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ABBY NOYCE: One of the things we've
been talking about when we're talking about how vision
and the visual system works is that the visual system is not
very interested in places where the world is all the same.
Clear blue sky, clear smooth surfaces, your visual system
looks at that and says I don't need a whole lot of information
to give you a perception about this.
But it really pays a lot of attention
to places where things change, where there's
motion, where there's edges, where color changes,
where there's lines.
And we're going to talk today a little bit
about the neural processes that manage to grab
onto these sorts of features.
So you guys talked yesterday about ganglion cells,
right, in the retina?
Yes, no.
AUDIENCE: Yes.
AUDIENCE: Are those rods and cones?
AUDIENCE: She mentioned it.
ABBY NOYCE: OK.
So what happens is--
we'll back up then.
So there's a bunch of different layers of cells in the retina,
right?
Here's the back of an eyeball.
Here's all of our photoreceptors, rods and cones.
And then in here, there's a bunch
of different layers of neurons.
Neurons, and there's some that do
horizontal processing and more.
Lots of neurons.
So the retina is this sheet, right?
It's got photoreceptors furthest to the back of the eye,
and then it does this preliminary processing.
And one of the things that is important
here is that this set of neurons that
is closest to the inside of the eye--
if you look straight in through somebody's pupil, this
would be the surface of the retina that's closest to you.
Remember, the photoreceptors are on the back,
and the layers of processing come forward--
are called ganglion cells.
And ganglion cells are the ones that
actually bundle all their axons together and line up and form
the optic nerve, right?
So ganglion cells have a particular kind
of receptive field.
A cell's receptive field is the part of the field of vision
that it responds to.
So for a given--
any given photoreceptor is responding usually
to a very small part of the visual field.
These have this very small and very simple receptive field.
They respond-- if there is light there,
they change their response.
If there is not, they go back to their baseline.
And some of them respond to different colors
preferentially, but for the most part,
they have these very simple receptive fields.
Ganglion cells, on the other hand,
because of what's called lateral inhibition,
have center-surround receptive fields.
So each ganglion cell is looking at a little bit
of the visual world and this is kind
of the classic representation of an on-center ganglion cell.
So this is a cell that would respond
if it's looking at a dark surface or a dark screen
like this, and it had a little white dot
right where the center of the receptive field is--
this is an excitatory center and an inhibitory surround.
So this ganglion cell would respond a whole lot
to this sort of stimulus.
Whereas if it's looking at just a blank field,
then the excitatory center is still going to get stimulated,
but the inhibitory surround is also going to get stimulated.
And so the excitatory and inhibitory stuff
that's feeding in cancels out.
So you can think of this as if we're looking in the fovea
in the center of your eye, where all the receptive fields are
really small because your visual system is doing detail,
it might look something like this.
We've got one cone.
That's not what it looks like, I know.
They've got all that squiggly stuff.
And this cone-- and we've got our ganglion cell down here.
And this cone is feeding in an excitatory impulse
to that ganglion cell, and that's
going to be the center of this ganglion cell's
receptive field.
But all of the cones that surround this cone
would be feeding inhibitory impulses to the ganglion cell.
And this is just straight up excitatory and inhibitory
synapses, right, like we talked about last week?
So that if just this cone gets stimulated,
then there's only an excitatory input coming into that ganglion
cell, and the ganglion cell will, in turn, fire.
If there's input coming in from both the center
and the surround, then the mix of excitatory and inhibitory
is going to actually decrease, probably average out to zero,
the way most of these are weighted.
If the cell is looking at a stimulus that just looks
like that, so that only the inhibitory part is stimulated
and the excitatory part isn't getting anything,
then it's going to actually decrease its baseline firing
rate.
So all these cells kind of have a base firing rate
that if they're not getting any kind of stimulation,
they just tick, tick, tick.
They just go every so often.
And then that rate will increase as the cell gets
more excitatory stimulation, or it can actually
decrease as the cell gets more inhibitory stimulation.
So ganglion cells, we get on-center.
We also get off-center ganglion cells.
These are inhibitory in the center and excitatory
in the surround.
Just like the on-center, the off-center cell,
if it's looking at a plain field like this,
it's not going to respond very much.
The excitatory and the inhibitory parts cancel out.
But this cell will be inhibited by the single white dot
and excited by the ring with the dark dot.
So they're responding to opposite sorts of simulations,
but in both cases, they're not responding to these kind
of featureless fields.
They're responding to places where the visual system
is doing something more-- where the visual stimulus is doing
something more interesting.
So like I said, so ganglion cells are the cells whose axons
run all the way down the optic nerve,
back into the lateral geniculate nucleus of the thalamus,
and from there, they get passed to a new set of--
they terminate there.
They synapse onto more neurons, and those neurons
go to primary visual cortex.
So did Xander show you guys the scintillating grid
illusion with the black squares and the white grid
in between them?
Did you talk about what causes that?
AUDIENCE: She said no one knows.
AUDIENCE: No, not that one.
That was the other one.
AUDIENCE: No, she said no one knows for that one.
Yeah.
ABBY NOYCE: So to some extent, it
can be explained by looking at the ganglion cells.
The other kind of classic example of this--
I wish I had my slides--
is what's called the mock band illusion, where
if you put rectangles of light gray and dark gray
next to each other, then right at the border between them,
the light gray one looks lighter and the dark gray one looks
darker compared to the rest of the rectangle.
And again, this kind of ganglion cell response
can explain some of that.
So that there scintillating grid, if you've got--
new piece of board.
You've got your boxes, right?
Box, et cetera.
That's a lot of chalk dust.
So think about what's happening--
think about what's happening to ganglion cells
with-- let's just think about on-center ganglion cells
as they hang out here.
