Stitched together

By scrambling and carefully piecing together snippets of foreign speech (example above in English; audio below, produced by the researchers, is in German) in a sound quilt, scientists found that a brain region called the superior temporal sulcus responded more strongly to longer chunks of sound. Fragment lengths were very small, ranging in length from 30 to 960 milliseconds.

Neuroscientist Tobias Overath of Duke University and colleagues designed “sound quilts,” long strings of speech made by stitching together shorter chunks ranging from 30 to 960 milliseconds. The superior temporal sulcus became more active as the chunks of sound quilt grew longer, brain scans revealed. The findings serve as a good example of how the brain uses precise timing information as it merges disparate sounds into meaningful words. The results “are not overtly about time, or timing as such,” says Overath. “But it’s very compelling evidence of how time actually does play a role in something as important and fundamental to us humans as speech.”

It’s unclear what the individual neurons in the superior temporal sulcus are doing to track the lengthening sounds, Overath says. Brain scanning methods, like the fMRI used in the Duke study, don’t have the resolution required to detect the behavior of single nerve cells. But studies on animals may provide clues about how some neurons keep time.

A recent study in rats offers a clear explanation of how some cells tick off the seconds. Rats were trained to press a lever for a sip of water. But the water would appear only after a certain amount of time had elapsed. The rats quickly learned not to waste their energy pushing the lever when it was too early, a behavior that revealed their timing abilities.

While the rats were timing the appearance of the water, neuro-scientist Gustavo Mello of the Champalimaud Centre for the Unknown in Lisbon, Portugal, and colleagues eavesdropped with electrodes on neurons in the rats’ striatum, a brain area thought to be important for time perception. Sure enough, the team found cells that fired off electrical signals in a sequence that spanned the entire waiting period, the team reported in May in Current Biology. These cells were keeping track of the seconds that ticked by.

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