Optical illusion

What does this optical illusion teach us about our brain. The black spot that seems to actually get bigger - guess what? - is still, but deceives our eye pupils.

If looking at the image above you have the impression that the black spot in the center is enlarging to the point of sucking you in, do not worry: the image is actually fixed and the hole in its center is not increasing in size, even if ours brain would like us to believe otherwise. In addition to being a curiosity to share with friends, optical illusions like this offer research groups important insights to better understand how our brain works and how we see the world, or at least think we see it.

The expanding black spot, for example, was the subject of recent research recently published in the scientific journal Frontiers in Human Neuroscience . The illusion was shown to 50 men and women without sight problems, while the research group detected with a particular instrument the movements of the eyes and in particular of the pupil, the small hole that allows the passage of light inside the eyeball.

The analysis revealed that the people who see the effect of the apparently expanding black spot more than others are also those whose pupils expand the most when they look at the image. The study also found that about 14 percent of the people involved saw the illustration for what it was: a static image with a dark spot in the center that was always the same size.

Your pupils are constantly dilating and narrowing, yours are doing as you read this article, to adapt your vision to the amount of light around us. In low light conditions, the pupils dilate to try to let in as much light as possible, while they shrink when there is a lot of light, for example when we are outside on a sunny day.

In the case of the optical illusion, the spot in the center is not getting darker nor are other lighting conditions changing, but the perception that it is expanding is due to how our brain sees things and causes the pupils to respond in unexpected ways. , writes the research team in the study. As one of the authors explained to the New York Times : “There is no reason why the pupil should change in this situation, because nothing is changing. But something has clearly changed in our minds. "

The mechanisms that determine this reaction, as well as those to other optical illusions, are not completely clear, but the research nevertheless exposes some hypotheses. The vision of the image has that effect because the way it is made, with a gradient that becomes darker and darker, induces a sensation similar to the one you get when you go from a bright place to a darker one, like a gallery. without lighting. The impression is therefore that of a darkness that progressively envelops us, and hence the feeling that the black spot is widening.

Our brain works by processing signals and detecting differences, then referring to previous experiences with similar characteristics. Observing the image recalls the sensation you get when you enter a dimly lit room, and from this comes the effect of seeing the image enlarge as if you were moving into that new environment.

Human beings, like all other animals, do not have systems in their organisms for measuring external stimuli and what is happening around them with great precision. Our eyes, for example, do not measure light as a camera would do by returning a precise data: they collect much more vague information, which is then transmitted to the brain where it is processed on the basis of other data collected by the other senses and experiences. . The result in this case is what we call vision and which has many more subjective elements than we imagine.

It is this subjectivity that causes the different perception of the “expanding black spot” effect of the image, and causes some people to see no expansion or motion effect. This is also the reason why some people are more prone to the effect when it is played with a background other than white. For example, in their study, the research team reported that the effect is most frequently seen when the background has magenta as the color.

In a certain sense, the stimuli our brain almost always responds to by trying to guess, trying to get as close as possible to the best solution. This system works in most cases and allows us to have, for example, the right coordination to drive or even more simply to remain standing without losing balance, but in some circumstances some contradictory stimuli - such as those deriving from an optical illusion - they can break the mechanism or make it work less efficiently.

The research team working on the image of the expanding black spot also speculated that the brain tries to predict the future when it receives the information about the illustration. The visual stimulus takes a few fractions of a second before reaching the brain, which will then have to process it and figure out what to do with that information. At the end of this process, however, other things have already happened around, so there is a minimum delay between reality and what we can perceive.

The hypothesis is that our mind tries to compensate for this delay, trying to predict what may happen in the next moments, then finding confirmations or contradictions when the new data arrives. This ability can be essential when dangers arise, for example, which require you to respond very quickly to avoid the worst. And you never know what you might encounter in the dark in a tunnel.