You Must Not Remember This

By Greg Miller
ScienceNOW Daily News
9 January 2008

The ability to suppress distracting or distressing memories helps people cope with everyday life, yet neuroscientists know little about how it works. Now researchers have gained some clues from a study that combined hypnosis and functional magnetic resonance imaging (fMRI) to investigate changes in brain activity as volunteers suppressed--and later recalled--memories of a recently viewed movie.

Some people can be made to suppress a particular memory by hypnotic suggestion, an effect called posthypnotic amnesia. Hoping to take advantage of this phenomenon, neuroscientist Yadin Dudai of the Weizmann Institute of Science in Rehovot, Israel, and colleagues had subjects watch a movie depicting a young woman going about her daily routine--making meals, talking on the phone, rollerblading with friends, and so on. A week later, the volunteers returned to the lab and, under hypnosis, were instructed to forget the movie until they heard the phrase "Now you can remember everything."

As the researchers had hoped, the hypnosis triggered memory suppression. After the subjects woke up, they took a quiz about the activities of the woman in the movie. They performed no better than chance, answering only half of the yes-no questions correctly. Immediately afterward, the volunteers heard the magic phrase and took the quiz again. This time they averaged about 80%26#37; correct, the same as a control group that wasn't susceptible to posthypnotic amnesia.

The fMRI scans, collected as subjects answered questions about the movie, revealed what was happening in the brain, the team reports in the 10 January issue of Neuron. Several regions, mostly in the occipital and temporal lobes, were unusually quiet when subjects suppressed memories. In contrast, activity in the rostrolateral prefrontal cortex, which has been implicated in memory retrieval (Science, 13 July 2007, p. 215), was elevated during memory suppression. Dudai hypothesizes that this region may have vetoed retrieval of the movie memories in the volunteers.

"It's a clever idea ? and an intriguing result," says Michael Anderson, a cognitive neuroscientist at the University of St. Andrews in Fife, U.K. He likes Dudai's team's hypothesis that the rostrolateral prefrontal cortex cuts off memory retrieval but adds that more work will be needed to clarify this region's role. Understanding the mechanisms of memory suppression is important, Anderson says, because it's a crucial part of maintaining emotional equilibrium. "We're often confronted with reminders of things we'd rather not think about."

Related site

A 2004 Science article about memory suppression