Auditory enhancement and the role of spectral resolution in normal-hearing listeners and cochlear-implant users

Detection of a target tone in a simultaneous multi-tone masker can be improved by preceding the stimulus with the masker alone. The mechanisms underlying this auditory enhancement effect may enable the efficient detection of new acoustic events and may help to produce perceptual constancy under varying acoustic conditions. Previous work in cochlear-implant (CI) users has suggested reduced or absent enhancement, due perhaps to poor spatial resolution in the cochlea. This study used a supra-threshold enhancement paradigm that in normal-hearing listeners results in large enhancement effects, exceeding 20 dB. Results from vocoder simulations using normal-hearing listeners showed that near-normal enhancement was observed if the simulated spread of excitation was limited to spectral slopes no shallower than 24 dB/oct. No significant enhancement was observed on average in CI users with their clinical monopolar stimulation strategy. The variability in enhancement between CI users, and between electrodes in a single CI user, could not be explained by the spread of excitation, as estimated from auditory nerve evoked potentials. Enhancement remained small, but did reach statistical significance, under the narrower partial-tripolar stimulation strategy. The results suggest that enhancement may be at least partially restored by improvements in the spatial resolution of current CIs.