The Biological Clock

Can August be almost over? Didn’t they play the Super Bowl just last week? It sometimes seems that with each passing year, the days and weeks zip by more quickly.

If you ever had this feeling, you are not imagining it. Studies of human time perception show that age-related changes in the nervous system alter ones sense of time; it really does seem to move more quickly with age.

At a meeting of the Society for Neuroscience in New Orleans recently, a psychologist, Dr Peter A. Mangan, reported on a study in which he asked people in different age groups to estimate when three minutes had passed by, silently counting one-one-thousnad, two-one-thousand, three-one-thousand, and so on. People in their early 20s were accurate within three seconds, and some got it exactly right. People in their 60s estimated that three minutes were up after three minutes and 40 seconds had passed. Middle-aged subjects fell in between, but, like the older people, all underestimated the passage of time.

An American psychologist, Dr Hudson Hoagland, first suspected the existence of an internal clock in the 1930s, when his wife ran a high fever. Mrs. Hoagland complained that her husband had been out of the room for a very long time when he had actually been gone for only a few moments. Curious, Dr Hoagland asked his wife to estimate when a minute had passed. After 37 seconds, she said that the time was up. And as her temperature rose, she counted faster.

A test of Subjective Time reveals that people's concepts of the relationship of past events vary widely with their ages. In this test four subjects, ranging in age from nine to 70 years, were given strips of paper (above) which represented their life-spans from birth to the present. They were asked to indicate five points in time from "yesterday" back to when they entered first grade. Although all of the subjects gave more weight to recent times - marking yesterday and last week farther to the left than they actually should be - time sense seemed to become more realistic with age. The nine-year-old marked yesterday, last week and six months ago at almost equal intervals, while the 70-year-old spaced them at roughly appropriate distances.

The idea that there is a clock measuring intervals in the range of second to minutes makes a lot of sense. The ability to estimate short durations of time is critical for learning and survival, said Dr john Gibbon of the New York State Psychiatric Institute and Columbia University.

The human interval clock is running all the time as people unconsciously monitor the timing of external events and respond to them. For example, Dr Meck said, “suppose you are sitting at a red light, waiting for it to turn green,” adding: “At a certain point, based on past experience, you will begin to put your foot on the accelerator in anticipation that the light is about to turn green. Unconsciously, you are counting the seconds, without looking at your watch. But if the light fails to turn green in the expected amount of time, you start fretting, wondering if it is working properly. If enough time passes, you may decide to run the red light.”

People use interval clocks when engaged in music or sports. Basketball players, Dr Meck points out, know that they will be penalized under certain circumstances if they hold the ball for longer than several seconds without passing or dribbling, the shot-clock violation. They keep track of the time in their brains rather than by checking the clock. Musicians use their interval clocks to simultaneous measure not only the beat, but the phase, the crescendos and innuendoes.

For the 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. The key to how this clock works — or fails to work — is dopamine. When the brain notices something new or rewarding, dopamine made in the 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.

The interval clock has drawn the interest of medical researchers. Dr Guinevere Eden, of Georgetown University Medical Centre, in Washington, said that dyslexia is basically a timing problem throughout the brain and that for dyslexics difficulty in reading is 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.

People with Parkinson’s disease lose cells that make dopamine and their interval clocks are thrown off, Dr Meck said. They have tremors, difficulty in starting movements, rigid muscles, and problems perceiving time accurately — all of which can be reversed with drugs that supply dopamine to the brain.