encoding

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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More evidence that older adults become less able to ignore distraction

December, 2010

A new study adds to the evidence that our ability to focus on one thing and ignore irrelevant information gets worse with age, and that this may be a crucial factor in age-related cognitive impairment.

A study involving young (average age 22) and older adults (average age 77) showed participants pictures of overlapping faces and places (houses and buildings) and asked them to identify the gender of the person. While the young adults showed activity in the brain region for processing faces (fusiform face area) but not in the brain region for processing places (parahippocampal place area), both regions were active in the older adults. Additionally, on a surprise memory test 10 minutes later, older adults who showed greater activation in the place area were more likely to recognize what face was originally paired with what house.

These findings confirm earlier research showing that older adults become less capable of ignoring irrelevant information, and shows that this distracting information doesn’t merely interfere with what you’re trying to attend to, but is encoded in memory along with that information.

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How we can control individual neurons

November, 2010

Every moment a multitude of stimuli compete for our attention. Just how this competition is resolved, and how we control it, is not known. But a new study adds to our understanding.

Following on from earlier studies that found individual neurons were associated with very specific memories (such as a particular person), new research has shown that we can actually regulate the activity of specific neurons, increasing the firing rate of some while decreasing the rate of others.

The study involved 12 patients implanted with deep electrodes for intractable epilepsy. On the basis of each individual’s interests, four images were selected for each patient. Each of these images was associated with the firing of specific neurons in the mediotemporal lobe. The firing of these neurons was hooked up to a computer, allowing the patients to make their particular images appear by thinking of them. When another image appeared on top of the image as a distraction, creating a composite image, patients were asked to focus on their particular image, brightening the target image while the distractor image faded. The patients were successful 70% of the time in brightening their target image. This was primarily associated with increased firing of the specific neurons associated with that image.

I should emphasize that the use of a composite image meant that the participants had to rely on a mental representation rather than the sensory stimuli, at least initially. Moreover, when the feedback given was fake — that is, the patients’ efforts were no longer linked to the behavior of the image on the screen — success rates fell dramatically, demonstrating that their success was due to a conscious, directed action.

Different patients used different strategies to focus their attention. While some simply thought of the picture, others repeated the name of the image out loud or focused their gaze on a particular aspect of the image.

Resolving the competition of multiple internal and external stimuli is a process which involves a number of different levels and regions, but these findings help us understand at least some of the process that is under our conscious control. It would be interesting to know more about the relative effectiveness of the different strategies people used, but this was not the focus of the study. It would also be very interesting to compare effectiveness at this task across age, but of course this procedure is invasive and can only be used in special cases.

The study offers hope for building better brain-machine interfaces.

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Electrical stimulation improves name recall, math skill

November, 2010

Studies involving gentle electrical stimulation to the scalp confirm crucial brain regions and demonstrate improved learning for specific knowledge.

In a study involving 15 young adults, a very small electrical current delivered to the scalp above the right anterior temporal lobe significantly improved their memory for the names of famous people (by 11%). Memory for famous landmarks was not affected. The findings support the idea that the anterior temporal lobes are critically involved in the retrieval of people's names.

A follow-up study is currently investigating whether transcranial direct current stimulation (tDCS) will likewise improve name memory in older adults — indeed, because their level of recall is likely to be lower, it is hoped that the procedure will have a greater effect. If so, the next question is whether repeating tDCS may lead to longer lasting improvement. The procedure may offer hope for rehabilitation for stroke or other neurological damage.

This idea receives support from another recent study, in which 15 students spent six days learning a series of unfamiliar symbols that corresponded to the numbers zero to nine, and also had daily sessions of (tDCS). Five students were given 20 minutes of stimulation above the right parietal lobe; five had 20 minutes of stimulation above the left parietal lobe, and five experienced only 30 seconds of stimulation — too short to induce any permanent changes.

