Bad Memories Turned to Happy Ones in Mice Brains
Bad Memories Turned to Happy Ones in Mice Brains
Memories are often
associated with emotions, and these feelings can change through new
experiences and over time. Now, using light, scientists have been able
to manipulate mice brain cells and turn the animals' fearful memories
into happy ones, according to a new study.
Memories are
encoded in groups of neurons that are activated together or in specific
patterns, but it is thought that neurons in different brain regions
encode different aspects of a memory
of an event. For example, the place where an event occurred and the
emotion associated with it may be stored in different places.
In
the new study, researchers examined whether it is possible to
selectively change one part of a memory — the emotion attached to it.
They made male mice form fearful memories by giving them painful
electrical shocks, or form pleasant memories by letting the animals
interact with female mice. [Why You Forget: 5 Strange Facts About Memory]
Later, using light to control the activity of neurons (a method called
optogenetics), the researchers evoked the fearful memories every time
the mice went to a certain corner of their cage, which led the mice to
avoid that corner. In mice that had formed pleasant memories, the
researchers used those memories to make a certain corner look attractive
to the rodents.
In the last
step, to reverse the associations between a place and an emotion, the
researchers evoked only the "place" part of the fearful memories, while
letting the mice interact with female counterparts. As a result, the
mice were no longer afraid of that specific corner of the cage.
The researchers were also able to do the reverse, and turn positive
memories to fearful ones, according to the study published today (Aug.
27) in the journal Nature.Shaping memory fragments
It is well-known that memories are subject to change, and may even get slightly rewritten each time we recall them during a new experience, studies suggest.
However, scientists do not entirely understand the brain mechanisms
that enable memories to change and that even allow us to feel different
emotions about those memories. Elucidating these mechanisms could help
scientists one day develop new therapies for conditions such as
depression and post-traumatic stress disorder.
In the new study, the researchers looked at neurons in a brain
structure called the hippocampus, which is thought to encode the context
of memories, such as where an event happened. The researchers also
looked at neurons in another brain structure, the amygdala, which is
believed to encode emotions.
The mice in the study were genetically engineered to make tracking their
memories easier. As the animals' fearful or pleasant memories were
formed, a light-sensitive protein was expressed in the neurons that
encoded the new memories. This way, the researchers were able to tag
these neurons, and later use light to reactivate the memory those brain
cells held.
The new
experiment worked because the scientists tackled the contextual and
emotional aspects of the memory separately. When the researchers
activated the neurons in the hippocampus,
it evoked the contextual part of the memory, while new events the mouse
was experiencing rewrote the emotional part of the memory. This led to a
new memory of the same place but with a different emotional
association, the researchers said.
Looking at the cells under the microscope, the researchers confirmed
that the relationship between the memory-holding neurons in the
hippocampus and those in the amygdala was altered after the scientists'
manipulations, suggesting that the connection between the two brain
regions is indeed malleable.
The new experiments followed previous studies by the same researchers last year, in which they implanted false memories in mice.
In those studies, the researchers activated neurons to make mice
remember a previous experience as the animals were undergoing a new and
different experience. This made mice form a memory of the mixture of the
two experiences, which in real life had never happened.
Email Bahar Gholipour or follow her @alterwired. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.
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