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Psychologist finds instance where ’two wrongs do make a right’


UO study shows the distinction between perception and action streams is oversimplified

A trusted mental map of your surroundings turns out to be slightly misaligned, skewing your orientation. Your ability to control the direction in which you move is similarly compromised, although in a manner opposite the map’s offset. Taken together, the errors cancel one another, and you end up exactly where you want to be. Contrary to the proverb, two wrongs do make a right. This exception is the rule when it comes to how our brain processes what our eyes see and where our body moves, according to a discovery by University of Oregon researchers Paul Dassonville and Jagdeep Kaur Bala that will appear in the November issue of the journal PLoS Biology.

Their study, funded by the National Science Foundation, challenged a dominant theory of how the brain processes vision. The theory holds that information from the eyes separates into two distinct streams: one to simply represent where we see things in the environment, and another to guide the physical movements of our body within that environment. Both processes have been thought to depend largely on accessing distinct maps of the environment within the brain, representing objects from varying locations in our field of vision by systematically varying activity in corresponding regions of the brain. "It’s starting to look like the distinction between perception and action streams is an oversimplification," says Dassonville, an assistant professor of psychology. "There appear to be as many as 40 different visual areas, many of which contain some type of spatial map, each with its own idiosyncratic pattern of errors. Different tasks draw on different subsets of maps."

To untangle these error-prone maps, the researchers employed an illusion known as the induced Roelofs effect. People were briefly shown a bright spot, surrounded by a large rectangular frame, and were asked to say where they thought the spot appeared. If the frame was shifted to the observer’s right or left, the perceived location of the spot was shifted in the opposite direction. However, when observers were asked to physically point toward the spot, they did so with very little error. "Perception is prone to the illusion, while action seems immune," explains Dassonville, echoing the commonly accepted axiom that separate visual systems drive perception and action.

However, the UO study revealed that slight manipulations of the illusion led to a wholly new realization. In fact, the illusion actually distorted observers’ perceptions of where "straight ahead" was with respect to their own bodies, ultimately affecting both perception and action. "The ’wrong’ perception of where the object appeared was offset by the opposing ’wrong’ pointing movement that was based on the body’s distorted sense of its own position. As a result, the ’right’ pointing movements are made in spite of an inaccurate sense of the visual scene," Dassonville explains.

While we still lack a complete picture of how the brain converts visual input into motor output, Dassonville says, these findings enhance our understanding of the most fundamental concepts of how we experience and interact with the world. Dassonville and Elizabeth Walter, a graduate student, are now using the functional magnetic resonance imaging (fMRI) resources of the University of Oregon’s Robert and Beverly Lewis Center for NeuroImaging to identify brain mechanisms related to the effects described in the PLoS Biology article.

Dassonville joined the UO faculty in 1999. In addition to his appointment in the psychology department, he is a member of the university’s Institute of Neuroscience. Bala is continuing her research on the effects of attention on visual perception in the laboratory of UO psychology professor Richard Marrocco.

PloS Biology, first published in 2003, is the flagship journal of the Public Library of Science, a nonprofit organization of physicians and scientists dedicated to widespread, free access to scientific and medical literature.

Melody Ward Leslie | EurekAlert!
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