Overview
Source: Laboratory of Jonathan Flombaum—Johns Hopkins University
One thing becomes very salient after basic exposure to the science of visual perception and sensation: what people see is a creation of the brain. As a result people may fail to see things, see things that are not there, or see things in a distorted way.
To distinguish between physical reality and what people perceive, scientists use the term awareness to refer to what people perceive. To study awareness, vision scientists often rely on illusions-misperceptions that can reveal the ways that the brain constructs experience. In 2001, a group of researchers discovered a striking new illusion called motion-induced blindness that has become a powerful tool in the study of visual awareness.1
This video demonstrates typical stimuli and methods used to study awareness with motion-induced blindness.
Procedure
1. Stimulus
- Doing a motion-induced blindness experiment requires software or a programming environment that can make simple animations and collect keypress responses.
- The basic motion-induced blindness stimulus involves three features: a square made up of bright blue crosses on a black background, a bright yellow disc towards one of the corners of the space occupied by the square, and a white disc in the centre of the screen to serve as a fixation point. Figure 1 shows a single frame of basic motion-induced blindness stimulus.
- Set the square of blue crosses to rotate continuously in one direction, as though the square is pegged down by a fixation point.
Figure 1: A single frame containing the motion-induced blindness stimulus. In the dynamic version, the square of blue crosses rotates around the central fixation point, while the yellow dot remains stationary.
2. Producing the illusion
- To experience the illusion, fixate the white disc in the center of the stimulus, but also attend to the yellow disc as you watch the motion of the blue crosses. The yellow disc should disappear from awareness, even though it never actually disappears from the screen.
3. Running an experiment
- To run a basic experiment, create a slightly different version of the stimulus. Rather than the one yellow disc in the upper left corner of the stimulus, include a yellow disc in the right corner and one on the bottom as well, for a total of three yellow discs. Figure 2 shows a single frame from such a stimulus.
Figure 2: A single frame of a simple motion-induced blindness experiment. Participants are asked to report the number of yellow discs absent from their awareness at each moment.
- Instruct a participant to fixate on the center point while attending to the stimulus widely.
- Tell the participant that the experiment will involve 5, 30-s trials. In each trial they should fixate and attend to the stimulus, and they should report when discs disappear from awareness as follows:
- If one disc is absent, hold down the 'J' key. If two are absent, hold down the 'K' key, and if three are absent, hold down the 'L' key. Adjust which keys are held down as discs enter and disappear from awareness. And when all the discs are perceived, be sure to release all the keys.
- The experiment program should record the participant's key presses.
What we see in our surroundings does not always match the reality of the physical world. Sometimes, our brains actually erase sensory information.
In certain situations, like driving on a busy and narrow highway at night, a driver might find himself staring into oncoming headlights. When this happens, the taillights of the car immediately in front of him can temporarily disappear.
This phenomenon is an example of motion-induced blindness, a perceptual illusion in which the brain discards part of the visual field when motion occurs simultaneously.
In this video, we describe the elements used to create the illusion in a laboratory setting based on the methods of Bonneh and colleagues. We will also determine the frequency at which stimuli disappear and provide additional scenarios where the brain alters awareness of the world.
In this experiment, participants observe a simple animation with three basic features: a square containing bright blue crosses on a black background, bright yellow discs within the orderly pattern, and a centered fixation point.
For every 30-s trial, participants are asked to fixate their eyes on the center point and attend to the stimuli as a whole while the background rotates in continuous motion.
During this time, they’ll report how many of the yellow discs vanish, which serves as the dependent variable. If one or more disappear, motion-induced blindness is expressed.
In this case, the yellow circles are invariant and don’t rotate as they should if they were on the same surface with the moving squares. Consequently, the brain concludes that they must not be real and removes them from awareness, thereby distorting physical reality.
As the first step, verify that stimuli have been accurately animated.
Then, greet a participant in the lab and have them sit comfortably in front of a monitor and keyboard.
To begin, explain that the participant should fixate on the white dot and attend to the yellow discs, while the square of blue crosses rotates. Indicate that the 'J' key should be held down when one yellow disc disappears, 'K' if two are absent, and 'L' for all three. If all objects are perceived, completely release keys.
Go ahead and turn off the room lights to reduce glare and start the program. Note that every participant should complete a total of five trials, each one lasting 30 s, with the yellow discs in a shifted location every time; during these instances, perception may change and the computer will record all responses behind the scenes.
