning, their short-term memory was unaffected. The researchers suggest that these particular LTD responses are critical for ‘post-learning information sculpting’, which they suggest is a step (hitherto unknown) in the consolidation process. This step, they postulate, involves modifying the new information to fit in with existing networks of knowledge.

Previous work by these researchers has found that mice genetically modified to have an excess of NR2B became ‘super-learners’. Until now, the emphasis in learning and memory has always been on long-term potentiation, and the role (if any) of long-term depression has been much less clear. These results point to the importance of both these processes in sculpting learning and memory.

The findings also seem to fit in with the idea that a major cause of age-related cognitive decline is the failure to inhibit unwanted information, and confirm the importance of keeping your mind actively engaged and learning, because this ratio is also affected by experience.

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Dopamine decline underlies episodic memory decline in old age

December, 2012

Findings supporting dopamine’s role in long-term episodic memory point to a decline in dopamine levels as part of the reason for cognitive decline in old age, and perhaps in Alzheimer’s.

The neurotransmitter dopamine is found throughout the brain and has been implicated in a number of cognitive processes, including memory. It is well-known, of course, that Parkinson's disease is characterized by low levels of dopamine, and is treated by raising dopamine levels.

A new study of older adults has now demonstrated the effect of dopamine on episodic memory. In the study, participants (aged 65-75) were shown black and white photos of indoor scenes and landscapes. The subsequent recognition test presented them with these photos mixed in with new ones, and required them to note which photos they had seen before. Half of the participants were first given Levodopa (‘L-dopa’), and half a placebo.

Recognition tests were given two and six hours after being shown the photos. There was no difference between the groups at the two-hour test, but at the six-hour test, those given L-dopa recognized up to 20% more photos than controls.

The failure to find a difference at the two-hour test was expected, if dopamine’s role is to help strengthen the memory code for long-term storage, which occurs after 4-6 hours.

Individual differences indicated that the ratio between the amount of Levodopa taken and body weight is key for an optimally effective dose.

The findings therefore suggest that at least part of the reason for the decline in episodic memory typically seen in older adults is caused by declining levels of dopamine.

Given that episodic memory is one of the first and greatest types of memory hit by Alzheimer’s, this finding also has implications for Alzheimer’s treatment.

Caffeine improves recognition of positive words

Another recent study also demonstrates, rather more obliquely, the benefits of dopamine. In this study, 200 mg of caffeine (equivalent to 2-3 cups of coffee), taken 30 minutes earlier by healthy young adults, was found to improve recognition of positive words, but had no effect on the processing of emotionally neutral or negative words. Positive words are consistently processed faster and more accurately than negative and neutral words.

Because caffeine is linked to an increase in dopamine transmission (an indirect effect, stemming from caffeine’s inhibitory effect on adenosine receptors), the researchers suggest that this effect of caffeine on positive words demonstrates that the processing advantage enjoyed by positive words is driven by the involvement of the dopaminergic system.

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Rest briefly after learning

October, 2012

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.

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Sleep learning making a comeback?

August, 2012

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.

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Quick therapy may reduce post-traumatic stress when delivered immediately

July, 2012

A pilot study supports the value of brief cognitive therapy for victims of traumatic events, when delivered as soon as possible after the event. The benefit appears greatest for sexual assault victims.

A new study has found that, when delivered quickly, a modified form of prolonged exposure therapy reduces post-traumatic stress reactions and depression.

The study involved 137 patients being treated in the emergency room of a major trauma center in Atlanta. The patients were chosen from survivors of traumatic events such as rape, car or industrial accidents, and shooting or knife attacks. Participants were randomly assigned to either receive three sessions of therapy beginning in the emergency department (an average of 12 hours after the event), or assessment only. Stress reactions were assessed at 4 and 12 weeks, and depression at baseline and 4 weeks.

Those receiving the therapy reported significantly lower post-traumatic stress at 4 weeks and 12 weeks, and significantly lower depression at 4 weeks. Analysis of subgroups revealed that the therapy was most effective in rape victims. In the cases of transport accidents and physical (non-sexual) assault, the difference between therapy and assessment-only was only barely significant (for transport at 4 weeks) or non-significant. In both subgroups, the effect was decidedly less at 12 weeks than at 4 weeks.

