The other phenomenon here is the equidistance tendency, again,
the physical object is the blue and
the perceived distance of the object is the red position.
The subtlety of this phenomenon is that when you are looking at something above
eye level, remember this one was at eye level, but
when you're looking at something above eye level, the perceptual distance, again,
seems to be further than the physical distance.
And when you're looking at objects below eye level,
the perceptual distance is oddly nearer than the physical distance.
These are subtle phenomena and they are a challenge to explain.
This one and this one are basically opposites and
how are you going to explain that?
Well, in all of these cases, like other aspects of visual perception that we
have been talking about, not only what we see does not correspond to
physical measurements but the implication and it's particularly obvious I think in
thinking about these queues to monocular depth, is that we learn these things.
So I think it's obvious if we go back to these examples that we didn't come into
the world knowing about perspective, we didn't come into the world knowing that
a near object occludes an object that's further away.
We didn't come into the world knowing you have things that are at a distance
are going to be fuzzier and bluer-looking and things that are nearby.
We learned all these things and
as we learn them, we use them as cues to judge distance.
In this more subtle case that presents a more specific challenge to
psychophysicist, seems that we also learn these cues.
But if you want to say and explain how we do them, you need to go back and get some
data as to how we actually experience the world when we look at eye level,
when we look above eye level, when we look at objects below eye level.
Well, that's not hard to do.
We can use our old friend, the laser scanner that we talked about before and
easily find out in this kind of analysis of scenes that
are reported digitally in an ordinary photograph and
compared to the laser range scan image.
Which you remember tells us the distance and
direction of every point in the scanned scene here,
we can get the information that shown in this next graph.
This is what's called a Contour plot and
it's a distribution of the distances from the image plane from the laser scanner.
As a function of elevation whether you're looking at the eye level, above our level,
below our level.
And on this axis are shown the probabilities of the images that you're
going to get in those three different circumstances, color-coded in terms of
their distance as a function of the angle at which you are looking at the world.
So here is the angle of elevation.
This would be eye level at zero and this would be looking up,
this would be looking down and you can see that there is a difference.
The frequency of occurrence of what you see in terms of the distance of objects
when you look at eye level, looking up, looking down, it's not all the same.
I mean, if you think about it, you might very well expect that, but
that's in fact what you see.
And when you make these kinds of graphs, you can predict what one should see.
Looking up, looking down, looking at eye level, and the outcome of that,
as in the outcome of so many of the other things that we've talked about at this
point in the course, is that [COUGH] you can predict very well the phenomenology of
the specific distance tendency and the equidistance tendency.
These rather subtle things and show that they are also learned.
You can predict what we see on the basis of the frequency or occurrence of
images that we experience just as we predict aerial perspective,
and motion parallax, as well as occlusion.
All of those things are also, in a more simple minded way,
things that we learn that we could predict if we
generated statistical information about those phenomena, as well.
About those distances, as well.
So let's end the lesson by coming back to
this slide of these more obvious cues to depth.
The bottom line of all of this is that we seem to
have learned these monocular cues to depth.
And they're very effective.
Let me just remind you if it's not obvious to you that when you close one eye
you don't have a particularly hard time judging distances and
people with one eye get along quite well.
There are even professional athletes who have gotten along
reasonably well with one eye.
And there are numerous people in the world with their injury have only one eye,
or not.
They can drive, they can participate in sports, they can do all kinds of things.
There's not much they can't do.
We'll talk about what they can't do in a minute when we get to binocular vision.
But the point is,
let's say with one eye using this information that we learned in life,
through our experience, is very effective in letting us get along in the world.
The big problem that people have with one eye, and
you can easily demonstrate this for yourself, that if you close one eye and
take a finger, and you see it here, you see it here, you see it here.
Here, it disappears with one eye, with two eyes, of course,
it doesn't disappear until it circles, or half-circles,
the full 180 degrees in the [NOISE] front of the visual space that confronts us.
People with one eye can only get to the point where
the nose blocks the view to the side of the closed eye.
