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TEDSalon London Spring 2011

Neil Burgess: How your brain tells you where you are

November 11, 2011

How do you remember where you parked your car? How do you know if you're moving in the right direction? Neuroscientist Neil Burgess studies the neural mechanisms that map the space around us, and how they link to memory and imagination.

Neil Burgess - Neuroscientist
At University College in London, Neil Burgess researches how patterns of electrical activity in brain cells guide us through space. Full bio

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Double-click the English subtitles below to play the video.
When we park in a big parking lot,
00:15
how do we remember where we parked our car?
00:17
Here's the problem facing Homer.
00:19
And we're going to try to understand
00:22
what's happening in his brain.
00:24
So we'll start with the hippocampus, shown in yellow,
00:26
which is the organ of memory.
00:28
If you have damage there, like in Alzheimer's,
00:30
you can't remember things including where you parked your car.
00:32
It's named after Latin for "seahorse,"
00:34
which it resembles.
00:36
And like the rest of the brain, it's made of neurons.
00:38
So the human brain
00:40
has about a hundred billion neurons in it.
00:42
And the neurons communicate with each other
00:44
by sending little pulses or spikes of electricity
00:47
via connections to each other.
00:49
The hippocampus is formed of two sheets of cells,
00:51
which are very densely interconnected.
00:54
And scientists have begun to understand
00:56
how spatial memory works
00:58
by recording from individual neurons
01:00
in rats or mice
01:02
while they forage or explore an environment
01:04
looking for food.
01:06
So we're going to imagine we're recording from a single neuron
01:08
in the hippocampus of this rat here.
01:11
And when it fires a little spike of electricity,
01:14
there's going to be a red dot and a click.
01:16
So what we see
01:19
is that this neuron knows
01:21
whenever the rat has gone into one particular place in its environment.
01:23
And it signals to the rest of the brain
01:26
by sending a little electrical spike.
01:28
So we could show the firing rate of that neuron
01:31
as a function of the animal's location.
01:34
And if we record from lots of different neurons,
01:36
we'll see that different neurons fire
01:38
when the animal goes in different parts of its environment,
01:40
like in this square box shown here.
01:42
So together they form a map
01:44
for the rest of the brain,
01:46
telling the brain continually,
01:48
"Where am I now within my environment?"
01:50
Place cells are also being recorded in humans.
01:52
So epilepsy patients sometimes need
01:55
the electrical activity in their brain monitoring.
01:57
And some of these patients played a video game
02:00
where they drive around a small town.
02:02
And place cells in their hippocampi would fire, become active,
02:04
start sending electrical impulses
02:07
whenever they drove through a particular location in that town.
02:10
So how does a place cell know
02:13
where the rat or person is within its environment?
02:15
Well these two cells here
02:18
show us that the boundaries of the environment
02:20
are particularly important.
02:22
So the one on the top
02:24
likes to fire sort of midway between the walls
02:26
of the box that their rat's in.
02:28
And when you expand the box, the firing location expands.
02:30
The one below likes to fire
02:33
whenever there's a wall close by to the south.
02:35
And if you put another wall inside the box,
02:38
then the cell fires in both place
02:40
wherever there's a wall to the south
02:42
as the animal explores around in its box.
02:44
So this predicts
02:48
that sensing the distances and directions of boundaries around you --
02:50
extended buildings and so on --
02:52
is particularly important for the hippocampus.
02:54
And indeed, on the inputs to the hippocampus,
02:57
cells are found which project into the hippocampus,
02:59
which do respond exactly
03:01
to detecting boundaries or edges
03:03
at particular distances and directions
03:06
from the rat or mouse
03:08
as it's exploring around.
03:10
So the cell on the left, you can see,
03:12
it fires whenever the animal gets near
03:14
to a wall or a boundary to the east,
03:16
whether it's the edge or the wall of a square box
03:19
or the circular wall of the circular box
03:22
or even the drop at the edge of a table, which the animals are running around.
03:24
And the cell on the right there
03:27
fires whenever there's a boundary to the south,
03:29
whether it's the drop at the edge of the table or a wall
03:31
or even the gap between two tables that are pulled apart.
03:33
So that's one way in which we think
03:36
place cells determine where the animal is as it's exploring around.
03:38
We can also test where we think objects are,
03:41
like this goal flag, in simple environments --
03:44
or indeed, where your car would be.
03:47
So we can have people explore an environment
03:49
and see the location they have to remember.
03:52
And then, if we put them back in the environment,
03:55
generally they're quite good at putting a marker down
03:57
where they thought that flag or their car was.
03:59
But on some trials,
04:02
we could change the shape and size of the environment
04:04
like we did with the place cell.
04:06
In that case, we can see
04:08
how where they think the flag had been changes
04:10
as a function of how you change the shape and size of the environment.
04:13
And what you see, for example,
04:16
if the flag was where that cross was in a small square environment,
04:18
and then if you ask people where it was,
04:21
but you've made the environment bigger,
04:23
where they think the flag had been
04:25
stretches out in exactly the same way
04:27
that the place cell firing stretched out.
04:29
It's as if you remember where the flag was
04:31
by storing the pattern of firing across all of your place cells
04:33
at that location,
04:36
and then you can get back to that location
04:38
by moving around
04:40
so that you best match the current pattern of firing of your place cells
04:42
with that stored pattern.