The therapy, carried out by trained therapists, involved participants describing the trauma they had experienced while the therapist recorded the description. The bulk of the hour-long session was taken up with reliving and processing the experience. There were three sessions spaced a week apart. The patients were instructed to listen to their recordings every day, and 85% were compliant. The therapists also explained normal reactions to trauma, helped the patients look at obtrusive thoughts of guilt or responsibility, and taught them a brief breathing or relaxation technique and self care.

While this study doesn’t itself compare the effects of immediate vs delayed therapy, the assumption that delivering the therapy so soon after the trauma is a crucial factor in its success is in line with other research (mainly to do with fear-conditioning in rodent and human laboratory studies). Moreover, while brief cognitive-behavioral therapy has previously been shown to be effective with people diagnosed with acute stress disorder, such therapy is normally begun some 2-4 weeks after trauma, and a study of female assault survivors found that although such therapy did indeed accelerate recovery compared with supportive counseling, after 9 months, PTSD severity was similar in both groups.

Another, severe, limitation of this study is that the therapy involved multiple items. We cannot assume that it was the repeated re-experiencing of the event that is critical.

However, this study is only a pilot study, and its findings are instructive rather than decisive. But at the least it does support the idea that immediate therapy is likely to help victims of trauma recover more quickly.

One final, important, note: It should not, of course, be assumed that simply having the victim describe the events — say to police officers — is in itself therapeutic. Done badly, that experience may itself be traumatic.

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Sleeping after learning is most effective

May, 2012

A new sleep study confirms the value of running through new material just before bedtime, particularly it seems when that material is being learned using mnemonics or by rote.

We know that we remember more 12 hours after learning if we have slept during that 12 hours rather than been awake throughout, but is this because sleep is actively helping us remember, or because being awake makes it harder to remember (because of interference and over-writing from other experiences). A new study aimed to disentangle these effects.

In the study, 207 students were randomly assigned to study 40 related or unrelated word pairs at 9 a.m. or 9 p.m., returning for testing either 30 minutes, 12 hours or 24 hours later.

As expected, at the 12-hour retest, those who had had a night’s sleep (Evening group) remembered more than those who had spent the 12 hours awake (Morning group). But this result was because memory for unrelated word pairs had deteriorated badly during 12 hours of wakefulness; performance on the related pairs was the same for the two groups. Performance on the related and unrelated pairs was the same for those who slept.

For those tested at 24 hours (participants from both groups having received both a full night of sleep and a full day of wakefulness), those in the Evening group (who had slept before experiencing a full day’s wakefulness) remembered significantly more than the Morning group. Specifically, the Evening group showed a very slight improvement over training, while the Morning group showed a pronounced deterioration.

This time, both groups showed a difference for related versus unrelated pairs: the Evening group showed some deterioration for unrelated pairs and a slightly larger improvement for related pairs; the Morning group showed a very small deterioration for related pairs and a much greater one for unrelated pairs. The difference between recall of related pairs and recall of unrelated pairs was, however, about the same for both groups.

In other words, unrelated pairs are just that much harder to learn than related ones (which we already know) — over time, learning them just before sleep vs learning early in the day doesn’t make any difference to that essential truth. But the former strategy will produce better learning for both types of information.

A comparison of the 12-hour and 24-hour results (this is the bit that will help us disentangle the effects of sleep and wakefulness) reveals that twice as much forgetting of unrelated pairs occurred during wakefulness in the first 12 hours, compared to wakefulness in the second 12 hours (after sleep), and 3.4 times more forgetting of related pairs (although this didn’t reach significance, the amount of forgetting being so much smaller).

In other words, sleep appears to slow the rate of forgetting that will occur when you are next awake; it stabilizes and thus protects the memories. But the amount of forgetting that occurred during sleep was the same for both word types, and the same whether that sleep occurred in the first 12 hours or the second.

Participants in the Morning and Evening groups took a similar number of training trials to reach criterion (60% correct), and there was no difference in the time it took to learn unrelated compared to related word pairs.

It’s worth noting that there was no difference between the two groups, or for the type of word pair, at the 30-minutes test either. In other words, your ability to remember something shortly after learning it is not a good guide for whether you have learned it ‘properly’, i.e., as an enduring memory.

The study tells us that the different types of information are differentially affected by wakefulness, that is, perhaps, they are more easily interfered with. This is encouraging, because semantically related information is far more common than unrelated information! But this may well serve as a reminder that integrating new material — making sure it is well understood and embedded into your existing database — is vital for effective learning.

The findings also confirm earlier evidence that running through any information (or skills) you want to learn just before going to bed is a good idea — and this is especially true if you are trying to learn information that is more arbitrary or less well understood (i.e., the sort of information for which you are likely to use mnemonic strategies, or, horror of horrors, rote repetition).

