Brain May Be Able to Suppress Memories

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Participants performed this "think/don't think" exercise 12 times for each picture.

Finally, at the end of the experiment, they were tested on whether they could still remember all the matched pairs -- something they had been able to do with ease before.

The result: Participants had much more trouble remembering paired photos they had tried not to think of in the second phase of the experiment. They had no trouble recalling the other photo match-ups, however.

"So, when individuals invoke this suppressive control, the items are recalled less frequently," Depue concluded. "It looks like individuals have gained some control over the [memories], and they actually are suppressed," he said.

How might the brain do this? According to fMRI brain images recorded during the think/don't think task, "what seems to occur is that the prefrontal cortex -- what neuroscientists consider the center of cognitive control -- becomes more active in the case of suppression," Depue said.

At the same time, "you see deactivation in areas of the visual cortex, which is normally active when someone is remembering a picture," he said.

Two other brain areas central to memory and emotion -- the hippocampus and the amygdala -- were also quieted down during the "don't think" test. "Those two areas are interconnected greatly and have to do with memorizing and retrieving emotional memories," Depue explained.

He said decisions made in the prefrontal cortex appear to be "modulating" the more primitive areas of the brain, regions that would normally leap to retrieve a memory.

"I liken it to meditation," Depue said. "If you meditate, you continually practice to clear your mind from thoughts. It could be that when you are meditating, you are using part of this specific mechanism to do that."

Another brain expert was intrigued by the findings.

"It looks like they are confirming that humans can actively suppress memory," said Paul Sanberg, director of the Center for Excellence for Aging and Brain Repair at the University of South Florida College of Medicine, in Tampa.