Motion After Effects

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Del curso dictado por Duke University

Visual Perception and the Brain

129 calificaciones

Duke University

Visual Perception and the Brain

129 calificaciones

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.

De la lección

Seeing Motion

Definitions2:29

Phenomena that Need Explaining1:35

Apparent Motion5:15

Motion After Effects3:39

The Inverse Problem for Motion3:27

Perceived Speed: The Flash-Lag Effect7:14

An Empirical Explanation of the Flash-Lag Effect9:57

Perceived Direction: Aperture Effects8:28

An Empirical Explanation12:30

Conoce a los instructores

Dale Purves
M.D.
Duke Institute for Brain Sciences

The second aspect of motion that I'm going to give short shrift to but

that you should be aware of is what are called motion after effects.

Motion after effects and again you'll be familiar with this you've probably seen in

school a demonstration of seeing let's say a red square on a white background.

And the teacher asks you to look at that for, let's say 30 seconds,

then takes the red square away, this could be done with pieces of paper of course,

takes the red square away and you continue looking at the blank white sheet of

paper and you see a green object, a green square in place of the red square.

And it's a very impressive phenomenon.

That's an after effect.

The one then is equally popular is the so-called waterfall after effect.

And it's the same general idea.

Here's a picture of a waterfall.

This happens to be Angel Falls in Venezuela,

if you're curious what that waterfall is, it's the highest one in the world.

But if you stare at this or any moving water, you can do this in your bath tub

for that matter, if you stare at moving water, going down by the force of gravity

for a period, again, of let's say 30 seconds or more.

And then you look away to the wall of the bathroom or

the towel on the wall or whatever it is.

The rocks near the waterfall they will appear to be moving

upwards the opposite direction.

That's the waterfall after effect and

like the red green after effect it's like color after effect, it's quite remarkable.

It's not really in the domain of the main kinds of problems that we

want to consider in motion because in a sense, just as TV and video

are not really terribly informative about half the way we see motion other than

in how we put together time which is kind of a different and mysterious subject.

So the color after effect and the water fall after effect are something that

you would call obviously normal in the sense that we don't stare for 30 seconds

objects, whether they be colored objects or waterfalls.

We just, our eyes are moving around with that I mentioned before,

three or four every second that's the normal way in which we look at the world.

So looking at something for a long time and seeing an aftereffect Is unusual or

abnormal and it has a general explanation and I don't know whether anyone's

ever confirmed this with physiological measurements, I don't think they have.

But the general idea is that neurons in your visual system the retina, thalamus,

Visual cortex that are being activated by the red square, by the moving water,

for 30 seconds, an abnormal period of staring, are in a sense,

let's put it in quotation marks, tired after that period of activity.

And the neurons that haven't been activated,

the neurons that are sensitive to green.

Or the neurons that are sensitive to motion moving upwards instead of downwards

are relatively more active and therefore, you see these after effects.

I think that's a very casual explanation of something that's been studied

in detail or enough physiologically to my knowledge.

And the explanation is probably as good as it needs to be.

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