Internal clock gives brain the time to learn

American Association for the Advancement of Science: Discovery aids understanding of vital brain function, reports Tom Wilkie

There is a metronome ticking away at the back of our brains, and American researchers have discovered its location.

Gourmet cooks do not set 20 egg-timers to keep track of when each dish will be ready, they keep track of time in their heads, Dr Warren Meck told the American Association for the Advancement of Science, meeting in Baltimore.

Dr Meck, of Duke University, North Carolina, has discovered that the timing mechanism, which everyone has to keep track of short intervals, is located in the same area of the brain that controls motor function and movement. The results have implications for patients with Parkinson's disease because the timing mechanism is located in precisely the area which is damaged in sufferers.

Dr Meck's experiments, in both humans and animals, show that the striatum, a portion of the brain which had once been thought only to control movement, also keeps track of timing short intervals, from seconds to minutes. It receives a steady stream of pulses from the brain's own metronome, located in an area known as the substantia nigra, and acts as gatekeeper to turn on and off the awareness of time intervals.

A sense of time is crucial to learning, Dr John Gibbon, of Columbia University, pointed out. In the classic instance of conditioned reactions in Pavlov's dogs, they learnt to associate the ringing of a bell with the appearance of food because the dogs could appreciate that the time interval between them was shorter than the intervals between feeding.

Dr Meck pointed out that in the animal world the ability to time short intervals is a key to survival. "Animals have to be able to determine if they are getting enough food to eat during a given time interval. They have to sense when the yield is no longer worth the effort and it's time to move on."

Timing is essential to the survival of human beings too, for they use their internal clock to judge if they have enough time to cross the street before a car hits them, Dr Meck added.

He and his colleagues used a technique known as functional magnetic resonance imaging (MRI) to measure which parts of the brain are activated when someone is keeping track of short time intervals.

MRI measures the very slight magnetic properties of water inside the body to create non- invasive images of the organs. In effect, it measures blood flow, and therefore activity in the brain, and turns that into maps of the active regions.

Volunteers were asked to estimate 11-second intervals and Dr Meck and his colleagues found that the most active regions of the brain for these motionless volunteers were the front cortex and the striatum, a portion thought to be involved only with motor skills.

The data in humans back up earlier experiments on rats. Dr Meck found that if the region of the rat brain which produces dopamine were damaged, then the rats lost their sense of time. In humans, damage to the corresponding area of the brain results in Parkinson's. When the rats were given injections of L-dopa, a drug used to treat Parkinson's patients, their ability to estimate short time intervals was restored.

Following on from Dr Meck's results, Dr Gibbon has found similar problems in estimating time intervals among patients with Parkinson's disease if they stopped taking L-dopa.

In a separate development, Professor Rae Silver believes she has identified how the brain controls not only short interval timing but also circadian rhythms: the body's response to cycles of light and dark. Dr Silver believes a group of cells located just behind the eyes, known as the suprachiasmatic nucleus (SCN) acts as the circadian pacemaker by emitting some sort of chemical.

The signal from the SCN instructs the pineal gland when to secrete melatonin into the bloodstream to induce sleep. It had been thought that all communication in the brain took place among connected nerve cells. But Dr Silver has evidence to show that the SCN operates by diffusing a chemical rather than through neuronal connections. She has not yet isolated the chemical, but believes it may one day enable humans to take a pill to reset body clocks for shift work or to avoid jet lag.