June 23, 2004

New Light on How the Brain Handles Brightness

Salk News

New Light on How the Brain Handles Brightness

La Jolla, CA – Studies by a Salk Institute research team on how we perceive the brightness of light may reveal how the brain is wired to handle the wide ranges of light stimulation we encounter every minute.

The study, by professor Terry Sejnowski and colleagues and published in the April 15 issue of Nature, shows that timing as well as the intensity of a light determine how we judge a light’s brightness. Scientists knew that brightness depends on such factors as scene context, shadows and three-dimensional perspectives, but these spatial arrangements did not explain fully how brightness is perceived.

The team found that the timing intervals between brief and long bright light flashes could create an optical illusion. Volunteers were asked to fixate their vision at a point on a computer screen. Then, two lights were flashed; one short, the other long, and the volunteers were asked which one was brighter. When the short light flashed at the beginning of the long-duration light it appeared to the volunteers to be dimmer, but when it flashed at the end of the long light the short light was reported as brighter.

The illusion showed that timing is as important as spatial influences in allowing the brain to measure brightness, which raises new questions on how nerve cell networks encode visual signals to mediate our perception of brightness. The scientists concluded that the illusion arose from nerve cell activity in the cerebral cortex, specifically in the area of the brain that handles higher visual functions.

The work is part of Sejnowski’s continuing goal to unravel how the complex networks of nerve cells in the brain handle perception, thought, language, consciousness and the other functions in the brain that make us uniquely human.

The Salk Institute for Biological Studies, located in La Jolla, Calif., is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and conditions, and the training of future generations of researchers. Jonas Salk, M.D., founded the institute in 1960 with a gift of land from the City of San Diego and the financial support of the March of Dimes Birth Defects Foundation.

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