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A new look at moving violations.

A new Look at moving violations

Stare at the cascading spray of a waterfall for a while, then shift your gaze to the trees on the bank. Like salmon swimming defiantly upstream, the trees appear to "swim" upward.

Psychologists call this the "motion aftereffect." In the laboratory, when a person views a moving visual pattern, the same pattern appears to drift in the opposite direction when it is stopped. Many researchers say this perception of reversed motion is an automatic response of the brain's visual system, and outside of an individual's control. In their view, brain cells that detect motion in a particular direction become so overworked when adapting to a moving pattern that their activity temporarily reverses when the pattern halts, giving motion to what an observer knows are firmly rooted trees.

But the motion aftereffect may not be automatic after all, according to a study in the March 1 NATURE. Paying attention to a feature embedded within a moving pattern markedly weakens motion reversals, reports Avi Chaudhuri of the salk Institute in La Jolla, Calif.

The finding suggests that much of the early processing of motion in the brain, such as that required for the motion aftereffect, depends on how an observer directs his or her attention to a moving object, writes Oliver Braddick of the University of Cambridge, England, in an accompanying commentary.

Chaudhuri had five observers monitor a sequence of rapidly changing letters and numbers in the center of a field of moving dots on a computer screen. Their task was to strike a key whenever a number appeared. Compared to experiments in which the same observers fixed their gaze on the letters and numbers without having to take any action, the duration of motion aftereffects dropped by almost 70 percent after attention was directed to the appearance of numbers.

However, paying attention to a feature not directly concerned with movement apparently does not influence motion aftereffects, Chaudhuri says. For example, observers who pressed a key when the background color of the dot display changed to red reported no weakening of motion reversals.

Color and motion properties of the same object may be processed through different pathways in the brain's visual system, Braddick suggests.
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Title Annotation:visual perception
Publication:Science News
Date:Mar 17, 1990
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