by David F. Salisbury
Seeing is believing, even when what we see is ambiguous or misleading.
New research has found that the brain continues to accept ambiguous
visual information about an object in motion even when it conflicts
with more reliable information that we can touch. The studies, which
appear in the June 7 issue of the journal Psychological Science and the
forthcoming June issue of the journal Cognitive, Affective, & Behavioral Neuroscience, provide new insights into the way the brain blends and balances information from different senses.
The research, conducted by Vanderbilt psychologists Randolph Blake,
Centennial Professor of Psychology, and Thomas W. James and Kenith V.
Sobel, research associates, found that the region of the brain that
specializes in processing visual movement—the middle temporal visual
center, or MT—also responds to motion that we feel. But they were
surprised to discover that when individuals were presented with an
ambiguous visual image and were able to touch that object, their brains
did not fuse the visual and touch inputs into a single, accurate
representation. Instead, the researchers found that the brain keeps the
two inputs separate and accepts a degree of “cognitive dissonance” when
the two conflict.
"This suggests that there is naturally a higher level of inconsistency
between seeing and feeling something moving than there is between
seeing and feeling something's shape,” says James.
To explore the relationship between vision and touch, the team
developed a procedure that relies on a well-known visual illusion
called the kinetic depth effect. The kinetic depth effect begins with a
group of dots arranged in a circular pattern on a computer screen. The
dots are programmed so that half move left to right and half move right
to left. The dots’ movements are choreographed so that they move as if
they are fixed to the surface of a transparent globe. This tricks the
brain into seeing them as forming a rotating, three-dimensional sphere.
The viewer cannot tell from the dots in which direction the sphere is
turning. So the brain splits the difference—the sphere appears to
rotate from right-to-left 50 percent of the time and from left-to-right
50 percent of the time.
The researchers set up a rotating Styrofoam ball that the subjects
could touch with both hands and projected the moving dot illusion into
their eyes in such a way that it appeared to be the same in every way
as the physical ball.
“Our thought was that if a person touched something turning in a
specific direction, then the perceptual system should fuse the tactile
and visual information, resulting in people ‘seeing’ a sphere rotating
in a direction that was consistent with the more reliable source of
sensory information,” says James.
They found instead that the tactile and visual information did not
fuse. The tactile information did, however, change the subjects’
perception of which direction the sphere was rotating. The number of
times that the subjects reported the visual sphere rotating in the same
direction as the physical sphere jumped from 50 percent to 65 percent.
Yet it still appeared to rotate in the opposite direction 35 percent of
the time, in direct contrast to what they were feeling with their hands.
The researchers went a step further and monitored brain activity in the
part of the brain most likely to be involved, the Middle Temporal
visual center, using functional magnetic resonance imaging. This
technique detects activity levels in different parts of the brain by
measuring blood flow. They recorded a reliable increase in activity in
MT when the globe was rotating compared to when it was stationary for
both vision and touch. But the activity stimulated by the visual
stimulus was four times greater than that generated by touch.
A central question remained unanswered: Why didn’t the visual and
tactile information fuse? The researchers speculated that the fact that
subjects could not see their fingers touching the rotating sphere might
have been enough to keep the fusion from taking place.
So they came up with another approach, which is described in the Cognitive, Affective, & Behavioral Neuroscience
article. They attached a wire-frame sphere to a motor to enable it to
slowly rotate. When viewed with one eye closed, it created the same
illusion as the moving dots. In this case, however, subjects were able
to reach out and directly touch the sphere as they were looking at it
to determine in which direction it was rotating.
The researchers were surprised to find that this procedure produced
almost precisely the same results as their first effort. Without
touching the sphere, subjects reported that it appeared to rotate in
each direction approximately 50 percent of the time. When the subjects
were touching the sphere, it appeared to them to rotate in the
direction consistent with what they were feeling 60 to 70 percent of
the time, while appearing to rotate in the opposite direction 30 to 40
percent of the time.
“It is surprising that an unreliable visual stimulus should be that
resistant to tactile input,” says James. “This points out that, in
terms of visual motion processing, the mechanism that produces
alterations in the perceptual state must be very powerful because it is
not over-ridden even by highly reliable tactile input.”
For more news about Vanderbilt research, visit Exploration, Vanderbilt’s online research magazine, at http://exploration.vanderbilt.edu.