miércoles, 15 de septiembre de 2010

Why some memores stick

Why some memories stick
Repetitive neural responses may enhance recall of faces and words.

Janelle Weaver

Faces that activate the same regions of the brain again and again are more likely to be remembered.
Pasieka / Science Photo Library

Practice makes perfect when it comes to remembering things, but exactly how that works has long been a mystery. A study published in Science this week1 indicates that reactivating neural patterns over and over again may etch items into the memory.

People find it easier to recall things if material is presented repeatedly at well-spaced intervals rather than all at once. For example, you're more likely to remember a face that you've seen on multiple occasions over a few days than one that you've seen once in one long period. One reason that a face linked to many different contexts — such as school, work and home — is easier to recognize than one that is associated with just one setting, such as a party, could be that there are multiple ways to access the memory. This idea, called the encoding variability hypothesis, was proposed by psychologists about 40 years ago2.

Each different context or setting activates a distinct set of brain regions; the hypothesis suggests that it is these differing neural responses that improve the memory. But neuroimaging research led by Russell Poldrack, a cognitive neuroscientist at the University of Texas, Austin, now suggests that the opposite is true — items are better remembered when they activate the same neural patterns with each exposure.

Neural rehearsal
Poldrack's team measured brain activity in 24 people using functional magnetic resonance imaging (fMRI). The subjects saw 120 unfamiliar faces, each one repeated four times at varying intervals during the fMRI scan. One hour later, they were shown the faces again, mixed with 120 new ones, and asked to rate the familiarity of each.

The researchers then looked at the brain responses that had been recorded when the subjects were first shown the faces, focusing on 20 brain regions associated with visual perception and memory. Faces that were later recognized evoked similar activation patterns at each repetition in nine of the regions, particularly those associated with object and face perception; faces that were later forgotten did not evoke such pattern to the same extent.

In a separate experiment, subjects in the fMRI scanner were shown 180 words, each repeated three times. Six hours later, they performed two memory tests. The remembered words elicited similar patterns at each repetition in 15 of the 20 brain regions that the researchers examined.

Explaining the brain
But Marvin Chun, a cognitive neuroscientist at Yale University in New Haven, Connecticut, says that the results do not invalidate the encoding variability hypothesis because Poldrack and his team were at a different type of situation. To directly test the hypothesis, the authors should have presented items in different contexts, he says.

What's more, attention-grabbing words or faces may elicit more reproducible patterns of activation when they are presented multiple times than do less striking items, says Rik Henson, a cognitive neuroscientist at the MRC Cognition and Brain Sciences Unit in Cambridge, UK. This effect could explain the results without refuting the encoding variability hypothesis, he adds.

"We can't rule that out," Poldrack says. To address this concern, he would have to further analyse subjects' brain responses to individual items. "It may well be the case that there is a version of the encoding variability hypothesis that is compatible with these data."

"If we push the theorists to think a little harder, and to try to incorporate neuroscience data into these theories, then I think that is a good thing, regardless of whether the encoding variability theory turns out to be right," he adds.

a.. References
1.. Xue, G. et al. Science doi:10.1126/science.1193125 (2010).
2.. Martin, E. Psych. Rev. 75, 421-441 (1968). | Article
Source: Nature

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