An illusion new to science shows that the pupillary light reflex, which controls the width of the pupil in anticipation of expected changes in light, depends on the perceived environment rather than the physical reality. Optical illusions aren’t mere gimmicks without scientific interest: researchers in the field of psychosociology study them to better understand the complex processes our visual system uses to anticipate and make sense of the visual world in a far more roundabout way than a photometer device, which simply registers the amount of photonic energy. Now, researchers show that the expanding hole illusion is so good at deceiving our brain that it even prompts a dilation reflex of the pupils to let in more light, just as would happen if we were really moving into a dark area. Researchers explored how the color of the hole (besides black: blue, cyan, green, magenta, red, yellow, or white) and of the surrounding dots affect how strongly we mentally and physiologically react to the illusion. On a screen they presented variations of the expanding hole image to 50 women and men with normal vision, asking them to rate subjectively how strongly they perceived the illusion.
While participants gazed at the image, the researchers measured their eye movements and their pupils’ unconscious constrictions and dilations. As controls, the participants were shown scrambled versions of the expanding hole image, with equal luminance and colors, but without any pattern. The illusion appeared most effective when the hole was black. 14% of participants didn’t perceive any illusory expansion when the hole was black, while 20% didn’t if the hole was in color. Among those who did perceive an expansion, the subjective strength of the illusion differed markedly. The researchers also found that black holes promoted strong reflex dilations of the participants’ pupils, while colored holes prompted their pupils to constrict. For black holes, but not for colored holes, the stronger individual participants subjectively rated their perception of the illusion, the more their pupil diameter tended to change. The researchers don’t yet know why a minority seem unsusceptible to the expanding hole illusion. Nor do they know whether other vertebrate species, or even nonvertebrate animals with camera eyes such as octopuses, might perceive the same illusion as we do.
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