04:44
That guides you back to the location that you want to remember.
04:46
But we also know where we are through movement.
04:49
So if we take some outbound path --
04:52
perhaps we park and we wander off --
04:54
we know because our own movements,
04:56
which we can integrate over this path
04:58
roughly what the heading direction is to go back.
05:00
And place cells also get this kind of path integration input
05:02
from a kind of cell called a grid cell.
05:06
Now grid cells are found, again,
05:09
on the inputs to the hippocampus,
05:11
and they're a bit like place cells.
05:13
But now as the rat explores around,
05:15
each individual cell fires
05:17
in a whole array of different locations
05:19
which are laid out across the environment
05:22
in an amazingly regular triangular grid.
05:24
And if you record from several grid cells --
05:29
shown here in different colors --
05:32
each one has a grid-like firing pattern across the environment,
05:34
and each cell's grid-like firing pattern is shifted slightly
05:37
relative to the other cells.
05:40
So the red one fires on this grid
05:42
and the green one on this one and the blue on on this one.
05:44
So together, it's as if the rat
05:47
can put a virtual grid of firing locations
05:50
across its environment --
05:52
a bit like the latitude and longitude lines that you'd find on a map,
05:54
but using triangles.
05:57
And as it moves around,
05:59
the electrical activity can pass
06:01
from one of these cells to the next cell
06:03
to keep track of where it is,
06:05
so that it can use its own movements
06:07
to know where it is in its environment.
06:09
Do people have grid cells?
06:11
Well because all of the grid-like firing patterns
06:13
have the same axes of symmetry,
06:15
the same orientations of grid, shown in orange here,
06:17
it means that the net activity
06:20
of all of the grid cells in a particular part of the brain
06:22
should change
06:25
according to whether we're running along these six directions
06:27
or running along one of the six directions in between.
06:29
So we can put people in an MRI scanner
06:32
and have them do a little video game
06:34
like the one I showed you
06:36
and look for this signal.
06:38
And indeed, you do see it in the human entorhinal cortex,
06:40
which is the same part of the brain that you see grid cells in rats.
06:43
So back to Homer.
06:46
He's probably remembering where his car was
06:48
in terms of the distances and directions
06:50
to extended buildings and boundaries
06:52
around the location where he parked.
06:54
And that would be represented
06:56
by the firing of boundary-detecting cells.
06:58
He's also remembering the path he took out of the car park,
07:00
which would be represented in the firing of grid cells.
07:03
Now both of these kinds of cells
07:06
can make the place cells fire.
07:08
And he can return to the location where he parked
07:10
by moving so as to find where it is
07:12
that best matches the firing pattern
07:15
of the place cells in his brain currently
07:17
with the stored pattern where he parked his car.
07:19
And that guides him back to that location
07:22
irrespective of visual cues
07:24
like whether his car's actually there.
07:26
Maybe it's been towed.
07:28
But he knows where it was, so he knows to go and get it.
07:30
So beyond spatial memory,
07:33
if we look for this grid-like firing pattern
07:35
throughout the whole brain,
07:37
we see it in a whole series of locations
07:39
which are always active
07:42
when we do all kinds of autobiographical memory tasks,
07:44
like remembering the last time you went to a wedding, for example.
07:46
So it may be that the neural mechanisms
07:49
for representing the space around us
07:51
are also used for generating visual imagery
07:54
so that we can recreate the spatial scene, at least,
07:58
of the events that have happened to us when we want to imagine them.
08:01
So if this was happening,
08:04
your memories could start by place cells activating each other
08:06
via these dense interconnections
08:09
and then reactivating boundary cells
08:11
to create the spatial structure
08:13
of the scene around your viewpoint.
08:15
And grid cells could move this viewpoint through that space.
08:17
Another kind of cell, head direction cells,
08:19
which I didn't mention yet,
08:21
they fire like a compass according to which way you're facing.
08:23
They could define the viewing direction
08:26
from which you want to generate an image for your visual imagery,
08:28
so you can imagine what happened when you were at this wedding, for example.
08:31
So this is just one example
08:34
of a new era really
08:36
in cognitive neuroscience
08:38
where we're beginning to understand
08:40
psychological processes
08:42
like how you remember or imagine or even think
08:44
in terms of the actions
08:47
of the billions of individual neurons that make up our brains.
08:49
Thank you very much.
08:52
(Applause)
08:54

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Neil Burgess - Neuroscientist
At University College in London, Neil Burgess researches how patterns of electrical activity in brain cells guide us through space.

Why you should listen

Neil Burgessis is deputy director of the Institute of Cognitive Neuroscience at University College London, where he investigates of the role of the hippocampus in spatial navigation and episodic memory. His research is directed at answering questions such as: How are locations represented, stored and used in the brain? What processes and which parts of the brain are involved in remembering the spatial and temporal context of everyday events, and in finding one's way about?

To explore this space, he and his team use a range of methods for gathering data, including pioneering uses of virtual reality, as well as computational modelling and electrophysiological analysis of the function of hippocampal neurons in the rat, functional imaging of human navigation, and neuropsychological experiments on spatial and episodic memory.

A parallel interest: Investigating our human short-term memory for serial order, or how we know our 123s.

The original video is available on TED.com
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