Optical illusion dilates the pupils – wissenschaft.de

When looking at this picture, do you notice the black center widening, as if you are entering a dark tunnel? Scientists say 86 percent of people think the image has this effect. Her more detailed study of this perceptual phenomenon shows that the optical illusion even produces a reaction in the eye: although there is no actual change in brightness, the pupils dilate to let in more light, as if it were actually getting darker. This explains that the pupils are not only passively adapting to incident light, but are controlled in a complex way, the researchers said.

We see movement where there is actually no movement: cogs that seem to rotate, areas swaying in the corner of the eye … Although they are actually static, certain patterns in the images give the impression of movement or change. These optical illusions may amaze us – but they are more than just a curiosity or a trick: perceptual phenomena can give scientists insight into how people respond to visual stimuli and create a picture of their environment. In this context, researchers led by Bruno Laeng of the University of Oslo are now looking at an interesting form of optical illusion that has yet to be scientifically described: the “expanding hole”.

Apparently widening darkness

The dynamic illusion is created by a circular shadow spot that turns black towards the center and is presented against a background of small dots. “For most people, this image has a strong optical flow sensation – as if the observer is entering a dark hole or tunnel,” says Laeng. As part of their research, he and his colleagues further investigated what factors lead to the phenomenon of perception and how the eyes react to an optical illusion. They presented 50 subjects with normal eyesight with different variants of the expanding hole and asked them for a subjective assessment of how strongly they perceived the illusion of movement. While the participants looked at the photo, the researchers also recorded unconscious narrowing or dilatation of the pupils with an eye tracker.

First of all, it turned out that the effect was strongest in the standard model with the shadow turning to black in the center. 86% of respondents saw an illusory expansion in this version. However, about 80 percent also noticed the effect when the hole had a white or colored shadow gradient with a bright center. In this case, it gives the impression of moving in an increasingly brighter area. However, the subjective descriptions of the subjects showed that the strength of the illusion varied significantly between individuals in all cases.

Students react in the same way as to changes in light

The perception of the illusion was reflected in the eye’s responses in an interesting way, the team reports further: researchers found that black holes caused participants’ pupils to dilate, while colored holes constricted the pupils. In the case of black holes, Laeng and his colleagues were also able to show that participants’ pupil diameter increased as they subjectively assessed their perception of illusion. What emerges is that the optical illusion prompts the brain to induce a change in the pupils to adapt to the expected changes in light, just as it does when we are actually moving in regions of new light.

“Here, using the ‘widening hole’ illusion, we show that the pupil reacts not only to the amount of light energy that actually reaches the eye, but also to how we perceive light – even if that ‘light’ is like in an illusion. it’s just a delusion, ”says Laeng. “The pupil dilation or constriction is therefore not a mechanism, such as a photocell, which triggers the opening of the door, for example. Rather, the eye adapts to perceived and even imaginary light, rather than just physical effects, “the scientist says.

For now, however, an interesting question remains: why does the minority appear reluctant to cheat? Scientists say there is further research potential. According to them, it would also be interesting to explain to what extent different species of animals perceive the same illusion as we do. “Future research may discover other kinds of physiological or physical changes that could shed light on the effects of the deception,” says Laeng.

Source: Frontiers, article in the journal: Frontiers in Human Neuroscience, doi: 10.3389 / fnhum.2022.877249

Leave a Comment