Motion-Induced Blindness
A scotoma describes a part of the visual field that does not contain input from photoreceptors. Humans have scotomas where the optic nerve exits the back of the retina to bring signals into the brain. Despite having these scotomas, the visual scenes that we view usually appear complete. In other words, we do not notice our scotomas. This lab activity demonstrates how the brain compensates for parts of the visual field that do not contain information.
Blind Spot Demonstration
You may have not heard the word scotoma before, but you may have heard the phrase “blind spot.” To demonstrate to yourself that you have a blind spot in the corner of your eye, you can do the following. Take a piece of paper and draw a small X on the right side. Measure about 12 centimeters (about 5 inches) to the left of the X and draw a circle the size of a dime (1 centimeter or 1/3 of an inch) at that location. Hold the paper in front of you and close your right eye. Look at the X and you should be able to see the dot out of the corner of your eye. By slowly moving the paper in front of you, the dot will seem to disappear at a certain point out of the corner of your eye. You have found your scotoma, or blind spot. You can find the blind spot for your other eye by rotating the paper 180 degrees and repeating the last two steps.
We all have scotomas caused by the optic nerve leaving the eye. Some people have scotomas caused by damage to the retina or damage to the visual cortex due to brain injury like a stroke. Despite these scotomas the brain is able to construct what appears to be an intact, complete scene by removing the scotoma from awareness.
How the Brain Compensates for Scotomas
In the first part of this activity, you will be presented with a movie stimulus. Focus on the flashing green circle in the center of the stimulus for the duration of the trial. You can press the play button to begin. What did you notice happen to the yellow dots as you looked at the green circle in the middle?
In the second part of this activity, you will be presented with a similar stimulus. As before, press play to begin and focus on the flashing green circle in the center of the stimulus for the duration of the trial. What happened this time to the yellow dots as you viewed the stimulus?
Motion-Induced Blindness
In the first part of this activity, most people see the yellow dots disappear when shown as part of a stimulus that is rotating. But they do not see the yellow dots disappear in the second part when they are shown in the context of a stationary stimulus. Since the yellow dots only disappear in the context of a moving stimulus, this phenomenon is called motion-induced blindness (MIB). The dots that vanish are still sensed in MIB, but perception of those areas is prevented from reaching consciousness.
Is Motion Needed for MIB?
The next part of the lab activity addresses whether motion is actually needed for MIB. You will be presented with another stimulus. This stimulus contains a green circle in the middle, surrounded by a circle of circles with arms of circles radiating outward similar to rays of a sun. As before, press the play button and focus on the flashing green circle in the center. What did you see?
Interpreting What You Perceived
People do not notice their scotomas despite having parts of their visual field where there is no input. New & Scholl of Yale University explain MIB by suggesting that when the visual system detects an element like the green center stimulus that is invariant in the presence of changes occurring in a large peripheral region of the visual field, the visual system may expunge, or get rid, of the stimulus from awareness. That humans ignore stationary elements of a moving stimulus could be an adaptation that helps compensate for parts of the visual field that do not contain information, such as those that have blind spots. In the last part of this activity, you may have noticed that stimuli do not have to actually move to produce MIB. They just need to be changing. In the last part of the activity, the change was not produced by motion but by increasing and decreasing luminance (i.e., brightness) of surrounding dots. When the majority of stimuli in the scene are changing, unchanging or stationary stimuli (e.g., the yellow dots) will be treated like scotomas and removed from awareness.