Abstract
To understand speech, the slowly varying outline, or envelope, of the acoustic stimulus is used to distinguish words. A small amount of information about the envelope is sufficient for speech recognition, but the mechanism used by the auditory system to extract the envelope is not known. Several different theories have been proposed, including envelope detection by auditory nerve dendrites as well as various mechanisms involving the sensory hair cells. We used recordings from human and animal inner ears to show that the dominant mechanism for envelope detection is distortion introduced by mechanoelectrical transduction channels. This electrical distortion, which is not apparent in the sound-evoked vibrations of the basilar membrane, tracks the envelope, excites the auditory nerve, and transmits information about the shape of the envelope to the brain.
Highlights
To understand speech, the slowly varying outline, or envelope, of the acoustic stimulus is used to distinguish words
While it is clear that the pattern of action potentials in the auditory nerve reflects the shape of the envelope[9,10,11,12], frequency components corresponding to the envelope have not been found in the sound-evoked vibrations of the basilar membrane at the base of the cochlea[13,14]
Here we examined the mechanism used by the inner ear to encode critical features of communication-relevant sounds
Summary
The slowly varying outline, or envelope, of the acoustic stimulus is used to distinguish words. We used recordings from human and animal inner ears to show that the dominant mechanism for envelope detection is distortion introduced by mechanoelectrical transduction channels This electrical distortion, which is not apparent in the sound-evoked vibrations of the basilar membrane, tracks the envelope, excites the auditory nerve, and transmits information about the shape of the envelope to the brain. A clear example of this comes from cochlear implant users, most of whom have excellent speech recognition when a few frequency bands of envelope information are presented through the implanted electrodes[6] This information is conveyed to the auditory brainstem nuclei, where some cells respond selectively to specific rates of envelope modulation[7], and a systematic gradient of temporally specific neurons is found in one of the principal nuclei, the inferior colliculus[8]. More recent modeling work emphasized neural mechanisms, such as rate adaptation in auditory nerve dendrites, as a mechanism for encoding envelopes[20]
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