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Optical illusions reveal how the brain works

Apparatus attached to thumb and forefinger allow psychologists to measure the grip aperture of subjects as they reach for the center circles of an Ebbinghaus array. Courtesy Peter Vishton. Common optical illusions are being used as tools to explore how the human brain works. A group of William and Mary psychologists are conducting experiments based on subjects’ reaction to the Ebbinghaus illusion.

Named for German psychologist Herman Ebbinghaus, the familiar illusion consists of two circles of equal size. The first is surrounded by a group of larger circles; the second, by smaller circles. A quirk of relative size perception makes the target circle within the group of big circles seem smaller to almost all people who view it.

A group of psychologists at William and Mary are conducting experiments using the Ebbinghaus illusion to explore how the human visual system processes information. Peter Vishton, assistant professor of psychology at William and Mary, is the lead author of a paper published recently in the journal Psychological Science.

In their paper, Vishton and his colleagues present evidence that when subjects reach—or even plan to reach—for a target disk, the effect of the illusion decreases. This could suggest, he explains, the presence of two distinct neurological visual systems, one governing perception and a second that regulates actions, such as reaching. Vishton and his colleagues favor another explanation, however.

“Our take on it is that maybe there’s not two visual systems that operate in parallel,” he said. “Instead, maybe there are two different modes of processing, so that when you reach for something, your visual system shifts operating characteristics.”

The experimenters used disks, rather than printed circles, in the center of the Ebbinghaus arrays. Subjects were fitted with position sensors—a wired glove on their thumb and forefinger. The position sensors allows researchers to monitor the subjects’ grip aperture, or the distance between thumb and forefinger, as they reach for the target disk. Measurement of the maximum grip aperture is vital, Vishton explains, as it is a precise indicator of how large the person believes the object to be.

“We humans are really good at this,” he said. “We scale the grip as we reach for it really precisely to the size of the target we’re reaching for. We kinda have to: If your fingers are off even a little bit, you won’t be able to make that kind of a nice smooth lifting action.”

The William and Mary studies showed that subjects’ grip apertures matched the real size—as opposed to the apparent size—of the target disks. In one experiment, researchers inserted an oversized disk into one side of the array so that the two target disks looked to be the same size. Once again, the subjects used grip apertures appropriate to the actual size of the object.

“Even if the object on the right looks the same size as the one on your left, somewhere in your head there is an accurate representation of just how big each is and that controls how big your grip aperture is,” Vishton explained. Tests also indicated that merely planning to reach for the object reduces the effect of the Ebbinghaus illusion.

Vishton’s co-authors on the Psychological Science article included Jennifer Stevens, also of William and Mary’s psychology department and several students.