Optical illusions reveal how the brain works
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.