When the participant has finished, thank them for taking part in the experiment.
To analyze the data, compute the percent of time that one, two, or all three yellow discs were not perceived by the participant and graph the results.
Notice that participants saw one disappear more often than two or three. If the brain believes that the dots may not really be there—but is also uncertain—then it makes sense that one will be deleted more frequently than all.
Now that you are familiar with the motion-induced blindness illusion, let’s look at a recent theory of why the brain deletes items from awareness, as well as insights into the functioning of the parietal cortex.
In 2008, researchers New and Scholl developed the Perceptual Scotoma theory to explain why motion-induced blindness happens. They suggested that the human brain mistakes the yellow dots on the screen for scotomas, which are injuries to the retina. People with scotomas should experience an empty space in their visual perceptions, but they do not.
The reason is that the brain learns to discount the empty space caused by the scotoma because it is invariant with respect to the rest of the outside world. That is, it must originate from inside the eye, and as a result, the brain removes the blank space from awareness.
This is also why an individual who wears glasses is not always aware that they are dirty; the brain removes the dirt specks!
In another study assessing conscious perception, Funk and Pettigrew used transcranial magnetic stimulation, or TMS, to investigate where motion-induced blindness is induced in the brain. They found that the disappearance and appearance of stimuli can be modified with TMS pulses to the parietal cortex, an area implicated in visuospatial attention.
By combining motion-induced blindness and TMS in patients with parietal cortex damage, especially those that experience visual extinction, it is possible that a therapeutic procedure could be found to alleviate symptoms.
You’ve just watched JoVE’s video on the motion-induced blindness illusion. Now you should have a good understanding of how to incorporate the elements and run the experiment, as well as how to analyze and assess the results.
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Results
Figure 2 shows typical results from a single observer. The moving blue crosses cause the brain to believe that the yellow discs may not really be there. But the more discs there are, the less the brain seems to trust that intuition. So only one disc is more likely to disappear compared to two or all three.
Figure 3: Percent of time stimuli are absent from awareness. Results are shown from one typical observer. One or more of the discs was absent from awareness nearly 40% of the time, with two or all of the three stimuli (yellow discs) disappearing simultaneously as well, albeit with less frequency.
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Applications and Summary
Motion-induced blindness demonstrates that the brain constructs awareness, and that it can decide what to include there or not. But why does this stimulus cause the brain to believe that the yellow discs are not actually there, deleting them from awareness? One of the applications of this relatively new technique emerges from a theory designed to answer that question.
The theory is known as the Perceptual Scotoma theory, proposed in 2008.2 A scotoma is the name for an injury inside the eye, in particular, an insult to retinal tissue. If a piece of the retina is damaged, the observer should in principle see the consequences in their perceptual awareness. They might see an empty space wherever in the visual field the scotoma is. They don't though. In fact, people are usually not aware they have scotomas at all. It's like wearing very dirty glasses. Often, one only realizes the glasses are dirty when they take them off. Why don't smudges and specs of dirt appear in perceptual awareness? Why don't scotomas as well? The answer is that the brain knows that scotomas are possible. And when it believes some stimulus is caused by a scotoma, it discounts it since it thinks it is not part of the outside world. The way it determines that something is a scotoma is if the stimulation is invariant with respect to the rest of the outside world. In motion-induced blindness, there is a clearly rotating surface-the square made up of crosses-but the yellow discs are invariant; for some reason, they don't rotate as they should if they were on the surface. Therefore, the brain concludes that they must not be, and instead, that they must be inside the eye, an aberration, perhaps caused by an injury. The same principle applies to dirt on someone's glasses. The brain notices that the specs of dirt move wherever the head moves, as they should, only if they are attached to the head in some way. So the brain deletes them from awareness, focusing its interests on what it thinks is in the world outside the observer.
This theory and the illusion of motion-induced blindness has made it possible for scientists to study the ways that the brain compensates and creates awareness when a scotoma occurs, when injury produces imperfections and in the human eye.
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References
- Bonneh, Y. S., Cooperman, A., & Sagi, D. (2001). Motion-induced blindness in normal observers. Nature, 411(6839), 798-801.
- New, J. J., & Scholl, B. J. (2008). "Perceptual Scotomas" A Functional Account of Motion-Induced Blindness. Psychological Science, 19(7), 653-659.
