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A New Study Explores the Brain's Built-In Clock

The neuroscience of keeping time is largely still a mystery.
Image: MIT

A computer keeps time thanks to an oscillator crystal. A bit of quartz with an applied electric field exhibits natural resonance, a fundamental frequency. This is exploited as a means to synchronize activity within a circuit, like the baton of an orchestra conductor.

The human brain doesn't feature an oscillator crystal, yet it's capable of keeping impeccable or at least pretty good time. The precise mechanisms have largely remained a mystery, but a group of neuroscientists from MIT and Columbia University may have the beginning of an answer. In a paper published this week in Current Biology, they describe how the lateral intraparietal cortex (LIC) region of the brain helps it to both interpret and reproduce time intervals, e.g. keep the beat, as it were.

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The LIC likely isn't the singular source of the brain's timing signals, the authors note, but it seems to be a critical part of the process of interval measurement. They tested this out using rhesus monkeys, observing brain activity as the primates interpreted interval-based sensory information and then converted that information in motor activity (movement).

The primate experiment was an adapted version of a game used with human subjects called "ready, set, go." The subject observes a "ready" signal followed by a "set" signal, and they're asked to provide a "go" signal at the same time interval from "set" and they observed between "ready" and "set."

"In many natural settings, sensory and motor aspects of timing are heavily intertwined," the study explains. "For example, in sports, music, and imitation, humans continuously measure time intervals and use those measurements to control the timing of their actions. To investigate the mechanisms that flexibly link sensory and motor timing capacities, we developed a time reproduction task for rhesus monkeys in which the animals measured an interval demarcated by two time markers and reproduced it by a proactive saccade [a type of eye movement]."

In previous, human versions of this experiment conducted by Mehrdad Jazayeri, the current paper's lead author, it had been shown that subjects did not simply attempt to reproduce the given interval, but they combined their own imperfect measurement of that interval with prior knowledge of what the interval could have been. As more and more intervals are observed, and this prior knowledge accumulates, the subjects were able to more accurately reproduce new intervals. The result is observation modulated by or fact-checked against prior experience.

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"When people reproduce time, they don't seem to use a timer."

"When people reproduce time, they don't seem to use a timer," Jazayeri offered in a statement. "It's an active act of probabilistic inference that goes on."

So, the next step was to repeat the experiment while monitoring neural activity directly. What the researchers found is that neural activity in the measurement phase of the experiment (as the monkey observes the interval between "ready" and "set") increases on a sharp curve before leveling off until the "set" signal is received. A bolt of anticipation and then a wait. The longer the interval between the first two signals, the flatter the slope winds up being at the end (at "set").

The slope at the end of the observation interval predicts the overall slope of increasing neural activity during the reproduction phase of the experiment (between "set" and "go"). This makes sense. A fast increase in activity correlates to a fast interval. If the gap is longer, the brain has more time to reach the required neural activity threshold to generate a response.

Jazayeri's research winds up asking more questions than it answers. One of these has to do with an unexplained dip in neural activity observed following the "set" signal. This dip seems to mask information about the interval being reproduced and how that information reappears is a mystery.

A follow-up experiment that's currently underway adds a second measurement interval, another opportunity for observation and refinement by the subject. Early results suggest that, as expected, subjects are able to use this additional information to further refine their estimate of the first timing interval. This is still just the beginning of understanding timing in the brain.