Guenther spent 15 years at BU mapping the brain's language centers, and he'd begun to pay attention to "formant frequencies," the acoustic changes that make an "oh" sound different from an "ah" sound. Guenther theorized that these frequencies are the key to how the brain encodes speech.

A split second before those impulses are sent, he believes, the brain forms a map of the sound that those muscle contractions are going to produce -- like a sound file on a computer.