Tuesday, February 22, 2011
There is no longer any doubt. Sleep does improve the gelling or consolidation of memory for recently encoded information. Research is now focusing on how this happens and what other factors interact with the sleep effect. At least two processes seem to be at work: 1) sleep protects new memories from disruption by the interfering experiences that are inevitable during wakefulness, and 2) sleep consolidates memories according to their relative importance and the learner’s expectations for remembering.
A good illustration of reducing interference comes from a study of napping at the University of Lübeck in Germany. The researchers knew about the extensive evidence that in wakefulness, new situations and stimuli can readily prevent new memories from consolidating. This is even true when learned material is recalled, because at that point the memory has to be reconsolidated and is therefore again vulnerable. The authors assumed that similar interference with memory formation could occur even after a sleep interlude.
To test the idea, they asked 24 volunteers to memorize the two-dimensional location of 15 pairs of cards with pictures of animals and everyday objects. During the study time, they were also exposed continuously to a slightly unpleasant odor, which was intended to be an associational cue.
Forty minutes later, the volunteers were asked to learn a second, slightly different set of card pairs. This second task was to act as an interfering disruptor of the initial learning. The difference is that after the first memorization session, half of the group stayed awake and the other half took a nap. For 20 minutes during the break after the first study session, the odor cue was presented with the intent of helping to reactivate the memory of the first session. The awake group got the odor cue for 20 minutes just before starting the second learning session, while the sleep group got the odor cue during the last 20 minutes of the nap (dreaming did not occur, because it normally requires more than 40 minutes of sleep to start appearing).
When both groups were tested for recall of the first set of cards, the sleep group remembered much better (85% correct versus 60% for the awake group). The explanation begins with the knowledge that when temporary memories (as for the first card set) are recalled, they are vulnerable to being destroyed by new mental activity (as with the second card set). In this study, memory was reactivated in both wakefulness and sleep by the odor cue. Yet, the memorization processes that apparently persisted during sleep made the original memories more resistant to disruption. By the time of the second interfering task some 40 minutes later, much of the initial learning had gelled during sleep, but less so during wakefulness.
These authors also performed brain imaging that showed that the nap group had mostly completed a shift in activity from the temporary processing area (in the hippocampus) to storage areas in the cortex. This was not true for the awake group. You might say that sleep enabled the information to be “uploaded from RAM to the hard drive” better than in the constant awake condition. Of course this computer metaphor breaks down in other respects. Biological memory is dynamic, readily degraded over time or changed by new experience. Also, recall of biological memory launches a reconstructive process whereby the memory can be reinforced or drastically altered.
The practical application, as I see it, is to take a short nap as soon as possible after trying to memorize something really important. For example, during a study session for a school exam, take a nap right away so that it has a better chance to consolidate than if you stayed awake and got exposed to many new interfering situations and stimuli.
Two new studies shed some light on prioritization of memory formation during sleep. We all have had the experience of improved memory if we know others expect us to remember. I guess such improvement occurs because we work harder at it, using more intensive rehearsal and perhaps using deliberate association strategies..But we now find out from a recent study that the sleep effect on improving memory formation benefits from the relevance of the learned information. Since sleep usually occurs significantly later than the learning and original encoding, this effect must arise from the consolidation process during sleep.
A recent study from this same German research lab has revealed that sleep helps memory formation the most if you know you will need the information later. That is, it seems that the brain prioritizes its consolidation operations during sleep to favor consolidation of information that is most important. The study tested 193 volunteers for recall of a variety of memory tasks. Some subjects were exposed to the learning material early in the day, when there would be no sleep involved. The others were exposed to the same material late, just before the night’s sleep. When subjects were told they would be tested later, they were more likely to remember if they had slept immediately after the learning. This was true for both procedural tasks (like finger-tapping sequences) or declarative tasks such as word matching or stating card-pair locations. Moreover, subjects who were told they would be tested later spent more total time in the deepest stage of Sleep (Stage IV) than did comparable subjects who were not told they would be tested later. Presumably, the brain is using Stage IV to accomplish this differential consolidation process.
In a recent study from a French group, the study focus was on sleep’s apparent ability to prioritize memory formation based on prior instructions to remember or forget items in a learning task. In the learning task, volunteers were shown 100 French words, one at a time. Fifty of these had accompanying instruction “to be remembered” and the other 50 “to be forgotten,” presented in a pseudorandom sequence that prevented more than three words of the same type being presented consecutively. After the training session, subjects were divided into two groups, one which was sent home to continue their normal activities and to sleep on their usual schedule for the next three nights. The other group was denied the first night’s sleep after training, where they stayed up all that night watching movies or playing games. Otherwise, this group was treated the same. On the fourth day, both groups were tested for recall with presentation the 100 of the original words and 100 new ones to serve as distracters. The task was to identify which words were in the original list.