The students were tested on the new number system at the end of each day. After four days, those who had experienced current to the right parietal lobe performed as well as they would be expected to do with normal numbers. However, those who had experienced the stimulation to the left parietal lobe performed significantly worse. The control students performed at a level between the two other groups.

Most excitingly, when the students were tested six months later, they performed at the same level, indicating the stimulation had a durable effect. However, it should be noted that the effects were small and highly variable, and were limited to the new number system. While it may be that one day this sort of approach will be of benefit to those with dyscalculia, more research is needed.

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Learning how to hear shapes

November, 2010

Researchers trained blindfolded people to recognize shapes through coded sounds, demonstrating the abstract nature of perception.

We can see shapes and we can feel them, but we can’t hear a shape. However, in a dramatic demonstration of just how flexible our brain is, researchers have devised a way of coding spatial relations in terms of sound properties such as frequency, and trained blindfolded people to recognize shapes by their sounds. They could then match what they heard to shapes they felt. Furthermore, they were able to generalize from their training to novel shapes.

The findings not only offer new possibilities for helping blind people, but also emphasize that sensory representations simply require systematic coding of some kind. This provides more evidence for the hypothesis that our perception of a coherent object ultimately occurs at an abstract level beyond the sensory input modes in which it is presented.

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[1921] Kim, J-K., & Zatorre R. J.
(2010).  Can you hear shapes you touch?.
Experimental Brain Research. 202(4), 747 - 754.

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One reason for practice tests to improve memory

November, 2010

Why does testing improve memory? A new study suggests one reason is that testing supports the use of more effective encoding strategies.

In an experiment to investigate why testing might improve learning, 118 students were given 48 English-Swahili translation pairs. An initial study trialwas followed by three blocks of practice trials. For one group, the practice trial involved a cued recall test followed by restudy. For the other group, they weren’t tested, but were simply presented with the information again (restudy-only). On both study and restudy trials, participants created keywords to help them remember the association. Presumably the 48 word pairs were chosen to make this relatively easy (the example given in the paper is the easy one of wingu-cloud). A final test was given one week later. In this final test, participants received either the cue only (e.g. wingu), or the cue plus keyword, or the cue plus a prompt to remember their keyword.

The group that were tested on their practice trials performed almost three times better on the final test compared to those given restudy only (providing more evidence for the thesis that testing improves learning). Supporting the hypothesis that this has to do with having more effective keywords, keywords were remembered on the cue+prompt trials more often for the test-restudy group than the restudy-only group (51% vs 34%). Moreover, providing the keywords on the final test significantly improved recall for the restudy-only group, but not the test-restudy group (the implication being that they didn’t need the help of having the keywords provided).

The researchers suggest that practice tests lead learners to develop better keywords, both by increasing the strength of the keywords and by encouraging people to change keywords that aren’t working well.

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[1929] Pyc, M. A., & Rawson K. A.
(2010).  Why Testing Improves Memory: Mediator Effectiveness Hypothesis.
Science. 330(6002), 335 - 335.

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Face-blindness an example of inability to generalize

October, 2010

It seems that prosopagnosia can be, along with perfect pitch and eidetic memory, an example of what happens when your brain can’t abstract the core concept.

‘Face-blindness’ — prosopagnosia — is a condition I find fascinating, perhaps because I myself have a touch of it (it’s now recognized that this condition represents the end of a continuum rather than being an either/or proposition). The intriguing thing about this inability to recognize faces is that, in its extreme form, it can nevertheless exist side-by-side with quite normal recognition of other objects.

Prosopagnosia that is not the result of brain damage often runs in families, and a study of three family members with this condition has revealed that in some cases at least, the inability to remember faces has to do with failing to form a mental representation that abstracts the essence of the face, sans context. That is, despite being fully able to read facial expressions, attractiveness and gender from the face (indeed one of the family members is an artist who has no trouble portraying fully detailed faces), they couldn’t cope with changes in lighting conditions and viewing angles.

