A lire sur: http://www.alnmag.com/news/2013/07/false-memories-implanted-mouse-brains
The phenomenon of false memory has been well-documented: In many court cases, defendants have been found guilty based on testimony from witnesses and victims who were sure of their recollections, but DNA evidence later overturned the conviction.
The phenomenon of false memory has been well-documented: In many court cases, defendants have been found guilty based on testimony from witnesses and victims who were sure of their recollections, but DNA evidence later overturned the conviction.
In a step toward understanding how these faulty memories arise, MIT
neuroscientists have shown that they can plant false memories in the
brains of mice. They also found that many of the neurological traces of
these memories are identical in nature to those of authentic memories.
“Whether it’s a false or genuine memory, the brain’s neural
mechanism underlying the recall of the memory is the same,” says Susumu
Tonegawa, the Picower Professor of Biology and Neuroscience and senior
author of a paper describing the findings in Science.
The study also provides further evidence that memories are stored
in networks of neurons that form memory traces for each experience we
have — a phenomenon that Tonegawa’s lab first demonstrated last year.
Neuroscientists have long sought the location of these memory
traces, also called engrams. In the pair of studies, Tonegawa and
colleagues at MIT’s Picower Institute for Learning and Memory showed
that they could identify the cells that make up part of an engram for a
specific memory and reactivate it using a technology called
optogenetics.
Lead authors of the paper are graduate student Steve Ramirez and
research scientist Xu Liu. Other authors are technical assistant Pei-Ann
Lin, research scientist Junghyup Suh, and postdocs Michele Pignatelli,
Roger Redondo and Tomas Ryan.
Seeking the engram
Episodic memories — memories of experiences — are made of
associations of several elements, including objects, space and time.
These associations are encoded by chemical and physical changes in
neurons, as well as by modifications to the connections between the
neurons.
Where these engrams reside in the brain has been a longstanding
question in neuroscience. “Is the information spread out in various
parts of the brain, or is there a particular area of the brain in which
this type of memory is stored? This has been a very fundamental
question,” Tonegawa says.
In the 1940s, Canadian neurosurgeon Wilder Penfield suggested that
episodic memories are located in the brain’s temporal lobe. When
Penfield electrically stimulated cells in the temporal lobes of patients
who were about to undergo surgery to treat epileptic seizures, the
patients reported that specific memories popped into mind. Later studies
of the amnesiac patient known as “H.M.” confirmed that the temporal
lobe, including the area known as the hippocampus, is critical for
forming episodic memories.
However, these studies did not prove that engrams are actually
stored in the hippocampus, Tonegawa says. To make that case, scientists
needed to show that activating specific groups of hippocampal cells is
sufficient to produce and recall memories.
To achieve that, Tonegawa’s lab turned to optogenetics, a new
technology that allows cells to be selectively turned on or off using
light.
For this pair of studies, the researchers engineered mouse
hippocampal cells to express the gene for channelrhodopsin, a protein
that activates neurons when stimulated by light. They also modified the
gene so that channelrhodopsin would be produced whenever the c-fos gene,
necessary for memory formation, was turned on.
In last year’s study, the researchers conditioned these mice to
fear a particular chamber by delivering a mild electric shock. As this
memory was formed, the c-fos gene was turned on, along with the
engineered channelrhodopsin gene. This way, cells encoding the memory
trace were “labeled” with light-sensitive proteins.
The next day, when the mice were put in a different chamber they
had never seen before, they behaved normally. However, when the
researchers delivered a pulse of light to the hippocampus, stimulating
the memory cells labeled with channelrhodopsin, the mice froze in fear
as the previous day’s memory was reactivated.
“Compared to most studies that treat the brain as a black box while
trying to access it from the outside in, this is like we are trying to
study the brain from the inside out,” Liu says. “The technology we
developed for this study allows us to fine-dissect and even potentially
tinker with the memory process by directly controlling the brain
cells.”
Incepting false memories
That is exactly what the researchers did in the new study —
exploring whether they could use these reactivated engrams to plant
false memories in the mice’s brains.
First, the researchers placed the mice in a novel chamber, A, but
did not deliver any shocks. As the mice explored this chamber, their
memory cells were labeled with channelrhodopsin. The next day, the mice
were placed in a second, very different chamber, B. After a while, the
mice were given a mild foot shock. At the same instant, the researchers
used light to activate the cells encoding the memory of chamber A.
On the third day, the mice were placed back into chamber A, where
they now froze in fear, even though they had never been shocked there. A
false memory had been incepted: The mice feared the memory of chamber A
because when the shock was given in chamber B, they were reliving the
memory of being in chamber A.
Moreover, that false memory appeared to compete with a genuine
memory of chamber B, the researchers found. These mice also froze when
placed in chamber B, but not as much as mice that had received a shock
in chamber B without having the chamber A memory activated.
The researchers then showed that immediately after recall of the
false memory, levels of neural activity were also elevated in the
amygdala, a fear center in the brain that receives memory information
from the hippocampus, just as they are when the mice recall a genuine
memory.
These two papers represent a major step forward in memory research,
says Howard Eichenbaum, a professor of psychology and director of
Boston University’s Center for Memory and Brain.
“They identified a neural network associated with experience in an
environment, attached a fear association with it, then reactivated the
network to show that it supports memory expression. That, to me, shows
for the first time a true functional engram,” says Eichenbaum, who was
not part of the research team.
The MIT team is now planning further studies of how memories can be distorted in the brain.
“Now that we can reactivate and change the contents of memories in
the brain, we can begin asking questions that were once the realm of
philosophy,” Ramirez says. “Are there multiple conditions that lead to
the formation of false memories? Can false memories for both pleasurable
and aversive events be artificially created? What about false memories
for more than just contexts — false memories for objects, food or other
mice? These are the once seemingly sci-fi questions that can now be
experimentally tackled in the lab.”
Source: MIT, Anne Trafton
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