How does this optical illusion fool our brains? A study reveals everything!

If you have the impression of entering a tunnel while looking at this image, it is because it is specially designed to fool your nervous system by exploiting a simple reflex.

Optical illusions are often fascinating; in addition to being entertaining for ordinary mortals, they are also very interesting experiences that allow you to explore the hidden mechanisms of vision and the nervous system in general. Recently, Norwegian researchers unlocked the secrets of a new optical illusion and explained to us what happens in our brain.

The illusion in question consists of a large black ellipse with blurred borders. It takes place in front of a background made up of smaller ellipses. And if you stare at the center of the image, you should quickly have the impression that the central ellipse is getting bigger, a bit like you’re getting closer to a tunnel.

This curious phenomenon seems to work for the vast majority of observers. But why is this the case? The researchers have their own idea. According to them, everything is played out at the level of a reflex which is supposed to intervene when the brain perceives a change in luminosity.

A problem of accommodation

It all starts with the retina, the layer of cells that lines the back of the eye to collect a light signal and convert it into an electrical signal. This structure is extremely powerful, as evidenced by the relative sharpness of human vision. But she is also very delicate. It only works properly under very specific conditions. Everyone knows that it is difficult to see clearly in the middle of the night and that staring at the Sun for a long time is a very bad idea.

To achieve the level of versatility they enjoy today, our eyes had to adapt. Over the course of natural selection, we have developed lots of evolutions that have accompanied the development of the retina. These are largely physical structures such as the eyelids which can close, or the diaphragm which allows the quantity of light which reaches the retina to be modulated; we then speak of accommodation.

The pupil dilates when the light drops…or when the brain senses it does. © Egot Vikhrev

A reaction to an imaginary darkness

However, these mechanisms are for the most part autonomous; no matter how hard you try to dilate your pupils in thought, you are not ready to succeed. On the other hand, you can achieve it very easily by closing your eyes.

When the retina detects a drop in light intensity, it automatically triggers a reaction in the autonomic nervous systema branch of the nervous system that manages all actions”automatic” like digestion or heartbeat. This results in a spontaneous reaction of the eye; the diaphragm opens to deliver just the right amount of light to the retina, no more, no less.

And it is on this same principle that this illusion works according to the researchers. “This hole is a highly dynamic illusion”, explains Bruno Laeng, a psychologist at the University of Oslo. “The illusion tricks the brain into thinking it’s facing a change in brightness that doesn’t really exist”, he specifies.

Indeed, the pattern of the image is specially calibrated to fool the ciboulot. As soon as the observer lays his eyes on this large black spot, the nervous system determines that he is facing the entrance to a dark area such as a tunnel or a cave. The patterns in the background are used to trick the brain into believing that it is moving towards this hole.

For the brain, the message is therefore very clear; the luminosity is about to decrease, and it is thus necessary to dilate the pupil in prevention of this change. But the change in question never happens, since in reality the luminosity has not changed at all. It is this inconsistency that gives the impression that the “black hole” is growing visibly. “This shows that the pupil reacts to the way we perceive light, even if it is imaginary”, sums up Laeng.

Still a few mysteries to unravel

From now on, the psychologist’s team will be interested in another equally interesting question. If this illusion works for the vast majority of people, some observers (about 86% during the experimental protocol) are completely insensitive to it; when they look at the image, they see nothing but a set of inanimate pixels.

A surprising inconsistency, since this illusion is based in a reflex anchored very deeply in the nervous system that all humans have. This suggests the intervention of another optical or nervous mechanism that the researchers will try to identify.

It could also be a simple error of interpretation on the part of the subjects. But the question remains just as interesting in this case, since the entire experience is based on the notion of perception. To try to see more clearly, they will therefore test this same experiment on animals, which will allow the interpretation to be pushed even further.

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