Newswise, August 25, 2015 — You see a man at the
grocery store. Is that the fellow you went to college with or just a guy who
looks like him?
One tiny spot in the brain has the answer.
Johns Hopkins University neuroscientists have
identified the part of the hippocampus that creates and processes this type of
memory, furthering our understanding of how the mind works, and what’s going
wrong when it doesn’t. Their findings are published in the current issue of the
journal Neuron.
“You see a familiar face and say to yourself, ‘I
think I’ve seen that face.’ But is this someone I met five years ago, maybe
with thinner hair or different glasses — or is it someone else entirely,” said James J.
Knierim, a professor of neuroscience at the university’s Zanvyl Krieger
Mind/Brain Institute who led the research.
“That’s one of the biggest problems
our memory system has to solve.”
Neural activity in the hippocampus allows someone to
remember where they parked their car, find their home even if the paint color
changes, and recognize an old song when it comes on the radio.
Brain researchers theorized that two parts of the
hippocampus (the dentate gyrus and CA3) competed to decide whether a stimulus
was completely new or an altered version of something familiar.
The dentate
gyrus was thought to automatically encode each stimulus as new, a process
called pattern separation. In contrast, CA3 was thought to minimize any small
changes from one experience to the next and classify the stimuli as being the
same, a process called pattern completion.
So, the dentate gyrus would assume
that the person with thinner hair and unfamiliar glasses was a complete
stranger, while CA3 would ignore the altered details and retrieve the memory of
a college buddy.
Prior work by Knierim’s group and others provided
evidence in favor of this long-standing theory.
The new research shows,
however, that CA3 is more complicated than previously thought — parts of CA3
come to different decisions, and they pass these different decisions to other
brain areas.
“The final job of the CA3 region is to make the
decision: Is it the same or is it different?” Knierim said.
“Usually you are
correct in remembering that this person is a slightly different version of the
person you met years ago. But when you are wrong, and it embarrassingly turns
out that this is a complete stranger, you want to create a memory of this new
person that is absolutely distinct from the memory of your familiar friend, so
you don’t make the mistake again.”
Knierim and Johns Hopkins postdoctoral fellows
Heekyung Lee and Cheng Wang, along with Sachin S. Deshmukh, a former assistant
research scientist in Knierim’s lab, monitored rats as they got to know an
environment and as that environment changed.
The team implanted electrodes in the hippocampus of
the rats. They trained the rats to run around a track, eating chocolate
sprinkles. The track floor had four different textures — sandpaper, carpet
padding, duct tape and a rubber mat. The rat could see, feel and smell the
differences in the textures.
Meanwhile, a black curtain surrounding the track
had various objects attached to it. Over 10 days, the rats built mental maps of
that environment.
Then the experimenters changed things up.
They
rotated the track counter-clockwise, while rotating the curtain clockwise,
creating a perceptual mismatch in the rats’ minds. The effect was similar,
Knierim said, to if you opened the door of your home and all of your pictures
were hanging on different walls and your furniture had been moved.
“Would you recognize it as your home or think you
are lost?” he said. “It’s a very disorienting experience and a very
uncomfortable feeling.”
Even when the perceptual mismatch between the track
and curtain was small, the “pattern separating” part of CA3 almost completely
changed its activity patterns, creating a new memory of the altered
environment.
But the “pattern completing” part of CA3 tended to retrieve a
similar activity pattern used to encode the original memory, even when the
perceptual mismatch increased.
The findings, which validate models about how memory
works, could help explain what goes wrong with memory in diseases like
Alzheimer’s and could help to preserve people’s memories as they age.
This research was supported by the National
Institutes of Health grants R01 NS039456 and R01 MH094146 and by the Johns
Hopkins University Brain Sciences Institute.
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