Discrimination of Stochastic Frequency Modulation by Cochlear Implant Users.

Abstract

Past work has shown that low-rate frequency modulation (FM) may help preserve signal coherence, aid segmentation at word and syllable boundaries, and benefit speech intelligibility in the presence of a masker. This study evaluated whether difficulties in speech perception by cochlear implant (CI) users relate to a deficit in the ability to discriminate among stochastic low-rate patterns of FM. This is a correlational study assessing the association between the ability to discriminate stochastic patterns of low-rate FM and the intelligibility of speech in noise. Thirteen postlingually deafened adult CI users participated in this study. Using modulators derived from 5-Hz lowpass noise applied to a 1-kHz carrier, thresholds were measured in terms of frequency excursion both in quiet and with a speech-babble masker present, stimulus duration, and signal-to-noise ratio in the presence of a speech-babble masker. Speech perception ability was assessed in the presence of the same speech-babble masker. Relationships were evaluated with Pearson product-moment correlation analysis with correction for family-wise error, and commonality analysis to determine the unique and common contributions across psychoacoustic variables to the association with speech ability. Significant correlations were obtained between masked speech intelligibility and three metrics of FM discrimination involving either signal-to-noise ratio or stimulus duration, with shared variance among the three measures accounting for much of the effect. Compared to past results from young normal-hearing adults and older adults with either normal hearing or a mild-to-moderate hearing loss, mean FM discrimination thresholds obtained from CI users were higher in all conditions. The ability to process the pattern of frequency excursions of stochastic FM may, in part, have a common basis with speech perception in noise. Discrimination of differences in the temporally distributed place coding of the stimulus could serve as this common basis for CI users.