Brain circuits run their own clocks

Timing is everything. But exactly how the brain keeps time, which it does very well, has been something of a mystery.

One widely held theory suggests that a single brain region acts as a centralised timekeeper ? possibly in the basal ganglia or cerebellum. However, a study now suggests that timekeeping is decentralised, with different circuits having their own timing mechanisms for each specific activity. The finding could help explain why certain brain conditions affect our sense of timing, and even raise the possibility of artificially manipulating time perception.

Geoffrey Ghose and Blaine Schneider, at the University of Minnesota in Minneapolis, investigated timing in the brain by training two rhesus macaques to perform tasks in which they moved their eyes between two dots on a screen at regular 1-second intervals. There were no external cues available to help them keep track of time.

After three months, the monkeys had learned to move their eyes between the two dots with average intervals of 1.003 and 0.973 seconds, respectively.

The researchers then used electrodes to record brain activity across 100 neurons in the lateral intraparietal cortex ? associated with eye movement ? while the monkeys performed the task.

The activity of these neurons decreased during the interval between each eye movement, and the rate of decrease correlated with the monkeys' timing. Using this information, Ghose and Schneider were able to predict the interval between eye movements by measuring the preceding decay rate. For example, in one task, a slower rate of decrease in the neurons' activity corresponded with a macaque overestimating of the length of a second. Likewise, if neuron activity decreased at a faster rate the monkeys moved their eyes before a second was up.

The researchers now want to study what goes on in this brain area while the monkeys are learning the task, to work out how these time intervals arise. This may help our understanding of why people with brain lesions or Parkinson's can have difficulty keeping time, says Ghose.

As well as indicating that brain circuits may have their own ability to keep time, the results also hint at how our perception of time can be altered during high emotional states.

Stress is associated with changes in the amount of neuromodulators such as adrenalin present in the brain. Adrenalin is known to affect the rate of decay of neuronal activity. "And in our model, a change in the activity decay rate is all you need to have a different sense of 'what time' it is," says Ghose. It might be possible to tweak an individual's sense of timing by altering these signals, he says.

The results support the idea that local neuron populations govern timing behaviour, says Catherine Jones at the University of Essex, UK. "Given the promising findings, it would certainly be of value to investigate human performance on this task."

Journal reference: PLoS Biology, DOI: 10.1371/journal.pbio.1001413

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