Memory and emotion - Thanks for no memory
Nov 13th 2003 From The Economist print edition
Some evidence about how and why memories are suppressed

ACCORDING to Freud's theory of repression, the mind hides memories of
traumatic events in places where they cannot easily be retrieved, in
order to prevent overwhelming anxiety. It is these "repressed
memories" that the memory-recovering techniques beloved of some
psychiatrists aim to unearth.

The existence of repressed memories is taken as a truism by
psychiatry. Unfortunately, it has never been verified by rigorous
scientific experiment. And that is not a matter of mere academic
interest, since memories apparently recovered by psychiatric
techniques such as hypnosis—particularly memories of childhood abuse—
have sometimes been enough to put people in prison, even when there
has not been any corroborating evidence. Moreover, even in cases
where an individual has undoubtedly witnessed something traumatic,
the reliability of his memories can be critical to convicting the
true perpetrator. Witnesses frequently disagree, and this may reflect
the way memory forms. Some actual data on the relationship between
unpleasant experiences and memory would therefore be welcome.


In this week's Proceedings of the National Academy of Sciences Bryan
Strange, of University College, London, and his colleagues provide
some. Rather than abuse their experimental subjects, though, they
merely showed them streams of words on a computer screen.



Totalless recall
Some of these words (murder, massacre and so on) had bad
connotations. Others (meeting, gathering and conference, for example)
were emotionally neutral. The subjects of the experiment, who did not
know in advance what was required of them, were asked to look at the
stream, which was presented one word at a time. Then, when they had
been shown it, they were asked to recall the words in it. In the
past, this technique has showed that emotionally charged words are
more likely to be recalled than neutral ones. What Dr Strange wanted
to look at was how well people remember neutral words adjacent to the
emotionally charged ones in the stream. He discovered that words
immediately preceding emotionally charged ones were less likely to be
remembered than normal.

Intrigued, he pushed a little further. Previous work had established
that emotion-associated enhancement of memory is caused, at least in
part, by the action of stress hormones, in particular norepinephrine,
on a part of the brain called the amygdala. He wondered if a similar
mechanism was at work in the emotion-associated memory loss the team
discovered.

The action of norepinephrine on the amygdala can be blocked by a drug
called propranolol. When the researchers repeated their experiments
on volunteers who had been dosed with this drug, they found, as
expected, that those volunteers did not remember emotional words any
better than neutral ones. In addition, however, they found that
memory for neutral words which preceded emotional ones improved.

The team was also able to draw on evidence from a patient who suffers
from Urbach-Wiethe disease, a rare genetic disorder that can cause
damage to the amygdala. They used brain-imaging techniques to confirm
that her amygdalas (people actually have two, one in each hemisphere
of the brain) were, indeed, damaged. They also measured her cognitive
functions—intelligence, attention and both short-term and long-term
memory—and found that these were normal. But her memory was not
affected by emotion; she remembered emotionally charged and neutral
words equally well, regardless of the order they were presented in.



The memory gap
The kind of memory Dr Strange studied is called explicit memory. It
concerns facts and experiences—knowledge that can be recalled by
conscious effort and can be reported verbally. Researchers believe
that explicit memory is formed in several steps. The first is
translating newly learned information into so-called neural
correlates. This does not involve permanent changes to the brain's
structure. In the second stage, consolidation, structural changes
such as the formation and destruction of connections between nerve
cells take place. This process involves the expression of genes and
the synthesis of new proteins, and Dr Strange suspects that emotion
interferes with these biochemical events. As a result, no memory is
formed.

Another line of evidence that supports this interpretation is work on
post-traumatic stress disorder (PTSD) carried out by Roger Pitman, of
Harvard University. Dr Pitman recently conducted a trial to see if
propranolol could prevent the development of this disorder, which
afflicts those who have been exposed to horrific events, such as
battles or plane crashes, with emotionally disturbing flash-back
memories. He reasoned that excessive amounts of stress hormones
released at the time of a traumatic event might be responsible for
overly strong memory formation. Because memory takes time to form, he
conjectured that drugs which block the action of these hormones soon
after the trauma might decrease the intensity of the memory. This
turned out to be true: a course of propranolol started shortly after
an acute traumatic event was able to reduce the symptoms of PTSD one
month later.

On the face of it, there is something slightly contradictory about
these results. It is odd that the amnesia observed by Dr Strange is
for events just before an emotionally charged incident, when what is
actually desirable is to wipe away any recollection of the incident
itself. But a simple laboratory experiment using what are, after all,
ultimately harmless words, is not the same as a case of child abuse
or the horrors of war. And it seems clear that the amnesia, as well
as the memory formation, is in some way a result of the stress
hormones.

What is undoubtedly true is that memory, like everything else in
biology, is an evolved, functional response. If individuals tend to
be better off by not remembering certain things, natural selection
will tend to construct their brains that way. Indeed, the existence
of post-traumatic stress disorder suggests that individuals are
better off without those memories. And in fact, most people do come
out of trauma with their psyches intact, so it is possible that what
has happened to PTSD sufferers is that the memory-prevention
mechanism has gone wrong.

Freud might thus have been right about the reason for what he thought
he had observed about trauma and memory. But it looks as though he
was wrong about the mechanism. The evidence, though limited at the
moment, suggests that memories are not repressed. Rather, they are
never formed in the first place. Obviously, no psychiatric technique
can recover something that was not there to start with. That is
something of which the courts should be acutely aware when they
assess the credibility of witnesses. It is also something
psychiatrists may care to ponder when they are trying to dredge
up "forgotten" childhood memories.

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