Overlap of competing voices may mask attentional effects on speech-like distortion product otoacoustic emissions

Whether selective auditory attention modulates the cochlear amplifier through the medial olivocochlear system remains debated. Using speech-like distortion-product otoacoustic emissions (DPOAEs), elicited by waveforms that follow the harmonics of natural speech, we previously found that selectively attending one of two competing talkers reduced the emissions related to the resolved harmonics of the female voice. Here, we attempted to reproduce this effect in a design that separated different frequency regimes and harmonic resolvability. Forty-four participants attended one of two competing talkers, a female and a male voice, presented to one ear, while speech-like DPOAEs related to both voices were recorded from the other ear. Stimuli were derived from resolved and unresolved harmonics in a low-frequency (up to 1.5 kHz) and a transitional (1.5 to 4 kHz) regime, which required pitch-shifting both voices so that their fundamental frequencies were much closer to each other than in the original study. Speech-like DPOAEs emerged for most stimulus types, although they were weak for some and absent for one. Their amplitude, however, did not depend on attention for any stimulus type. Further time-domain metrics showed only isolated attentional effects of opposite direction for two stimulus types, and a frequency-domain metric showed none. A cochlear excitation model suggests that, due to the closely spaced fundamental frequencies of the two voices, the emissions most comparable to the original study presumably arose at cochlear locations shared by both voices, where attention would modulate both voices alike.