Learners will be introduced to the problems that vision faces, using perception as a guide. The course will consider how what we see is generated by the visual system, what the central problem for vision is, and what visual perception indicates about how the brain works. The evidence will be drawn from neuroscience, psychology, the history of vision science and what philosophy has contributed. Although the discussions will be informed by visual system anatomy and physiology, the focus is on perception. We see the physical world in a strange way, and goal is to understand why.
Perceived Direction: Aperture Effects
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Del curso dictado por Duke University
Visual Perception and the Brain
142 calificaciones
De la lección
Seeing Motion
Conoce a los instructores
- Dale PurvesM.D.
Duke Institute for Brain Sciences
So know I emphasized at the outset of this module, that there were two parameters
that define motion, either objectively or subjectively, speed and direction.
So the flash lag effect is an opportunity, there are other phenomenon,
that can be explained in this same way.
But it's an opportunity to consider how empirically one can understand
the discrepancy between the speed in the world, or on the retina, and
the speed that we actually see.
In this lesson, we're going to consider the perceived direction.
Again, using a particular phenomenon that I mentioned earlier, aperture effects.
And now let me explain what aperture effects really are.
So what you're going to see in this panel, right now it's just a black background but
what you're going to see is a line moving from left to right across the image plane,
and then super imposed on that line is going to be an aperture.
An aperture just means a limited space through which you see something.
You can imagine this is being a peephole in the case of a circular aperture.
We're going to talk later about a vertical slit.
That's another kind of aperture.
So there are an infinite number of different types of apertures that one can
play with.
But we're going to impose a circular aperture on this and
you're going to see how the direction of the line
moving from left to right is altered by the application of the circular aperture.
A very significant amount about 45 degrees so that instead of seeing the same line,
the real line is moving off to right.
Instead of seeing that line moving left to right you're going to see it
moving downwards into the right.
So let's look at that.
I'll show it a couple of times so that you get the idea.
Here's the line moving left to right.
Now exactly the same line moving left to right, but now were seeing it through
a circular aperture, we're seeing it through a circular peephole.
Let's look at that again.
Line moving left to right, circular aperture,
and the direction of the same line is now shifted significantly.
You originally saw the line without the aperture moving this way.
Now you see it moving down and to the right when the aperture is applied.
So this is a phenomenon that's again been known for a long time and
the person who made this famous was psychologist named Hans Wallach.
A very interesting guy who put this phenomenology of aperture effects
into play in psychology in the 1930s.
So Wallach was a graduate student in Germany, in Berlin,
in 1934 when he was doing his PhD thesis on this subject.
And the apparatus that he had then was, of course,
not what we have today, that I just showed you, generated by computer.
But he used assistants to actually move rods behind a circular aperture and
tested people how they saw it and described these phenomenon.
He moved to the United States in 1935.
He was aware that bad things were going to happen in Nazi Germany and
he immigrated to the States to Swarthmore College which is a small liberal
arts college outside of Philadelphia where he spent his career and did many things.
He was a very productive psychologist.
But he is the major figure in
describing and beginning to understand aperture effects.
He didn't have a good explanation for them, but
he certainly described them in detail.
And what he showed was that when just as with what we saw in the computer screen,
is that when the bar was moving from left to right, carried by or
moved by one of his assistants behind the screen.
It was seen moving downwards into the right even though it's real
course of motion was left to right just as we saw.
And he was aware that in fact many different
directions of motion could satisfy this aperture and what I mean by satisfying
it is just moving across this aperture without revealing the ends.
If you had a bar moving across this screen and
one of the ends was revealed, well, you would immediately see it moving left or
right and would no longer see the aperture effect because the aperture effect
depends on having the ends of the object,
the ends of the bar, or the line in this case, being occluded.
So he was aware about an object moving in any one of these direction
over this semicircle could satisfy the aperture.
They would have to be of different lengths as we get to in a few minutes.
But there are many speeds and directions of motion that could satisfy,
create this appearance or this reality of a line moving across
the aperture of that is perceptually seen in this strange shifted way.
So this was Wallach's contribution, and he studied different kinds of apertures.
And this is just one of his
efforts in his thesis work, which we're talking about now.
So a circular aperture wasn't the only aperture that he and
many others since to have tested and
then I'm going to show you an example of another aperture, a vertical aperture,
that's causes a very different phenomenon that's equally challenging to explain.
How do you explain the perception of motion when in this case it's
even more radically discrepant than the object seen through the circular aperture.
So here again, we're going to see a line moving left to right across the screen.
But now, the aperture is going to be a vertical slit instead
of a circle that we've just been talking about.
So same general idea but then I look at the direction of motion that you see in
response to the application of a vertical aperture instead of circular aperture.
So line again, moving left to right, in this direction vertical aperture,
and now the line is seen moving down.
This is really, now we've shifted the apparent direction of motion by
90 degrees, this is not a trivial effect.
I think if you think about it you would probably be familiar with this kind of
effect because its called the barberpole illusion in psychology textbooks, and
those of you who've seen a barberpole or blue and
red stripes of a barberpole, they are doing the same thing.
You see the blue and red stripes moving up and down.
But the real direction of motion of the stripes is left to right.
The barberpole itself, in that case, is the equivalent of the vertical aperture.
So the problem that confronts us is how do you explain
these phenomena, how can you explain these aperture effects,
which are again, major shifts in the apparent direction of
motion that you see depending on how you're looking at it, for context?
Just as the context affected lightness, brightness, color, geometry.
So the context is critically important here and affects the motion we see for
very similar reasons.
So the challenge in the next lesson is going to be, how can you explain this?
Can you explain it empirically in the same terms that we used to explain the flash
lighting effect which is a phenomenon that concerns the speed of motion.
Now, we're talking about the direction of motion.
Again, the same discrepancies exist,
can you explain these discrepancies in similar terms?
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