Optical fibres implanted in a mouse's brain activated memory forming cells
False memories have been implanted into mice, scientists say. A team was able to make the mice wrongly associate a benign
environment with a previous unpleasant experience from different
surroundings.The researchers conditioned a network of neurons to respond to light, making the mice recall the unpleasant environment.Reporting in Science, they say it could one day shed light into how false memories occur in humans.
The brains of genetically engineered mice were implanted with
optic fibres in order to deliver pulses of light to their brain. Known
as optogenetics, this technique is able to make individual neurons
respond to light.
Unreliable memory
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Dr Xu Liu Massachusetts Institute of TechnologyOur memory changes every single time it's being recorded. That's why we can incorporate new information into old memories and this is how a false memory can form...”
Just like in mice, our memories
are stored in collections of cells, and when events are recalled we
reconstruct parts of these cells - almost like re-assembling small
pieces of a puzzle.
It has been well documented that human memory is highly
unreliable, first highlighted by a study on eyewitness testimonies in
the 70s. Simple changes in how a question was asked could influence the
memory a witness had of an event such as a car crash.
When this was brought to public attention, eyewitness
testimonies alone were no longer used as evidence in court. Many people
wrongly convicted on memory statements were later exonerated by DNA
evidence.
Xu Liu of the RIKEN-MIT Center for Neural Circuit Genetics
and one the lead authors of the study, said that when mice recalled a
false memory, it was indistinguishable from the real memory in the way
it drove a fear response in the memory forming cells of a mouse's brain.
How a memory was implanted in a mouse
- A mouse was put in one environment (blue box) and the brain cells encoding memory were labelled in this environment (white circles)
- These cells were then made responsive to light
- The animal was placed in a different environment (the red box) and light was delivered into the brain to activate these labelled cells
- This induced the recall of the first environment - the blue box. While the animal was recalling the first environment, they also received mild foot shocks
- Later when the mouse was put back into the first environment, it showed behavioural signs of fear, indicating it had formed a false fear memory for the first environment, where it was never shocked in reality
The mouse is the closest animal
scientists can easily use to analyse the brain, as though simpler, its
structure and basic circuitry is very similar to the human brain.
Studying neurons in a mouse's brain could therefore help
scientists further understand how similar structures in the human brain
work.
"In the English language there are only 26 letters, but the
combinations of letters make unlimited words and sentences, this is also
true for memories," Dr Liu told BBC News.
Evolving memories
"There are so many brain cells and for each individual memory,
different combinations of small populations of cells are activated."
These differing combinations of cells could partly explain
why memories are not static like a photograph, but constantly evolving,
he added.
Erasing memories?
Mice have previously been trained to believe they were somewhere else, "a bit like the feeling of deja-vu we sometimes get", said Rosamund Langston from Dundee University.A possibility in the future is erasing memories, she told BBC News.
"Episodic memories - such as those for traumatic experiences -
are distributed in neurons throughout the brain, and in order to make
memory erasure a safe and useful tool, we must understand how the
different components of each memory are put together.
"You may want to erase someone's memory for a traumatic event
that happened in their home, but you certainly do not want to erase
their memory for how to find their way around their home."
"If you want to grab a specific
memory you have to get down into the cell level. Every time we think we
remember something, we could also be making changes to that memory -
sometimes we realise sometimes we don't," Dr Liu explained.
"Our memory changes every single time it's being 'recorded'.
That's why we can incorporate new information into old memories and this
is how a false memory can form without us realising it."
Susumu Tonegawa, also from RIKEN-MIT, said his teams' work
provided the first animal model in which false and genuine memories
could be investigated in the cells which store memories, called
engram-bearing cells.
"Humans are highly imaginative animals. Just like our mice,
an aversive or appetitive event could be associated with a past
experience one may happen to have in mind at that moment, hence a false
memory is formed."
Silencing fear
Neil Burgess from University College London, who was not
involved with the work, told BBC News the study was an "impressive
example" of creating a fearful response in an environment where nothing
fearful happened.
"One day this type of knowledge may help scientists to
understand how to remove or reduce the fearful associations experienced
by people with conditions like post traumatic stress disorder."
But he added that it's only an advance in "basic
neuroscience" and that these methods could not be directly applied to
humans for many years.
"But basic science always helps in the end, and it may be
possible, one day, to use similar techniques to silence neurons causing
the association to fear."
'Diseases of thought'
Mark Mayford of the Scripps Research Institute in San Diego,
US, said: "The question is, how does the brain change with experience?
That's the heart of everything the brain does.
He explained that work like this could one day further help
us to understand the structure of our thoughts and the cells involved.
"Then one can begin to look at those brain circuits, see how
they change, and hopefully find the areas or mechanisms that change with
learning."
"The implications are potentially interventions for diseases
of thought such as schizophrenia. You cannot approach schizophrenia
unless you know how a perception is put together."
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