Why do some people or animals freeze when they sense a dangerous situation instead of running away from it? How can movement be created from the use of a still picture? How do we perceive items in a room as being stationary, even if we are scanning the room and the images we receive are moving?
The answers to questions such as these can be answered through the corollary discharge theory.
Movement Perception Requires 3 Specific Signals
The human eye offers four different ways to perceive moment.
- Retinal Motion. This occurs when an object happens to physically move on the retina itself.
- Apparent Movement. It is the stimulus that is provided by stationary items that are placed in slightly different locations. If you close one eye, then close the other eye, the item seems to move even though it is standing still.
- Induced Movement. When one object moves, it can result in the perception of another object moving. If clouds are moving in front of the moon, it can seem like the moon is also moving. This same effect can be obtained while driving at night and watching the moon.
- Motion Aftereffect. If you look at an object with spirals are three-dimensional shaping, the eyes can make it seem like the item seen is moving. This is evidence that there are direction selective neurons. To create balance, the mind attempts to perceive motion in the opposite direction.
Based on these different methods of perceiving movement, the corollary discharge theory shows that movement perceptions are based on three specific signals that are produced by the eye.
1. Image Movement Signals. This is the movement of an image which can stimulate the receptors which are placed across the retinal.
2. Motor Signals. These are the signals which are used to send signals from the eyes to the muscles of the eye.
3. Corollary Discharge Signal. This is a copy of the motor signal.
When the brain sends a command to move the eyes, a copy of that signal is sent to what is referred to as the “comparator.” Then the image motion signal is sent to calculate real motion that is occurring from the motions which is being perceived.
What Happens to the Images that People See?
Let’s say that you want to look a mountain range that is on the horizon. You begin at the right end of the range, moving your eyes along the view toward the left end of the range. When this happens, a motor signal is initiated. That is what causes you to move your eyes toward the right to begin looking at the view.
Then a copy of the signal to look toward the right is sent.
As you’re looking and scanning, your brain is receiving two distinct sets of signals: one from the right eye and the other from the left. The comparator subtracts the leftward image from the rightward signal and this allows you to recognize that the mountain range is not actually moving as you are looking at it.
Now let’s say there is a car coming towards you. This requires your vision to track the option. The same process occurs as when looking at the mountain range, but with one key difference. There is no image motion tracking occurring because you’ve already decided to track the car headed toward you. The comparator once again subtracts perceived movement from the image. Since the item is moving, the brain recognizes that real motion has occurred.
How You Can Test the Corollary Discharge Theory
There are four different ways that you can experience the effects of corollary discharge. Three of them can be tested at home, while the fourth option is one you probably don’t want to have happen to you.
Here are the three tests that you can make at home with the corollary discharge theory.
- Letting your eyes move in the dark. When you give a command to the eye to move in the dark, you’ll see that a bleached patch stays stationary on the retina as the eye moves. This occurs because an afterimage stays in place after the muscle are commanded to move the eyes.
- Pushing on your eye while looking at a spot. This option shows that there isn’t a corollary discharge signal. You’re not commanding the eyes to move. What you are doing is physically causing items to move, which means an image movement signal is created.
- Move your eyes to follow a target. Try to watch a bird in the sky. In this instance, an image movement signal is not generated because you are staying focused on your target. At the same time, a signal is being sent to the eye muscles so that they can continue to track the bird as it is flying.
What is the fourth option to consider with the corollary discharge theory? It occurs when there is a paralysis of the eye muscles. In this specific instance, an image movement signal would not be generated because the eye cannot move. Despite the paralyzed effect, however, a corollary discharge signal is generated because the brain attempts to command the eyes to move.
In the corollary discharge theory, it isn’t the actual movement that is important. It is the signal which is sent to the eye muscles to get them to move in the first place.
What Is the Evidence for the Corollary Discharge Theory?
We can see that the proposals in the corollary discharge theory are accurate because of direct observation. When damage to the medial superior temporal area occurs within a human, it leads to the perception of movement when looking at a stationary object if the eyes move.
We also know that the real-movement neurons that monkeys have will respond only with some stimuli moves. They no not respond if the eyes of the money move. This allows us to understand that nearly all neurons are direction-selective.
What makes the eye think that something is moving when it really is not are the times when there is a 0% coherence in place. There is a preferred direction that the neurons travel, so any series of images which follows that preferred direction will cause a perception of motion. That is why looking at a series of increasing or decreasing dots, spirals, or similar objects leave the eye with an impression of movement, though none exists.
The corollary discharge theory shows us that eye movements and what is being seen are directly related to each other. What the eye sees causes a signal to be sent, which initiates muscle movement, which then alters what is being seen. Then the process repeats itself throughout each day within every environment.
It is a process which allows us to understand what is safe and what is not safe, allowing us to move out of the way instead of being frozen in one’s tracks.