Nerves Beat to Own Uneven Drum

You might think there's little that's random about the start of a 100-meter race. But the time it takes an individual sprinter to react to the starting gun can vary randomly by up to a tenth of a second. Now scientists have a better understanding of why it's impossible for sprinters--or anybody else--to control reaction times precisely: Individual nerve cells that govern reactions vary in how long it takes them to prod you into action, says a report in today's issue of Science.

In the study, neuroscientist Jeffrey Schall of Vanderbilt University and grad student Doug Hanes trained monkeys to do a reaction-time task in which the animals fix their eyes on a dot in the middle of a blank computer screen and, when that dot disappears and another appears to the right or the left, shift their gaze to the new dot. The researchers recorded the firing of single neurons in a brain region that controls eye movements.

In hundreds of trials, the monkeys' reaction times varied from roughly 150 to 400 milliseconds, and the response time depended on how fast the neurons increased their firing rate to a level that seemed to trigger the eye movement. It's as if a bucket must be filled to the brim for a response to occur, but the water flow is different every time you turn on the tap, says Schall. ``To see [this variability] at a single-cell level is really incredible,'' says Gordon Logan, a psychologist at the University of Illinois, Urbana-Champaign.

The new view of a neural threshold for reactions might just improve the understanding of certain neurological conditions. For example, Parkinson's patients have slowed reaction times in eye-movement tests, something that could arise, says Schall, if ``the threshold has been raised to a higher level, or the cells accumulate activity abnormally slowly.''