Movement Perception

The illustrations that, undoubtedly you have been looking at demonstrate that motion perception is very complex. Recall that we perceive motion if we hold our heads and eyes still as a moving object passes in front of us. If we decide to hold our heads still and let our eyes follow the object we still see it move. Finally, we could even decide to hold our eyes steady and move only our head to follow an object. The interesting thing is all three modes of viewing a moving object result in about the same perception.

So far we have been concerned with perceiving real movement. By real movement I mean that the physical stimulus is actually moving and we perceive it as moving. It is possible to perceive motion when the stimulus is not moving. An example is the motion after effect (MAE)demonstration that was loaned to me by Dr. Ben Bauer, Trent University.

Here is a demonstration you can observe for yourself. If you have the opportunity to view a waterfall, (e.g.. Niagara Falls) look at the falling water for about a minute and then allow your gaze to fall on any stationary object. A building would be excellent. If you do this, the texture of the building, perhaps even the windows will appear to move up. Waterfalls usually are not readily available. However, you can easily build your own MAE apparatus. Take a round paper plate. Draw a dozen or so heavy lines radiating out from the middle of the plate. Then with a pin attach the plate through its center to the eraser end of a pencil. Now spin the plate at a moderate speed. Don't spin it so fast that the lines become an indistinct blur. After viewing the spinning plate for about a minute stop it and continue to look at the radiating lines. What do you suppose you will see? If you see what most people notice the radiating lines, which are actually stationary, will appear to rotate in the direction opposite to that which you spun the plate originally. If that is way you saw you witnessed the MAE. It is useful to try this demonstration with the paper plate because it will convince you that there are no special tricks involved with the MAE demo I mentioned above.

The phenomenon of Motion After Effects (MAE) has been studied intensively by visual scientists for many years. One explanation of how the MAE works is the following.  The visual system has motion detectors that, like most neurons, undergo spontaneous activity.   You normally do not see motion when there is none because the spontaneous activity is in balance.  However, when you viewed the downward motion of the black bars you adapted the motion detectors for motion in the downward direction.  When the real motion stopped, the spontaneous activity was no longer in balance, the upward spontaneous activity being slightly stronger and thus the black bars appear to drift upward.   The adaptation effect lasts  for a short time,  the motion detection system quickly becomes balanced again and the apparent movement stops.

Another example of motion being seen, when there is no physical motion, is the phi phenomenon. To those unacquainted with the field of vision research this phenomenon is probably unknown. However, all of you have seen it. The simplest demonstration of the phi phenomenon is to have two illuminated spots of light about 6 to 8 inches apart. When these lights alternately go on and off one usually sees a single spot of light moving back and forth.

This principle is used in many movie marquees where one sees a pattern of lights moving around the display. In fact, there is no physical motion, only a series of lights going on and off. Then, of course there are the movies. Movies are a series of single frames presented in rapid succession. No one would doubt the perception of movement seen in the cinema. Yet, if you analyze the strips of film that yield these images all you would see is a series of frames each with a slightly different image. When they are rapidly projected on to the viewing screen motion is seen.

A similar technique is used with cartoons. The illustrator actually draws a series of pictures. When they are rapidly presented to the viewer motion of the cartoon characters is seen.

There are two other instances when movement is perceived. Have you ever sat in a train or bus station patiently waiting to get moving? Then all of a sudden, low and behold there you go. Or are you? You feel no vibration, something feels wrong. Then you notice that it is the vehicle (train or bus) right next to you that is moving and it just felt as if you were moving. This is called induced motion.

Finally, (and this is an experiment you can try at home) view a small very dim light in an otherwise completely dark room. Make sure that the light is in a fixed position and not moving. After sometime in the dark, the small light will appear to move somewhat randomly. This is called autokinetic movement.

Here is another little experiment you can try. Look around your surroundings freely moving your eyes. As you move your eyes around are the stationary objects moving? Probably not. Now look at some object and with your finger rapidly press against your eyeball by pushing on your eyelid. (Don't push directly against the white (sclera) area). As you force your eye to move you will probably notice that whatever you are looking at starts to jump around. So you can see that it makes a difference whether you move your eyes normally or cause them to move in an unusual manner.

Electrophysiologists are scientists who insert tiny electrode into the brain of experimental subjects. They have discovered that there are cortical neurons which are specialized for movement. In fact, these neurons often are so specialized that they will respond best when the motion is in a specific direction. E. Bruce Goldstein presents a neural model in his textbook, which shows how the early retinal neural processing could occur which results in a signal being sent to the brain which say that movement has occurred in a specific direction.

Table of Contents
Table of Contents [When not using frames]