That matching of corresponding image points in the two eyes
is not simply explained by anything that anyone has been able to come up with so
far and we'll go into that in a minute.
But first let's understand in a little bit more detail how and
why the images of points that are near or
further away, project differently onto your two retinas.
That difference is called binocular disparity, and let me explain it to you.
So these black dots are the point of fixation.
These, in both of these right and left panels,
are indicating the point that your two eyes are looking at.
And that of course means that your center of your fovea,
the center of your line of sight, is in alignment with these points.
Two eyes are converging on these points.
Those points that you
are converging on that are generally in you
with perception of the same depth are called points on the horopter.
The horopter is a fancy Greek word but it simply means all the points
that you see as being equidistant in depth from you, the observer,
when you're looking at a specific point like the black point in these diagrams.
So there are a whole series of points on the horopter that look to you
as if they're the same depth.
And you can hold a pencil out here, look at it and
judge yes there's points here that will be in the same depth as the pin,
there are points here that will be in the same depth.
That's going to form a plane in space.
It's actually a very complex plane,
when people have actually studied this psychophysically.
But you get the general idea, that there is a plane in space, complex
though it may be that defines all the points that you perceive as equidistant
from the point of fixation, from the point that you are actually focusing,
converging your two eyes on because it's a point of interest to you at that moment.
Now what happens when you're focusing on a point on the horopter, like this.
And other points are newer or further from the horopter.
So again, horopter points perceived as equally distant.
So whereas these points in the dotted lines,
fall of course, on the center of the fovea.
The points that are away from the dotted lines.
So this point projects to points on the two retinas p and
q here that are away from the foveas.
And importantly, they're away in opposite directions.
So p in this retinal image and q in this retinal image.
Are displaced oppositely.
They are both displaced towards the temporal side of the eyeball or
the visual field.