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  • When we park in a big parking lot,

  • how do we remember where we parked our car?

  • Here's the problem facing Homer.

  • And we're going to try to understand

  • what's happening in his brain.

  • So we'll start with the hippocampus, shown in yellow,

  • which is the organ of memory.

  • If you have damage there, like in Alzheimer's,

  • you can't remember things including where you parked your car.

  • It's named after Latin for "seahorse,"

  • which it resembles.

  • And like the rest of the brain, it's made of neurons.

  • So the human brain

  • has about a hundred billion neurons in it.

  • And the neurons communicate with each other

  • by sending little pulses or spikes of electricity

  • via connections to each other.

  • The hippocampus is formed of two sheets of cells,

  • which are very densely interconnected.

  • And scientists have begun to understand

  • how spatial memory works

  • by recording from individual neurons

  • in rats or mice

  • while they forage or explore an environment

  • looking for food.

  • So we're going to imagine we're recording from a single neuron

  • in the hippocampus of this rat here.

  • And when it fires a little spike of electricity,

  • there's going to be a red dot and a click.

  • So what we see

  • is that this neuron knows

  • whenever the rat has gone into one particular place in its environment.

  • And it signals to the rest of the brain

  • by sending a little electrical spike.

  • So we could show the firing rate of that neuron

  • as a function of the animal's location.

  • And if we record from lots of different neurons,

  • we'll see that different neurons fire

  • when the animal goes in different parts of its environment,

  • like in this square box shown here.

  • So together they form a map

  • for the rest of the brain,

  • telling the brain continually,

  • "Where am I now within my environment?"

  • Place cells are also being recorded in humans.

  • So epilepsy patients sometimes need

  • the electrical activity in their brain monitoring.

  • And some of these patients played a video game

  • where they drive around a small town.

  • And place cells in their hippocampi would fire, become active,

  • start sending electrical impulses

  • whenever they drove through a particular location in that town.

  • So how does a place cell know

  • where the rat or person is within its environment?

  • Well these two cells here

  • show us that the boundaries of the environment

  • are particularly important.

  • So the one on the top

  • likes to fire sort of midway between the walls

  • of the box that their rat's in.

  • And when you expand the box, the firing location expands.

  • The one below likes to fire

  • whenever there's a wall close by to the south.

  • And if you put another wall inside the box,

  • then the cell fires in both place

  • wherever there's a wall to the south

  • as the animal explores around in its box.

  • So this predicts

  • that sensing the distances and directions of boundaries around you --

  • extended buildings and so on --

  • is particularly important for the hippocampus.

  • And indeed, on the inputs to the hippocampus,

  • cells are found which project into the hippocampus,

  • which do respond exactly

  • to detecting boundaries or edges

  • at particular distances and directions

  • from the rat or mouse

  • as it's exploring around.

  • So the cell on the left, you can see,

  • it fires whenever the animal gets near

  • to a wall or a boundary to the east,

  • whether it's the edge or the wall of a square box

  • or the circular wall of the circular box

  • or even the drop at the edge of a table, which the animals are running around.

  • And the cell on the right there

  • fires whenever there's a boundary to the south,

  • whether it's the drop at the edge of the table or a wall

  • or even the gap between two tables that are pulled apart.

  • So that's one way in which we think

  • place cells determine where the animal is as it's exploring around.

  • We can also test where we think objects are,

  • like this goal flag, in simple environments --

  • or indeed, where your car would be.

  • So we can have people explore an environment

  • and see the location they have to remember.

  • And then, if we put them back in the environment,

  • generally they're quite good at putting a marker down

  • where they thought that flag or their car was.

  • But on some trials,

  • we could change the shape and size of the environment

  • like we did with the place cell.

  • In that case, we can see

  • how where they think the flag had been changes

  • as a function of how you change the shape and size of the environment.

  • And what you see, for example,

  • if the flag was where that cross was in a small square environment,

  • and then if you ask people where it was,

  • but you've made the environment bigger,

  • where they think the flag had been

  • stretches out in exactly the same way

  • that the place cell firing stretched out.

  • It's as if you remember where the flag was

  • by storing the pattern of firing across all of your place cells

  • at that location,

  • and then you can get back to that location

  • by moving around

  • so that you best match the current pattern of firing of your place cells

  • with that stored pattern.

  • That guides you back to the location that you want to remember.

  • But we also know where we are through movement.

  • So if we take some outbound path --

  • perhaps we park and we wander off --

  • we know because our own movements,

  • which we can integrate over this path

  • roughly what the heading direction is to go back.

  • And place cells also get this kind of path integration input

  • from a kind of cell called a grid cell.

  • Now grid cells are found, again,

  • on the inputs to the hippocampus,

  • and they're a bit like place cells.

  • But now as the rat explores around,

  • each individual cell fires

  • in a whole array of different locations

  • which are laid out across the environment

  • in an amazingly regular triangular grid.

  • And if you record from several grid cells --

  • shown here in different colors --

  • each one has a grid-like firing pattern across the environment,

  • and each cell's grid-like firing pattern is shifted slightly

  • relative to the other cells.

  • So the red one fires on this grid

  • and the green one on this one and the blue on on this one.

  • So together, it's as if the rat

  • can put a virtual grid of firing locations

  • across its environment --

  • a bit like the latitude and longitude lines that you'd find on a map,

  • but using triangles.

  • And as it moves around,

  • the electrical activity can pass

  • from one of these cells to the next cell

  • to keep track of where it is,

  • so that it can use its own movements

  • to know where it is in its environment.

  • Do people have grid cells?

  • Well because all of the grid-like firing patterns

  • have the same axes of symmetry,

  • the same orientations of grid, shown in orange here,

  • it means that the net activity

  • of all of the grid cells in a particular part of the brain

  • should change

  • according to whether we're running along these six directions

  • or running along one of the six directions in between.

  • So we can put people in an MRI scanner

  • and have them do a little video game

  • like the one I showed you

  • and look for this signal.

  • And indeed, you do see it in the human entorhinal cortex,

  • which is the same part of the brain that you see grid cells in rats.

  • So back to Homer.

  • He's probably remembering where his car was

  • in terms of the distances and directions

  • to extended buildings and boundaries

  • around the location where he parked.

  • And that would be represented

  • by the firing of boundary-detecting cells.