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Sleep disturbances hurt memory consolidation

May, 2012

Even minor disturbances during sleep, such as that experienced by those with mild apnea, interfere with memory consolidation, and thus learning.

Now that we’ve pretty much established that sleep is crucial for consolidating memory, the next question is how much sleep we need.

A new study compared motor sequence learning in 16 people with mild obstructive sleep apnea to a matched control group (also attending the sleep clinic). There were no significant differences between the groups in total sleep time, sleep efficiency and sleep architecture (time spent in the various sleep stages), subjective measures of sleepiness, or performance on a psychomotor vigilance task (a task highly sensitive to sleep deprivation).

Nor were there any differences in learning performance during the training phase on the motor task.

But the interesting thing about consolidation is that skills usually improve overnight — your performance the next day will usually be better than it was at the end of your training. And here there was a significant difference between the groups, with the controls showing much greater overnight improvement on the motor sequence task. For sequences learned in the morning and tested 12 hours later on the same day, however, there were no differences between the groups.

So given all the factors relating to sleep that were the same between the two groups, what was the factor behind the group consolidation difference? It turns out it was (principally) the arousal index (arousals were scored on the basis of abrupt shifts in EEG frequency that last at least 3 seconds with 10 seconds of stable sleep preceding), and to a lesser extent the apnea-hypopnea index.

It seems likely, then, that arousals from sleep may (depending, presumably, on timing) interrupt the transfer of labile memories from the hippocampus to the neocortex for long-term storage. Thus, the more arousals you have, the more likely it is that this process will be interrupted.

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Sleep preserves your feelings about traumatic events

January, 2012

New research suggests that sleeping within a few hours of a disturbing event keeps your emotional response to the event strong.

Previous research has shown that negative objects and events are preferentially consolidated in sleep — if you experience them in the evening, you are more likely to remember them than more neutral objects or events, but if you experience them in the morning, they are not more likely to be remembered than other memories (see collected sleep reports). However, more recent studies have failed to find this. A new study also fails to find such preferential consolidation, but does find that our emotional reaction to traumatic or disturbing events can be greatly reduced if we stay awake afterward.

Being unable to sleep after such events is of course a common response — these findings indicate there’s good reason for it, and we should go along with it rather than fighting it.

The study involved 106 young adults rating pictures on a sad-happy scale and their own responses on an excited-calm scale. Twelve hours later, they were given a recognition test: noting pictures they had seen earlier from a mix of new and old pictures. They also rated all the pictures on the two scales. There were four groups: 41 participants saw the first set late in the day and the second set 12 hours later on the following day (‘sleep group’); 41 saw the first set early and the second set 12 hours later on the same day; 12 participants saw both sets in the evening, with only 45 minutes between the sets; 12 participants saw both sets in the morning (these last two groups were to rule out circadian effects). 25 of the sleep group had their brain activity monitored while they slept.

The sleep group performed significantly better on the recognition test than the same-day group. Negative pictures were remembered better than neutral ones. However, unlike earlier studies, the sleep group didn’t preferentially remember negative pictures more than the same-day group.

But, interestingly, the sleep group was more likely to maintain the strength of initial negative responses. The same-day group showed a weaker response to negative scenes on the second showing.

It’s been theorized that late-night REM sleep is critical for emotional memory consolidation. However, this study found no significant relationship between the amount of time spent in REM sleep and recognition memory, nor was there any relationship between other sleep stages and memory. There was one significant result: those who had more REM sleep in the third quarter of the night showed the least reduction of emotional response to the negative pictures.

There were no significant circadian effects, but it’s worth noting that even the 45 minute gap between the sets was sufficient to weaken the negative effect of negative scenes.

While there was a trend toward a gender effect, it didn’t reach statistical significance, and there were no significant interactions between gender and group or emotional value.

The findings suggest that the effects of sleep on memory and emotion may be independent.

The findings also contradict previous studies showing preferential consolidation of emotional memories during sleep, but are consistent with two other recent studies that have also failed to find this. At this stage, all we can say is that there may be certain conditions in which this occurs (or doesn’t occur), but more research is needed to determine what these conditions are. Bear in mind that there is no doubt that sleep helps consolidate memories; we are talking here only about emphasizing negative memories at the expense of emotionally-neutral ones.