So an on-center ganglion cell that's
scooched right up against the edge of one of these boxes,
so center-surround, its center is going to get
inhibited by the white part of the grid that it's on.
The center is going to get excited by the white part
of the grid that it's on, right?
And half of the surround is going
to be inhibited but the other half isn't,
so the excitatory part still trumps the inhibitory part.
So this ganglion cell will actually
respond more than a ganglion cell
that's only looking at the white part
because of how part of its inhibitory surround
is not being exposed to the white.
Part of its inhibitory surround is
looking at the black portion of the square.
And one of the things that people
think is happening with this here scintillating grid
illusion is that ganglion cells that are here and here and all
around it, that are schooched right
up against the edges of the squares
are firing more than a ganglion cell that's
out here in the middle of the intersection.
So it ends up looking like the bits that
are between the squares are brighter than the bit that's
in the middle.
So that's one theory.
And I like it as an example of how
you can take this really simple properties
of a particular class of nerve cells and what it responds to,
line a couple of them up together,
and hey, look, a perceptual effect,
and perceptual effects are cool.
AUDIENCE: Do we know which ones are on-center
and which are off-center?
ABBY NOYCE: You can test it.
I mean, the way people find these is--
and you've got both.
The way people have found them is
you've got your classic anesthetized cat lying
on a table, and you're showing it little dots of light.
And you're sticking an electrode into the cat's retina,
actually or-- yeah, probably directly into the retina
or maybe into the optic nerve and picking up the axons--
and measuring what sorts of stimuli are causing these cells
to fire and which aren't.
Which is how you find that each cell has a very specific region
of the visual field that it looks at,
and it responds in one of these two characteristic patterns.
So ganglion cells, scintillating grid.
So people knew for quite some time,
probably from the early 20th century,
that ganglion cells have this receptive field pattern.
And people were trying to figure out how cells in visual cortex
respond and how cells in the thalamus
and the lateral geniculate nucleus respond.
Remember, your information pathway goes like eye to V1,
right?
So the thalamus here is taking in the sensory input,
doing a little bit of processing,
and passing it along to cortex.
There's also actually an awful lot of information
that comes from all of these other parts of visual cortex
that goes back to the thalamus.
So there's a lot of top-down influence on the thalamus,
and nobody is exactly sure what that processing is doing.
Some theories are kind of like it
lets you compare what you're currently looking at to what
you were looking at a moment ago,
so you can see things like motion and changes
and direct your attention to it.
LGN also seems to be involved.
You've got a reflex, more or less.
So if you hear a loud noise off to one side,
you'll usually turn and look at it.
And that seems to be auditory information coming
into the lateral geniculate nucleus,
coming into the thalamus, at least
from the auditory system, which tells the lateral geniculate
nucleus to send you a signal to direct
your visual attention over where you heard the noise coming
from.
All right.
So cells in the lateral geniculate nucleus
have on-center and off-center receptive fields
with, this is called center-surround antagonism,
that pattern, where the center is doing one thing
and the surround is doing the opposite.
And the cells in the lateral geniculate nucleus
have pretty much that same pattern of response.
So a given cell will respond to a given thing.
Cells in the visual cortex, however, don't.
They do something different.
You guys, OK?
So the classic example of this is two guys
named David Hubel and--
shoot, what was his name?
Like Thorston or something.
Thorin, no, that's somebody else--
and Wiesel.
Hubel and Wiesel were researchers
in the '50s, who were working with anesthetized cats,
trying to find receptive fields for cells in the cat's
primary visual cortex.
So cat on table so that it's facing a screen.
The cat's anesthetized so it holds still,
and it has to look at the screen.
And they put little microelectrodes
into these parts of visual cortex
and tried showing the cat different kinds of stimuli,
dots, lights dots, dark dots, dots
in different parts of the visual field.
all of the stuff that will make cells in the lateral geniculate
nucleus so the ganglion cells in the retina fire like mad,
right?
And it didn't work.
And eventually, they managed to find one cell
that sometimes when they showed it a dot,
it responded to different parts of the visual field.
And they didn't really think it was
the dot of light they were showing it
that was the key stimulus.
So eventually, they managed to figure out
that what was triggering the cell to respond was,
as they pulled slides in and out of the side projector
they were using--
old school technology here--
that as the edge of the slide kind of
cast a shadow that moved across the screen,
the cell was responding to that, to a line,
the particular angle, in a particular part
of the receptive field, moving in a particular direction
or moving perpendicular to how the line was oriented.
So and once they had found that, they said, hey, look,
these cells are responding to something completely different
than everything in the LGN and in the retina.
And pretty quickly, other people started working on this.
And they've managed to find three different main types
of cells in V1, and all of them are dependent, really picky
about orientation.
They're all basically line detectors
of one kind or another.
And the simplest one are cells called simple cells, which
respond to a line of a particular orientation
or an edge of a particular orientation
in a particular part of the visual field.
So they're like all of-- like the cells
earlier in the system, like the ganglion cells
and the lateral geniculate cells,
they're very picky about where in space their stimuli are.
And these cells look for lines of a particular angle.
So how do you go from cells that look for this--
how do you take that kind of information
and pass it along and build a line detector using
this kind of preliminary cell?
AUDIENCE: Say if you're looking at one thing and then a line
moving, I guess, would cause multiple cells to fire,
like just one after another.
ABBY NOYCE: Yeah, good.
So yeah, it looks like a lot of what's
happening with motion detecting is
where you get sequential firing over a particular area
of the cortex.
Frogs actually have cells in their retina,
really early in their visual processing
system that look for just a small dot moving around.
They've got fly detectors to help them find their food.
So they've got cells that respond
to motion of a small stimulus.
We don't have those.
AUDIENCE: I think I read that frogs also they
can't detect their food if it's absolutely still.
So if it's a fly on the wall and it's not--