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 While lab rodents were used in the research, it adds to growing
evidence that the malleability of memory might be exploited to treat
disorders such as post-traumatic stress.
In both studies, scientists focused on a phenomenon called
reconsolidation. Discovered in the 1990s, it refers to the fact that
when a memory is retrieved, its physical manifestation in the brain
is so "labile," or changeable, that it can be altered. False
memories can form, and the associated emotions can flip.
"Recalling a memory is not like playing a tape recorder," said
Susumu Tonegawa of the Massachusetts Institute of Technology, who
led one of the studies. "It's a creative process."
The MIT team decided to see how creative. They gave male mice a
small electric shock when the animals wandered into one part of a
cage, creating a memory linking that place to pain. In a different
part, mice got to cavort with females, so they remembered that spot
quite fondly.
 The mice had been engineered so specific brain neurons could be
activated with light, a technique called optogenetics. Using lasers,
the scientists reactivated the where, what and when of the memories,
which are encoded in the hippocampus.
While the shock memory was active and labile, the mice got to play
with females. While the memory of socializing was active, they got a
shock.
That changed brain wiring, the scientists reported in the journal
Nature. The memory of the shock became physically connected to
neurons encoding pleasure; the memory of socializing connected to
neurons encoding fear.
"We could switch the mouse's memory from positive emotions to
negative, and negative to positive," Tonegawa told reporters.
More research is needed before anything similar could be used in
people, MIT's Roger Redondo said, "but the circuits appear to be
very similar" as in mice.
In a separate study, researchers at Boston's McLean Hospital also
exploited the malleability of reactivated memories to erase them
completely.
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After training rats that a flash of light precedes a shock, the
researchers turned on the light, reactivating the memory. They
immediately gave the animals xenon gas, an anesthetic that blocks
molecules involved in memory formation.
That apparently jammed the machinery needed for memory
reconsolidation, psychologist Edward Meloni and colleagues reported
in the journal PLOS One: The rats forgot that light precedes a
shock. Similarly trained rats not given xenon remembered just fine.
Psychologist Elizabeth Phelps of New York University called both
studies "interesting advances."
But clear ethical issues involved in manipulating human memory
remain, even for therapeutic purposes.
"I think we are still a long way from translating this research to
good clinical interventions," since memories that contribute to PTSD
are "likely much more complex" than in mice and rats, Phelps said.
(Reporting by Sharon Begley; editing by Gunna Dickson)
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