How does our brain measure time?

Depending on how we fill our time, we perceive it differently: sometimes hours pass, and sometimes minutes feel like hours. But if we focus on it consciously, we can estimate small timescales in the range of seconds to minutes with quite accuracy. How does our brain do that? A study in rats now provides a clue: If researchers manipulated neural activity in the striatum, a region of the brain, the rodents’ perception of time was altered. On the other hand, the speed of their movements remained unchanged. The findings could also contribute to understanding diseases such as Parkinson’s.

Our brain measures time in different scales. Our most famous internal clock controls our circadian rhythm and determines when we feel tired, when we wake up and how our metabolism adjusts to the time of day. A little research is how our brain estimates time scales smaller than seconds to minutes. One hypothesis is that it depends on the regular activity patterns of particular groups of neurons, like the ticking of a clock. But unlike a clock, such neurons can “tick” sometimes faster, sometimes slower and thus alter the perception of time. However, it has so far been difficult to test this hypothesis experimentally.

Nervous waves to set the time

Now, a team led by Tiago Montero of the Champalimaud Foundation in Lisbon has shown in mice that the brain actually depends on the activity of neurons in a brain region called the striatum when small time scales are estimated. Montero’s colleague Joseph Patton compares the pattern of this neural activity to a stone falling into water: “The stone creates waves that propagate across the surface in a repeatable pattern. By examining the patterns and locations of these waves, you can infer when and where the stone fell into the water,” he explains. “Just as the speed at which waves move can vary, so can the pace at which these activity patterns progress in populations of neurons.”

To prove that the speed of these neural “waves” is actually related to time-dependent decisions, the researchers trained rats to discriminate between different time periods. If the thirsty mice waited a set time after the cue, they were rewarded with a drop of water. Depending on the signal, they had to estimate whether the time period was longer or shorter than 1.5 seconds. Meanwhile, the researchers measured activity in the animals’ striatum, a part of the brain’s basal ganglia involved in motor control that has previously been linked to time-dependent decisions.

When the internal clock moves faster or slower

And indeed: if the rats estimated a longer period of time, faster neural activity was observed in the striatum, if they estimated a shorter, slower activity. Next, the team tested whether this association was based on a causal relationship. “To do this, we needed a way to experimentally manipulate these dynamics as the animals make time estimates,” explains Montero. “We used temperature to alter the rate of neuronal dynamics without disturbing the pattern.” The team implanted a small thermoelectric device into trained mice that heated or cooled the striatum with the push of a button.

The first measurements, in still anesthetized rats, showed that the speed of nerve waves did indeed increase when heated and slowed when cooled. “Temperature thus gave us a key with which we can stretch or compress neural activity over time,” says Montero’s colleague Felipe Rodriguez. “We used this manipulation with the behavioral experiment.” The result: “When we cooled the striatum, the rats estimated the interval to be shorter. If we heated it, they held it for longer.”

Initiate and control movements

While the animals’ estimates of time varied, the speed of their movements remained constant. “It got us thinking about the nature of controlling behavior in general,” Patton says. “Even the simplest organisms face two fundamental challenges in controlling movement. First, they have to choose between various possible actions — for example, in which direction they want to move. Second, once they have decided on an action, they must be able to modify and control it in a way Continuous to ensure it is implemented effectively.” What and when to do. On the other hand, the constant control of movement is left to other brain structures.

In another experiment, the team manipulated the temperature of the cerebellum, which is also involved in controlling movement. Here, temperature changes ensured that the movement speed also changed. “This division of labor between the two brain systems is pertinent to movement disorders such as Parkinson’s disease,” Patton says. In Parkinson’s disease, the ability to initiate movements, but not to perform them, is impaired. The results of our internal timekeeper can also help us better understand these diseases. In future studies, the team also wants to know how circuits in the brain generate timekeeping waves and how they help us respond to our environment.

Source: Tiago Monteiro (Champalimaud Foundation, Lisbon, Portugal) et al., Nature Neuroscience, Available here. doi: 10.1038/s41593-023-01378-5

© wissenschaft.de – Elena Bernard