Brain Aspects of a single memory can be scattered
throughout the outer “cortex” of the brain
Storing information so that you can easily find it
again is a challenge. From purposefully messy
desks to indexed filing cabinets, we all have our
preferred systems. How does it happen inside our
brains?
Somewhere within the dense, damp and intricate
1.5kg of tissue that we carry in our skulls, all of
our experiences are processed, stored, and –
sometimes more readily than others – retrieved
again when we need them.
It’s what neuroscientists call “episodic memory”
and for years, they have loosely agreed on a
model for how it works. Gathering detailed data
to flesh out that model is difficult.
But the picture is beginning to get clearer and
more complete.
A key component is the small, looping structure
called the hippocampus, buried quite deep
beneath the brain’s wrinkled outer layer. It is
only a few centimetres in length but is very well
connected to other parts of the brain.
People with damage to their hippocampus have
profound memory problems and this has made it
a major focus of memory research since the
1950s.
Quick learning
It was in the hippocampus, and some of its
neighbouring brain regions, that scientists from
the University of Leicester got a glimpse of new
memories being formed , in a study published this
week .
They used a rare opportunity to record the fizz
and crackle of single human brain cells at work,
in epilepsy patients undergoing brain surgery.
Individual neurons that went crazy for particular
celebrities, like Clint Eastwood, could be “trained”
to respond to, for example, the Statue of Liberty
as well – as soon as the patients were given a
picture of Clint in front of the statue.
It seemed that single brain cells, in the
hippocampus, had been caught in the act of
forming a new association. And they do it very
fast.
But that outer wrapping of the brain – the cortex
– is also important. It is much bigger than the
hippocampus and does myriad jobs, from sensing
the world to moving our limbs.
When we have a particular experience, like a trip
to the beach, different patches of the cortex are
called up to help us process different elements:
recognising a friend, hearing the seagulls, feeling
the breeze.
So traces of that experience are rather scattered
across the cortex. To remember it, the brain
needs some sort of index to find them all again.
And that, neuroscientists generally agree, is
where the hippocampus comes in.
“Think of the [cortex] as a huge library and the
hippocampus as its librarian,” wrote the
prominent Hungarian neuroscientist Gyorgy
Buszaki in his 2006 book Rhythms of the Brain.
The elements of our day at the beach might litter
the cortex like specific books along miles of
shelving; the hippocampus is able to link them
together and – if all goes well – pull them off the
shelf when we want to reminisce.
Completing patterns
Another brand new study, out this week in the
journal Nature Communications, looks inside the
brain using fMRI imaging to see this filing system
in action.
By getting people to learn and remember
imaginary scenarios while inside a brain scanner,
Dr Aidan Horner and his colleagues at University
College London collected the first firm evidence
for “pattern completion” in the human
hippocampus.
Pattern completion is the mechanism behind a
phenomenon we all recognise, when one
particular aspect of a memory – the smell of salt
in the air, perhaps – brings all the other aspects
flooding back.
“If you have an event that involves the Eiffel
tower, your friend and, say, a pink balloon… I can
show you a picture of the Eiffel tower, and you
remember not only your friend, but also the pink
balloon,” Dr Horner told the BBC.
While his volunteers had just this sort of
experience inside the scanner, Dr Horner saw
interplay between different parts of the cortex,
associated with different parts of a memory, and
the hippocampus.
The brain activity flowed in a way that showed
“pattern completion” was indeed underway – and
the cortex and the hippocampus were working
just like the library and the librarian in Prof
Buzsaki’s analogy.
“If I cue you with the location, and I get you to
explicitly retrieve the person, what we also see is
activation in the region that’s associated with the
object for that event,” Dr Horner explained. “So
even though it’s task-irrelevant, you don’t have
to retrieve it, it seems that we still bring that
object to mind.
“And the extent to which we see that activation in
the ‘object’ region correlates with the
hippocampal response. So that suggests that it’s
the hippocampus that’s doing the pattern
completion, retrieving all these elements.
“It’s able to act as an index, I suppose, by
linking these things together – and doing it very
very quickly, that’s the key thing.”
If the cortex were left to make its own
connections between the fragments of a memory,
he added, it would be far too slow.
“That’s clearly not a system we want, if we’re
going to remember a specific event that happens
once in a lifetime.”
Dr Horner said the findings also dovetail nicely
with the single-neuron, celebrity-spotting results
from the Leicester study.
“We can look across the cortex and the
hippocampus, and we can relate it to recollection.
But what they can do is say look, these cells [in
the hippocampus] have learned really quickly.
“So that’s the sort of underlying neural basis of
what we’re looking at, at a slightly broader
scale.”
Science, it seems, is finally managing to unpick
the way our brains record our lives. It is a
remarkable, beautiful, fallible system.
Building some sort of memory storage like this is
regarded as one of the next key challenges for
researchers trying to build intelligent machines.

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