Questionnaires revealed any strategies the subjects used in trying to remember “to be remembered” words and trying to ignore “to be forgotten” words. No subject intensively rehearsed the original items during the three-day interval, but of course casual rehearsal was going on. Generally, subjects made associations of “to be remembered” words with memories of personal events or with short stories or sentences. Mental images were much less used. Of course, no such rehearsals occurred with “to be forgotten” words.
Upon testing, both groups had about the same degree of correct recall for “to be remembered” words. But the sleep-deprived groups remembered more of the words they were not supposed “to be forgotten.” Thus, it would seem that during sleep, the brain preserved its ability to remember words that were expected to be remembered and discriminated against remembering words that were unimportant. Recall that the instructions to remember or forget were given at the time of initial encoding. Thus, the brain must have preserved these instructions and followed them in the consolidation process during sleep. Though the authors did not mention it, the poor ability of sleep-deprived subjects to discriminate between the two categories of words could have arisen because being awake for a whole day after learning interfered with remembering and following instructions at the time of encoding.
Diekelmann, S., Büchel, Born, J., and Rasch, Björn. 2011. Labile or stable: opposing consequences for memory when reactivated during wakefulness and sleep. Nature Neuroscience. Jan. 23. doi: 10.1038/nn.2744
Rauchs, G. et al. 2011. Sleep contributes to the strengthening of some memories over others, depending on hippocampal activity at learning. J. Neuroscience. 31 (7): 2563-2568.
Wilhelm, I. et al. 2011. Sleep selectively enhances memory expected to be of future relevance. J. Neuroscience. 31 (5): 1563-1569.
Thursday, February 03, 2011
Are you as smart as a rat? A rat can learn a lot of things with just the first attempt. For example, in the old days, exterminators used to use poisons such as strychnine. What they discovered was that some rats who ate the bait were never killed. If they survived the seizures of the first exposure, they learned not to eat that particular bait again. It’s called “bait shyness.” As a result, exterminators now use a different poison, Warfarin, that does not kill right away. The rat slowly bleeds to death over many days and does not realize any connection between eating that bait and getting sick. The explanation for the difference is illustrated in Figure 1.
Figure 1. One-try learning by rats being fed poison. In the top example, using strychnine, the rat gets sick soon after eating the bait. If it survives, it remembers an association between eating that particular kind of food and getting very sick, and it won’t eat the poison ever again. If, however, it takes a while to become sick, as with the anti-coagulant Warfarin, the rat is unable to make a connection between being sick and eating the bait. So, it keeps eating the bait every time it gets hungry.
Similar one-try learning has been demonstrated in two common types of experiments. In one type, the test apparatus is a large box, the floor of which is an electrified grid. In the middle of the floor, there is a safe-island platform that is not electrified. When an untrained rat is placed on this platform, it immediately runs to the walls, because rats feel vulnerable out in open spaces. But of course, the rat gets a learning experience of having its feet shocked. If you take the rat out, put it back in the home cage and re-test it the next day, the rat stays on the safe platform. Despite its natural inclination, it stays on the platform because it learned —in just one try —not to step off that platform.
Another example is a water maze. Put a rat or mouse in a tub of water and it swims desperately about hoping to find some escape. If there is a platform at one end it can climb up on, the rat, once it sees it, swims immediately to the platform and climbs up on it. If there is some kind of indicator of where the platform is, such as a light above it, and you fill the tub with a milky liquid where the platform cannot be seen, an untrained rat swims around until it accidentally finds the platform. Take the rat out, put it in the home cage, and re-test the next day, and the rat swims immediately to where the light and the safe platform is. Learning has occurred in just one try.
In all such learning situations there is one huge caveat. That deals with what happens immediately after the one-try learning. If some new learning situation occurs at that time, the learning will be disrupted and not formed into a lasting memory. For example, the memory will not form if in the foot shock or swim maze case immediately after the learning the investigator puts the rat in another learning situation or even just some distracting situation, such as putting the rat into a cage with strange rats rather than returning it to the home cage. Re-testing the next day will indicate that the rat never learned. Actually, it just forgot, because new stimuli immediately after learning interfere with forming a lasting memory.
This is the most common explanation of failure for humans to remember new learned events. After all learning events, a certain amount of uninterrupted time is needed to “consolidate” the short-term memory into a more lasting one.
Now consider how the rats might learn these things in one try. They have no language. The must surely rely on what they see. That is they must be making an association with something they see out in space: a certain kind of food that made them sick, a grid of bars that shocked their feet, a light cue showing where a safe platform was located. So, objects and where they are in space are powerful memory aids.
Memory gimmicks often use some kind of mapping technique, such as associating what you want to remember with location of objects in a room. I have discussed these in my book, Thank You Brain for All You Remember. Now I have a new and better image-mapping technique for one-try learning. I include it free in a revision of my e-book for students, Better Grades, Less Effort. The technique can be applied to most anything, is easy to use, and the maps are adjustable for any number of objects or ideas to be remembered. Moreover, sequential ordering is built-in. You can get the ebook for only $2.49 in all formats from Smashwords.com (http://www.smashwords.com/books/view/24623).