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[2672] Baran, B., Pace-Schott E. F., Ericson C., & Spencer R. M. C.
(2012).  Processing of Emotional Reactivity and Emotional Memory over Sleep.
The Journal of Neuroscience. 32(3), 1035 - 1042.

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What transient amnesia tells us about autobiographical memory and brain plasticity

November, 2011
  • Brain scans of those suffering from transient global amnesia indicate a permanent role of the hippocampus in autobiographical memory, and demonstrate the brain’s ability to self-repair.

When a middle-aged woman loses her memory after sex, it naturally makes the headlines. Many might equate this sort of headline to “Man marries alien”, but this is an example of a rare condition — temporary, you will be relieved to hear — known as transient global amnesia. Such abrupt, localized loss of autobiographical memory is usually preceded by strenuous physical activity or stressful events. It generally occurs in middle-aged or older adults, but has been known to occur in younger people. In those cases, there may be a history of migraine or head trauma.

Following an earlier study in which 29 of 41 TGA patients were found to have small lesions in the CA1 region of the hippocampus, scanning of another 16 TGA patients has revealed 14 had these same lesions. It seems likely that all the patients had such lesions, but because they are very small and don’t last long, they’re easy to miss. The lesion is best seen after 24-72 hours, but is gone after 5-6 days.

At the start of one of these attacks, memory for the first 30 years of life was significantly impaired, but still much better than memory for the years after that. There was a clear temporal gradient, with memory increasingly worse for events closer in time. There was no difference between events in the previous year and events in the previous five years, but a clear jump at that five-year point.

The exact location of the lesions was significant: when the lesion was in the anterior part of the region, memory for recent events was more impaired.

The hippocampus is known to be crucially involved in episodic memory (memory for events), and an integral part of the network for autobiographical memory. In recent years, it has come to be thought that such memories are only hosted temporarily by the hippocampus, and over a few years come to be permanently lodged in the neocortex (the standard consolidation model). Evidence from a number of studies of this change at the five-year mark has been taken as support for this theory. According to this, then, older memories should be safe from hippocampal damage.

An opposing theory, however, is that the hippocampus continues to be involved in such memories, with both the neocortex and the hippocampus involved in putting together reconsolidated memories (the multiple trace model). According to this model, each retrieval of an episodic memory creates a new version in the hippocampus. The more versions, the better protected a memory will be from any damage to the hippocampus.

The findings from this study show that while there is indeed a significant difference between older and more recent memories, the CA1 region of the hippocampus continues to be crucial for retrieving older memories, and for our sense of self-continuity.

Interestingly, some studies have also found a difference between the left and right hemispheres, with the right hippocampus showing a temporal gradient and the left hippocampus showing constant activation across all time periods. Such a hemisphere difference was not found in the present study. The researchers suggest that the reason may lie in the age of the participants (average age was 68), reflecting a reduction in hemispheric asymmetry with age.

There’s another message in this study. In these cases of TGA, memory function is restored within 24 hours (and generally sooner, within 6-10 hours). This shows how fast the brain can repair damage. Similarly, the fact that such tiny lesions have temporary effects so much more dramatic than the more lasting effects of larger lesions, is also a tribute to the plasticity of the brain.

The findings are consistent with findings of a preferential degeneration of CA1 neurons in the early stages of Alzheimer's disease, and suggest a target for treatment.

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Testing

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

The importance of retrieval cues

An imaging study has revealed that it is retrieval cues that trigger activity in the hippocampus, rather than, as often argued, the strength of the memory. The study involved participants learning unrelated word pairs (a process which included making up sentences with the words), then being asked whether various familiar words had been previously seen or not — the words being shown first on their own, and then with their paired cue word. Brain activity for words judged familiar on their own was compared with activity for the same items when shown with context cues. Increased hippocampal activity occurred only with cued recall. Moreover, the amount of activity was not associated with familiarity strength, and recollected items were associated with greater activity relative to highly familiar items.
[1102] Cohn, M., Moscovitch M., Lahat A., & McAndrews M P.
(2009).  Recollection versus strength as the primary determinant of hippocampal engagement at retrieval.
Proceedings of the National Academy of Sciences. 106(52), 22451 - 22455.
http://www.eurekalert.org/pub_releases/2009-12/uot-dik120709.php

