New principle guides memory dynamics
August 22, 2003
Weizmann Institute finding may lead to new treatments for
psychological trauma
Rehovot, Israel -- Is it possible to intentionally forget specific
memories, without affecting other memories? Many would undoubtedly be happy
to learn that unpleasant memories might be erased. This ability could be
especially significant when it comes to the kind of traumatic memories that
are debilitating to those experiencing them. It may well be that in the
future, we will be able to wipe out, or at least dim, certain types of
memories with controlled accuracy. A new fundamental rule governing the
workings of the brain, recently discovered by a team of scientists in the
Weizmann Institute of Science, headed by Prof. Yadin Dudai of the
Neurobiology Department, constitutes a step towards reaching this goal.
Every memory that we acquire undergoes a "ripening" process (called
consolidation) immediately after it is formed. In this process, it becomes
impervious to outside stimulation or drugs that would obliterate it. Until
recently, the accepted dogma was that for each separate item of memory,
consolidation occurs just once, after which the time window that allows for
"memory erasing" closes (usually about an hour or two after the memory is
acquired).
However, evidence has lately come to light that a memory is open to
disruption for a short period following each time this memory is recalled.
If this is true, it means that it would be possible to recall a memory and,
immediately after the act of remembering, to activate a "memory eraser" and
wipe it out, even though years may have passed since the original memory was
formed.
Research into the subject took place in leading labs around the world,
but the results were indecisive, as in some cases it was found possible to
erase old memories upon recall, while in others no evidence for this was
found.
Prof. Dudai's group have now identified a new principle guiding the
activity of the brain's memory systems, which sheds light on how memories
are recalled and stabilized, and which can explain the puzzling
discrepancies in the findings.
This principle delineates the conditions in which the recalled memory
becomes re-sensitized to the activity of the "memory erasers." In order to
understand the rule, think of the bits of information stored in our
memories, each with many associations, some of which conflict with others.
For instance, a certain food can bring up memories of taste delicious or
disagreeable; a person can be remembered in pleasant or unpleasant contexts,
and so on.
When we next taste the food or see the person, all of the associated
memories are called up in the blink of an eye, but in the end, only one of
those memories will dictate our reaction (e.g. become dominant.) This memory
will decide whether we will eat the food or reject it, or whether we will
smile at our acquaintance or ignore him.
Prof. Dudai's team found that only that recalled memory that won the
competition for dominance was re-exposed to the time window of sensitivity
to memory erasers, and it is this memory that must be consolidated once
again before being reinstalled in the long-term memory.
In other words, the winner, in the appropriate circumstances, may lose
all. Put succinctly, one can say the stability of the recalled memory is
inversely correlated with its dominance. This discovery is likely to assist
in the future in developing new methods of wiping out unwanted memories, and
thus of treating some kinds of psychological trauma.
Research that deals with the physical basis of the processes and
mechanisms of memory, especially those that involve chemical or other
intervention, relies on animal subjects. Prof. Dudai and his team carried
out their research with rats and fish, which are especially suited for this
type of research. The rats learned to remember flavors; the fish learned to
remember flashes of light, and in both instances, the animals were trained
to associate them with conflicting memories. That is, the tastes were
sometimes good and sometimes bad, and the light sometimes signaled danger
and sometimes didn't.
In both species, it was possible to show that the dominant memory that
which won out over other associated memories and determined subsequent
behavior was the only one that could be erased by giving the appropriate
drug within a few minutes of the memory's recall. The fact that the closer
we get to the "basic hardware" of memory, the more similarities exist
between different animals, including humans, paves the way to the
possibility that certain drugs found to be effective in eliminating memories
in animals will also work on humans. Studies on humans, however, are yet to
be conducted.
The results of the study were published today in the scientific journal
Science. Other than Prof. Dudai, participating in the study were research
students Mark Eisenberg, Tali Kobilo, and Diego Berman.
Prof. Yadin Dudai's research is supported by: Abe and Kathryn Selsky
Foundation; Nella and Leon Benoziyo Center for Neurosciences; Lester Crown
Brain Research Fund; Abramson Family Brain Research Program; Carl and
Michaela Einhorn-Dominic Brain Research Institute; and Murray H. & Meyer
Grodetsky Center for Research of Higher Brain Functions.
Prof. Dudai holds the Sara and Michael Sela Professorial Chair of
Neurobiology.
Weizmann Institute
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