Have you ever wondered how your visual system interprets complex motion perception information? To understand this phenomenon, let’s consider three different situations that produce three unique experiences on the retina.
Stationary Viewer—Stationary Environment
First, consider a stationary viewer, looking at a stationary environment, in which one object is moving—a person walking across your field of view. Our perception is obvious in this situation: The world is stationary and the man is moving.
Consider a second situation, however, in which the entire background seems to move leftward across the retina.
There are at least two possibilities for a sensory input like this: It could be that everything—the whole room—is moving to the left; but if you watched a movie of this, that wouldn’t be your perception. You would perceive it as a stationary world with a rightward moving point of view.
You produce this type of display yourself whenever you move your head to the right. In most situations, our visual input and the movement of images projected onto our retina are a combination of these two types of inputs: The motion of things in the environment and the motions of our retina through the environment.
Now consider a third situation, one in which a man is walking, but he remains stationary in the middle of the retina.
This is a transcript from the video series Understanding the Secrets of Human Perception. Watch it now, on The Great Courses Plus.
As he remains in the middle of the retina, however, the entire background drifts leftward across that retina. You produce visual inputs like this yourself whenever you follow a moving target with your eyes.
Just because something moves across your retina doesn’t mean that it’s actually moving in the world. Conversely, just because something is stationary on your retina doesn’t mean that it’s actually stationary in the world.
What’s moving and what’s not?
How do we figure out what’s moving and what’s not? At least part of the answer to this question has to do with a close linkage between the motor systems that move our eyes and the sensory systems that process visual information.
When your brain sends a command to your eyes, telling them to move from one place to another, it also sends a copy of that command to your visual cortex. This copy of that motor command is a corollary discharge. By subtracting out the visual motion caused by your eye movements, your visual cortex is then able to figure out what’s moving and what’s stationary.
There are at least two good sources of evidence of corollary discharge. You can do one of the experiments right now on your visual system. Close one eye, and with your index finger, reach up with your finger and gently press on your lower eyelid. As you do, you will cause your eye to jiggle a little bit as you press on it.
This isn’t how you usually control your eye movements (with your finger), so there’s no corollary discharge—no copy of the message—sent from your arm onto the visual cortex. Since there’s no corollary discharge signal to tell the visual system to subtract out the motion caused by this eye movement, you’ll notice a very particular perception as you press on your eye: The whole world will seem to move up and down!
As you move your eyeball down a little, the projection of the world shifts up. If that happens without your eye moving, it would mean that the world has moved up, and that’s what you perceive.
Muscles Provide the Capacity to Learn?
There’s a famous experiment—one conducted in a medical laboratory—that nails this corollary discharge phenomenon.
In the 1950s, American psychology was dominated by B. F. Skinner and the behaviorist school of thought. One of the tenets of their theoretical approach was that no thought, perception, or learning can take place without involving the body itself.
To test this, a volunteer participated in a somewhat frightening procedure. He agreed to be put on a ventilator while he was injected with drugs that paralyzed his motor system. He needed to be on the ventilator because lungs were paralyzed as well.
If the body was needed to control thought and memory, then his thought and memory should have ceased as well until the drugs wore off.
During the procedure, other experimenters showed him images, words, and math problems. Later on, they planned to test if he’d been able to recall what they had shown him and the answers to the math problems once the paralysis had worn off and he was able to speak again.
For what it’s worth, the man was able to think and remember in this state. We now know very clearly that thought can take place without the muscles being involved.
But something else unexpected happened that the subject reported after the experiment. As soon as he became paralyzed, he noticed that the world seemed to jump around wildly, up and down, and side to side.
Eventually, he figured out that this happened whenever he tried to move his eyes. Remember that his muscles were paralyzed, including the muscles that control his eyes, but the corollary discharge system within the central nervous system was still active.
When he tried to move his eyes, a signal was sent to his visual cortex telling it to expect a particular eye movement. When it didn’t take place, the visual cortex inferred that the objects in the environment had moved in the same direction as the eyes.
Think this through for a moment: If you moved your eyes 10 degrees to the right, and if the world around you rotated 10 degrees to the right as well at that same moment in time, what would you see? You would see the same thing before and after the eye movement, right?
Twelve degrees up with the eyes and 12 degrees of world movement; same visual input before and after the eye movement. This paralyzed man had this exact experience. No matter where he moved his eyes, the visual input didn’t change.
The only inference that the visual cortex could make was that the world must be moving in exact synchrony with the man’s eyes—that’s what the visual system inferred and so that’s what the man perceived.
Common Questions About Motion Perception
The primary cells in the eye that resolve motion perception are the rods and the cones. The rods, which are largely in the periphery, detect light very well and detect motion much better than the cones, which are better at visual resolution.
Motion aftereffect is a function of motion perception where neuronal structures wind down after dealing with a triggering motion. They are still functioning after we see the initial motion generator and it takes a period of time for them to reset.