I’m reminded of the phenomenon of perfect pitch, which is characterized by an inability to generalize across acoustically similar tones, so an A in a different key is a completely different note. Interestingly, like prosopagnosia, perfect pitch is now thought to be more common than has been thought (recognition of it is of course limited by the fact that some musical expertise is generally needed to reveal it). This inability to abstract or generalize is also a phenomenon of eidetic memory, and I have spoken before of the perils of this.

(Note: A fascinating account of what it is like to be face-blind, from a person with the condition, can be found at: http://www.choisser.com/faceblind/)

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People learn better when brain activity is consistent

October, 2010

A new way of analyzing brain activity has revealed that memories are stronger when the pattern of brain activity is more closely matched on each repetition.

An intriguing new study has found that people are more likely to remember specific information if the pattern of activity in their brain is similar each time they study that information. The findings are said to challenge the long-held belief that people retain information more effectively when they study it several times under different contexts, thus giving their brains multiple cues to remember it. However, although I believe this finding adds to our understanding of how to study effectively, I don’t think it challenges the multiple-context evidence.

The finding was possible because of a new approach to studying brain activity, which was used in three experiments involving students at Beijing Normal University. In the first, 24 participants were shown 120 faces, each one shown four times, at variable intervals between the repetitions. They were tested on their recognition (using a set of 240 faces), and how confident they were in their decision, one hour later. Subsequent voxel-by-voxel analysis of 20 brain regions revealed that the similarity of the patterns of brain activity in nine of those regions for each repetition of a specific face was significantly associated with recognition.

In the second experiment, 22 participants carried out a semantic judgment task on 180 familiar words (deciding whether they were concrete or abstract). Each word was repeated three times, again at variable intervals. The participants were tested on their recall of the words six hours later, and then tested for recognition. Fifteen brain regions showed a higher level of pattern similarity across repetitions for recalled items, but not for forgotten items.

In the third experiment, 22 participants performed a different semantic judgment task (living vs non-living) on 60 words. To prevent further encoding, they were also required to perform a visual orientation judgment task for 8 seconds after each semantic judgment. They were given a recall test 30 minutes after the session. Seven of the brain regions showed a significantly higher level of pattern similarity for recalled items.

It's interesting to observe how differences in the pattern of activity occurred when studying the same information only minutes apart — a difference that is presumed to be triggered by context (anything from the previous item to environmental stimuli or passing thoughts). Why do I suggest that this finding, which emphasizes the importance of same-context, doesn’t challenge the evidence for multiple-context? I think it’s an issue of scope.

The finding shows us two important things: that context changes constantly; that repetition is made stronger the closer context is matched. Nevertheless, this study doesn’t bear on the question of long-term recall. The argument has never been that multiple contexts make a memory trace stronger; it has been that it provides more paths to recall — something that becomes of increasing importance the longer the time between encoding and recall.

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Change in our understanding of memory development

September, 2010

Children’s slowly developing memory for past events may not be due to the slow development of the prefrontal cortex, as was thought, but to changes in the hippocampus.

Children’s ability to remember past events improves as they get older. This has been thought by many to be due to the slow development of the prefrontal cortex. But now brain scans from 60 children (8-year-olds, 10- to 11-year-olds, and 14-year-olds) and 20 young adults have revealed marked developmental differences in the activity of the mediotemporal lobe.

The study involved the participants looking at a series of pictures (while in the scanner), and answering a different question about the image, depending on whether it was drawn in red or green ink. Later they were shown the pictures again, in black ink and mixed with new ones. They were asked whether they had seen them before and whether they had been red or green.

While the adolescents and adults selectively engaged regions of the hippocampus and posterior parahippocampal gyrus to recall event details, the younger children did not, with the 8-year-olds indiscriminately using these regions for both detail recollection and item recognition, and the 10- to 11-year-olds showing inconsistent activation. It seems that the hippocampus and posterior parahippocampal gyrus become increasingly specialized for remembering events, and these changes may partly account for long-term memory improvements during childhood.

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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.

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[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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