University of Maryland researchers have found that a particular resonance pattern in the brain's auditory processing region is the key to its ability to discriminate speech.
In the June 21 issue of the journal Neuron, David Poeppel and Huan Luo report that the inherent rhythm of neural activity called "theta band" specifically reacts to spoken sentences by changing its phase.
The researchers also note that the natural oscillation of this frequency provides further evidence that the brain samples speech segments about the length of a syllable.
According to them, this is the first study to represent that such a broad neural response is central to perceiving the highly complex dynamics of human speech, for previous studies had explored the responses of individual neurons to speech sounds, but not the response of the auditory cortex as a whole.
During the course of study, the researchers asked volunteers to listen to spoken sentences such as "he held his arms close to his sides and made himself as small as possible," and scanned their brains simultaneously using magnetoencephalography.
The researchers found that the theta band, which oscillates between four and eight cycles per second, changed its phase pattern with unique sensitivity and specificity in response to the spoken sentences.
When boffins degraded the intelligibility of the sentences, the theta band pattern lost its tracking resonance with the speech.
The researchers conclude that the brain discriminates speech by modulating the phase of the continuously generated theta wave in response to the incoming speech signal.
They also note that the time-dependent characteristics of the theta wave suggest that the brain samples the incoming speech in "chunks" that are about the length of a syllable from any given language.