Cecità indotta dal moto
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Overview
Fonte: Laboratorio di Jonathan Flombaum—Johns Hopkins University
Una cosa diventa molto saliente dopo l’esposizione di base alla scienza della percezione visiva e della sensazione: ciò che le persone vedono è una creazione del cervello. Di conseguenza, le persone potrebbero non riuscire a vedere le cose, vedere le cose che non ci sono o vedere le cose in modo distorto.
Per distinguere tra la realtà fisica e ciò che le persone percepiscono, gli scienziati usano il termine consapevolezza per riferirsi a ciò che le persone percepiscono. Per studiare la consapevolezza, gli scienziati della visione spesso si affidano a illusioni-percezioni errate che possono rivelare i modi in cui il cervello costruisce l’esperienza. Nel 2001, un gruppo di ricercatori ha scoperto una nuova sorprendente illusione chiamata cecità indotta dal movimento che è diventata un potente strumento nello studio della consapevolezza visiva. 1
Questo video mostra gli stimoli tipici e i metodi utilizzati per studiare la consapevolezza con la cecità indotta dal movimento.
Procedure
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.
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.
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.
Transcript
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.
Thanks for watching!