Brain's Clock Has Millisecond Hand

Were it not for our brain's ability to sense time--even on the millisecond scale--Morse code would sound like one long beep. Now researchers are uncovering the first clues into how this mental clock works, even when we're not sending a telegraph.

For decades, neuroscientists believed the brain had a specific region dedicated to keeping track of time. Recently, this view has begun to change. Instead of a single internal chronometer, many researchers now believe that the natural firings of neurons throughout the brain may give us our ability to sense the passage of time. Exactly how this works, however, has remained a mystery.

To probe the inner workings of our mental clock, University of California (UC), Los Angeles, neuroscientist Dean Buonomano and Uma Karmarkar, now a post-doctoral fellow at UC Berkeley, focused on an idea called state-dependency. The concept goes like this: Imagine you are dripping red paint into a can of white paint. The first drop falls into pure white paint, but with each successive drop, the red paint enters a pinker and pinker solution.

Buonomano and Karmarkar hypothesized that something similar goes on in the brain. When you hear the first "beep" of Morse code, for example, a specific set of neurons fire. This causes the brain to be in a different state than it was before it heard the sound. As a result, when the next beep sounds, the brain clicks into another state. Keeping track of the differences between these states allows the brain to mark time, the team showed in a computer model that replicates neuron interactions.

To confirm the model, the scientists had human volunteers judge the interval between two beeps. Some beeps were played close together, others farther apart. Most subjects could easily tell the difference, but when the researchers played a "distracter" tone before the beeps, the volunteers had difficulty judging the time interval, the team reports in today's issue of Neuron. That's because the distracter tone confuses the brain's initial reference point, says Buonomano. It's the difference between throwing pebbles into a still pond or one that's already rippled, he notes. Only in the still pond can you correctly perceive the pattern of your pebble's ripples.

"This is a beautiful paper," says Warren Meck, a cognitive neuroscientist at Duke University in Durham, North Carolina. "[It] has important implications for our everyday perception of the temporal relationships among all of the sights and sounds that we process," he says.

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