by light directed at the surround.
So this center surround organization has been
a characteristic that's known, really, since the 1950s, or
before in different animals, but known in mammals since the 1950s.
And you can see how this organization,
receptive fields, might provide an explanation with this kind of effect.
Again in sort of the domain of things aren't working properly or quite properly.
This is a sacrifice that the emphasis on detecting
edges by virtue of this organization of receptive fields,
that a side effect of that is the difference that we see.
So, again, look at this boundary.
And these are receptive fields of a single cell
indicated in different positions in respect to this boundary.
So here it's completely the cell's responsiveness,
the cell's receptive field is now completely in the dark area.
And now it's a little bit in the dark area, the centers of the dark area, but
there's a little bit of the surround that's in the light area and so
on down through this array of diagratic representations, the receptive fields.
And you can see that at each one of these positions, A,
B, C, D here that are represented in this graph in terms
of the response rate, how excited or not is the cell.
That the cell's excitation is going to vary as a function of its
position across this light-dark boundary.
It fires a little bit less at B for reasons that should be obvious.
The inhibitory surround
is a little bit more in the light area than in the dark area.
That's going to change the firing rate of the cell,
the baseline firing rate of the cell, and it's going to be less.
Here at C, halfway in, halfway out, comes back to normal,
by normal I just mean the baseline firing rate.
And here at D, where the inhibition is largely out but
the center is completely out, there's going to be an increase in firing rate.
And then when it's completely out in the light area,
a decrease back to something that approaches baseline.
So what is all this about?
Well, this is a way of saying that, look,
because cells' receptive fields of neurons in the primary visual cortex
are organized in this particular way that one's known for a long time.
That you're going to have a difference in a firing rate of the cell
as a function of its position across a light dark boundary.
And we're interested in light dark boundaries because they indicate contrast
boundaries, the edges of objects that we're particularly interested in.
Seeing the world, it's well known that we are particularly interested in boundaries.
I showed you that before in talking about the way in which
the eye moves by eye movements all the time.
And you will remember that the eye moves to focus on contrast boundaries,
not on areas.
Remember we looked at a face,
not on areas like the cheeks that don't have much contrast,
they focus on where's there a lot of contrast like the eyes, for example.
So this would be an explanation of this phenomenon