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%
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."

Source: Science
http://sciencenow.sciencemag.org/cgi/content/full/2008/109/5?etoc