Attention to speech modulates distortion product otoacoustic emissions evoked by speech-derived stimuli in humans

Humans are remarkably skilled at understanding speech in noisy environments. While segregation of different audio streams is mostly accomplished in the auditory cortex, neural feedback connections run from the cortex to the brainstem and to the cochlea. The latter organ not only houses the mechanosensitive hair cells, but also possesses an active process enabling it to amplify sound in a frequency-dependent manner. A physiological correlate of the active process are distortion-product otoacoustic emissions (DPOAEs) that can be measured non-invasively from the ear canal. Here we employed speech-like DPOAEs, measured in response to stimuli derived from natural human speech and thus reflecting the harmonic spectral structure of voiced speech. We show that these emissions are modulated by selective attention to one of two competing voices, as well as by intermodal attention. Specifically, speech-like DPOAEs evoked by stimuli related to resolved harmonics of a voice were significantly reduced when that voice was attended compared to when it was ignored. No such effect was observed for stimuli related to unresolved harmonics of the target voice when the competing voice's harmonics in that range were unresolved as well, indicating that attentional modulation is specific to those components of voiced speech that are spectrally resolved. Our findings support the hypothesis that the cochlea's active process already shapes selective attention to speech in noise. Moreover, the speech-like DPOAEs that we developed open up further possibilities for investigating the contribution of the cochlear active process to auditory scene analysis in naturalistic settings.