Making student self-testing an effective study tool

A series of four experiments with 150 college students using Swahili-English vocabulary words has revealed that repeated retrieval was a very effective learning strategy. However, when subjects were given control over their own learning, they did not attempt retrieval as early or as often as they should to promote the best learning. The findings are thought to reflect a powerful metacognitive illusion that occurs during self-regulated learning — namely, that easy retrieval tends to make students believe they have “learned” it before the material is properly mastered, leading to premature termination of the study practice.
[285] Karpicke, J. D.
(2009).  Metacognitive Control and Strategy Selection: Deciding to Practice Retrieval During Learning.
Journal of Experimental Psychology: General. 138(4), 469 - 486.
http://www.eurekalert.org/pub_releases/2009-12/pu-sse121009.php

Longer high-stakes tests may result in a sense of mental fatigue, but not in lower test scores

A study involving 239 freshman college students who took three different versions of the SAT Reasoning Test, of progressively longer lengths (3.5, 4.5, 5.5 hours), has revealed that although the students reported higher levels of mental fatigue with longer tests, performance was not affected. In fact, the average performance for both the standard and long tests was significantly higher than for the short test. Moreover, the fatigue experienced was less related to the length of the exam (and to the amount of sleep they’d had) than it was to personality traits. Those with higher levels of achievement motivation and competitiveness felt less fatigue, and those with higher levels of neuroticism and anxiety felt more.
[1127] Ackerman, P. L., & Kanfer R.
(2009).  Test length and cognitive fatigue: an empirical examination of effects on performance and test-taker reactions.
Journal of Experimental Psychology. Applied. 15(2), 163 - 181.
Full text available at http://www.apa.org/journals/releases/xap152-ackerman-kanfer.pdf
http://www.eurekalert.org/pub_releases/2009-06/apa-lht052809.php

Why we don't always learn from our mistakes

A study of the tip-of-the-tongue (TOT) phenomenon suggests that most errors are repeated because the very act of making a mistake, despite receiving correction, constitutes the learning of that mistake. The study asked students to retrieve words after being given a definition. If that produced a TOT state, they were randomly assigned to spend either 10 or 30 seconds trying to retrieve the answer before finally being shown it. When tested two days later, it was found that they tended to TOT on the same words as before, and were especially more likely to do so if they had spent a longer time trying to retrieve them The longer time in the error state appears to reinforce that incorrect pattern of brain activation that caused the error.
[225] Warriner, A B., & Humphreys K. R.
(2008).  Learning to fail: reoccurring tip-of-the-tongue states.
Quarterly Journal of Experimental Psychology (2006). 61(4), 535 - 542.
http://www.physorg.com/news126265455.html

Testing strengthens recall whether something's on the test or not

The simple act of taking a test appears to help you remember everything you learned, even if it isn't tested. In a series of three experiments, researchers found undergraduates tested after being given 25 minutes to study a long article about the toucan bird recalled more a day later than those given further information about the toucan in an extra study session, or those who had neither experience. In the second experiment, students were given two articles to read, one of which was tested and one of which was not. Again, the one tested was remembered significantly better a day later. The third experiment revealed that later recall was better the more time the student had spent on answering questions in the first test. This relation was especially pronounced for students with lower performance on the test, and those who were encouraged to guess did significantly better on the second test than students who were discouraged from guessing.
[1188] Chan, J. C. K., McDermott III K. B., & Roediger H. L.
(2006).  Retrieval-Induced Facilitation: Initially Nontested Material Can Benefit From Prior Testing of Related Material.
Journal of Experimental Psychology: General. 135(4), 553 - 571.
http://www.eurekalert.org/pub_releases/2006-11/apa-tsr110606.php

Repeated test-taking better for retention than repeated studying

A study indicates that testing can be a powerful means for improving learning, not just assessing it. The study compared students who studied a prose passage for about five minutes and then took either one or three immediate free-recall tests, receiving no feedback on the accuracy of answers, with students who received no tests, but were allowed another five minutes to restudy the passage each time their counterparts were involved in a testing session. While the study-only group performed better on the test after the last session, they performed worse when tested 2 days later, and dramatically worse after one week. Note that the study-only group had read the passage about 14 times in total, while the repeated testing group had read the passage only 3.4 times in its one-and-only study session. It also appears that students who rely on repeated study alone often come away with a false sense of confidence about their mastery of the material.
[272] Roediger, H. L., & Karpicke J. D.
(2006).  Test-enhanced learning: taking memory tests improves long-term retention.
Psychological Science: A Journal of the American Psychological Society / APS. 17(3), 249 - 255.
http://www.eurekalert.org/pub_releases/2006-03/wuis-rtb030606.php

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