When the mind rests, memories become stronger, NYU study suggests

Fri Jan 29, 7:52 PM
By Anne-Marie Tobin, The Canadian Press

TORONTO - When you learn something new and want to commit it to memory, it's often suggested that you take a little time to let it sink in.

This advice assumes new meaning in an intriguing study that scrutinized the brain before, during and after a task, and discovered interesting levels of activity in certain areas of the brain in a rest phase after the task. The researchers found that more brain activity during rest predicted better performance in a subsequent memory test.

"We wanted to look to see whether there were times of relative rest during the day that would allow your brain to sort of replay your recent experiences, and if that occurred, whether that would actually have any impact on memory," says Lila Davachi, an assistant professor of psychology and neural science at New York University, and senior author of the paper published this week in the journal Neuron.

By way of background, she explains that as memories get older, they become consolidated - stronger and more resistant to being forgotten.

"It's thought to be due to the fact they become more distributed in the brain so that instead of one part of the brain representing that memory, making it relatively sensitive and weak to changes, the memories over time become distributed and supported by multiple regions of the brain," she says.

"There's very good evidence that this kind of redistribution or consolidation of memory happens during sleep."

She and her team decided to look at the possible role of rest during the day.

The study began with baseline brain scans of 16 people aged 22 to 34 in a functional MRI. There were scans as they went through tasks that involved looking at images of faces, objects and scenes, and further scans during rest periods.

"Over a 20-minute period they performed an active task where we were showing them pictures of faces and objects and they were actively having to incorporate the two visual stimuli in each trial and make a response," Davachi says in an interview from New York.

"After that we simply told them to lay awake and rest in the scanner for another eight minutes, and that was our critical scan."

The researchers focused on brain regions that are believed to play a role in memory.

"What we found during the rest scan after the task is that the hippocampus was interacting more closely with the cortex than what we saw before the task," Davachi says.

"And it was interacting with the part of the cortex that we know was active during the task ... So in other words this part of the cortex - part of the visual cortex - was responding on every trial to these faces and objects. And it looked like after the task was done, as subjects were resting, that activity seemed to still exist during the rest scan."

There were differences among individuals, however.

Some people had the hippocampus communicating very well with the cortex during the rest scans, and some less so.

All the subjects performed a memory test after being in the scanner, even though they hadn't been informed that this would be expected of them.

The researchers found that the magnitude of the hippocampal-cortical correlations during the rest phase predicted later memory performance. In other words, those with the most activity in those brain areas during the rest phase remembered the most about the earlier visual stimuli.

"That's sort of the measure that we're reporting that links these correlations with memory - that they actually predict memory performance on a later memory test," says Davachi.

"I can't explain why there is variation, but the variation allows us to conclude that there is a relationship between the correlations and memory."

In a post-study questionnaire, no subjects reported they had been thinking about any of the preceding stimuli during the rest scans.

Morris Moscovitch, a senior scientist at Toronto's Rotman Research Institute at Baycrest, says the results are interesting and exciting.

"What they've shown is really quite terrific," says Moscovitch, who's also a professor of psychology at the University of Toronto.

"What they've shown is that you form connections between various regions while you're encoding. The correlations between them during the rest period, not only do they increase, but they also predict how good your memory is going to be for objects and faces."

He says the findings go along with the idea that if you learn something you should probably take a little time afterward to let it sink in.

"The simple lesson I guess to learn is that after you've had a period of intense learning, try not to now start learning something else intensely again."

Although the questionnaire responses indicated people weren't thinking about the images during their rest phase, it's possible that the test subjects were rehearsing, either consciously or unconsciously, he adds.

Davachi notes that more study is needed. For instance, it's not known whether lying in a bed on a scanner in a dark room with eyes open is equivalent to the sort of rest one might get during a coffee break.

"It's possible that any given subject in different situations may or may not exhibit those correlations."

But the data suggest that moments of relative rest during the day - taking a walk, having a coffee break - might allow the brain to perform an active process that may encourage memory formation, she says.

"This multi-tasking environment that we live in - I think for me the most important issue that brings up is basically what are we doing to our brains in this environment? Are we not allowing the brain to sort of take its time to consolidate what we want to remember?" she asks.

"This is kind of a gift, to offer the possibility that actually taking a break may be something you can do that will enhance memory."

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