After reflexively reaching out to
grab a hot pan falling from the stove, you may be able to withdraw your hand at
the very last moment to avoid getting burned. That is because the brain's
executive control can step in to break a chain of automatic commands. Several
new lines of evidence suggest that the same may be true when it comes to the
reflex of recollection—and that the brain can halt the spontaneous retrieval of
potentially painful memories. Within the brain, memories sit in a web of
interconnected information. As a result, one memory can trigger another, making
it bubble up to the surface without any conscious effort. When you get a
reminder, the mind's automatic response is to do you a favor by trying to
deliver the thing that's associated with it, neuroscientists say at the
University of Cambridge. But sometimes we are reminded of things we would
rather not think about. Humans are not helpless against this process, however.
Previous imaging studies suggest that the brain's frontal areas can dampen the
activity of the hippocampus, a crucial structure for memory, and therefore
suppress retrieval.
In an effort to learn more, researchers
recently investigated what happens after the hippocampus is suppressed. They
asked 381 college students to learn pairs of loosely related words. Later, the
students were shown one word and asked to recall the other—or to do the
opposite and to actively not think about the other word. Sometimes between
these tasks they were shown unusual images, such as a peacock standing in a
parking lot. They found that the participants' ability to subsequently recall
the peacocks and other strange pictures was about 40 percent lower if they had
been instructed to suppress memories of words before or after seeing the
images, compared with trials in which they had been asked to recall the words.
This provides further evidence that a memory-control mechanism exists and
suggests that trying to actively forget a particular memory can negatively
affect general memory. They call the phenomenon an “amnesic shadow” because it
apparently blocks recollection of unrelated events happening around the time of
decreased hippocampal activity. The results may explain why some people who
have experienced trauma have poor memory of everyday events.
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