People have been keeping track of time for much longer than wristwatches have been around. How does the brain do it? Designing experiments to reveal how the brain senses time is devilishly tricky, but new work with monkeys shows that neurons in a part of the brain involved with spatial processing might double as timekeepers.
The experiment involved monkeys looking at lights, which stimulates several parts of the brain. First off, the brain begins to process flashes and other sensory information in areas collectively known as the sensory cortex. If the eyes respond by darting one way or another, other regions kick in. One of these is an area known as the lateral intraparietal area, or LIP, which helps the brain decide to take action. The LIP offered a key clue about the brain's sense of timing.
Neurobiologists Matthew Leon and Michael Shadlen of the University of Washington in Seattle noticed that changing the timing of light flashes during an experiment changed the way LIP neurons responded. They decided to investigate the role of these neurons in perceiving elapsed time. The researchers trained rhesus monkeys to compare two flashes of light and report whether the second "test" flash was longer or shorter than the first. Over numerous trials, the first flash stayed the same, while the second was either much shorter than, much longer than, or nearly the same as the first. The monkeys moved their eyes to either a red or green light to indicate whether they thought the second flash was longer or shorter. Meanwhile, the scientists recorded the electrical activity of individual neurons in the LIP. Some of the neurons were involved in identifying a flash as being longer, and these neurons became even more active while the light stayed on, the team reports in the 24 April issue of Neuron. That suggests that the neurons' activity level tracks how much time has elapsed.
"This opens up a new chapter in neurobiology," says Randy Gallistel, an experimental psychologist at Rutgers University in Piscataway, New Jersey. Although there have been a number of behavioral studies on how animals learn duration, he says, this is one of the first studies to look at which neurons process time information.