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.
