People use interval clocks when engaged in music or sports. Basketball players, Dr. Meck points out, keep track of time in their brains, knowing that they will be penalized under certain circumstances if they hold the ball for longer than several seconds without dribbling or passing. Musicians simultaneously measure not just the beat but the phrase, the crescendos and innuendoes. When jazz players shade the time in violation of strict beats, it makes the music interesting, Dr. Gibbon said.

In the case of the basketball player, different parts of the brain are working on different tasks. Cells in the visual system are looking for places to run or pass the ball. Cells in the motor system are controlling movements. Cells in the auditory system are listening for information from teammates on what to do next. Each of these specialized cell circuits carrying out different jobs tend to oscillate or fire at different rates. Some might be firing 5 times a second, others up to 40 times a second. It is as if they are operating independently on different time scales, yet the basketball player's brain must integrate them so that he or she can decide what to do with the ball.

For this task of coordination, Dr. Meck and his assistant have nominated a structure in the midbrain called the striatum, which is loaded with spiny neurons, so called because their projections are thick with spines. Such neurons are well connected in that each one -- and there are thousands of them -- is linked to tens of thousands of other cells via dendrites coming from other parts of the brain. The dendrites are the slender spines that help brain cells communicate. They detect oscillations or cell firing rates that occur all over the brain, Dr. Meck said, ''and the question has been what the heck do they do with them?''

''We think spiny neurons integrate these signals,'' Dr. Meck said, and, based on previous experience of what is important, select those that are beating at the same frequency and synchronize them. This collective timing signal is sent to higher cortical areas where, in a grand loop from the brain's basal ganglia to its frontal lobes, perceptions and actions are coordinated and acted upon. When a person is performing several tasks at once and needs to measure time, spiny neurons parcel out the tasks, Dr. Meck said.

The key to how this clock works -- or fails to work -- is dopamine. When the brain notices something new or rewarding, dopamine made in a nearby region called the substantia nigra is released into the spiny neurons, which become excited and begin to integrate time signals. In this way, the brain learns to anticipate events seconds or minutes into the future.

Animal and human experiments support the existence of the short-interval circuit, Dr. Meck said. For example, rats trained to press a lever at regular intervals to get food lose the ability when their dopamine-producing cells are removed. When the rats are given a synthetic form of dopamine, the ability is restored. In brain imaging experiments by Dr. Sean Hinton at Duke, people were asked to estimate when 11 seconds were up and to squeeze a ball just before and after this interval. The loop from the midbrain, where the source of dopamine and spiny neurons reside, to the higher cortex was activated each time they estimated 11 seconds had gone by.

The interval clock has drawn the interest of medical researchers. Dr. Guinevere Eden of Georgetown University Medical Center in Washington said that dyslexia was basically a timing problem throughout the brain and that for dyslexics difficulty in learning to read was just one manifestation of a more widespread defect. Some dyslexics have a problem with time, she said. They come late to appointments and have trouble keeping rapidly moving events in proper chronological order.