The role of continuous low-frequency harmonicity cues for interrupted speech perception in bimodal hearing

Abstract

Low-frequency acoustic cues have been shown to enhance speech perception by cochlear-implant users, particularly when target speech occurs in a competing background. The present study examined the extent to which a continuous representation of low-frequency harmonicity cues contributes to bimodal benefit in simulated bimodal listeners. Experiment 1 examined the benefit of restoring a continuous temporal envelope to the low-frequency ear while the vocoder ear received a temporally interrupted stimulus. Experiment 2 examined the effect of providing continuous harmonicity cues in the low-frequency ear as compared to restoring a continuous temporal envelope in the vocoder ear. Findings indicate that bimodal benefit for temporally interrupted speech increases when continuity is restored to either or both ears. The primary benefit appears to stem from the continuous temporal envelope in the low-frequency region providing additional phonetic cues related to manner and F1 frequency; a secondary contribution is provided by low-frequency harmonicity cues when a continuous representation of the temporal envelope is present in the low-frequency, or both ears. The continuous temporal envelope and harmonicity cues of low-frequency speech are thought to support bimodal benefit by facilitating identification of word and syllable boundaries, and by restoring partial phonetic cues that occur during gaps in the temporally interrupted stimulus.

FIG. 1.

Spectrograms illustrating the five stimulus types used in experiments 1 and 2, for the first 1.5 s (underlined) of the sentence “Do you want to have a barbeque this evening.” The original (unprocessed) stimulus is also shown (upper left panel).

FIG. 2.

(Left) Mean percent-correct word recognition scores across 5 listening conditions for 12 YNH subjects. (Right) Scores for the bimodal conditions expressed as percentage-point gain. Error bars indicate ±1 standard error of the mean.

FIG. 3.

(Left) Mean percent-correct word recognition scores across three listening conditions for eight YNH subjects. (Right) Comparison of benefit across two listening conditions that provided continuity information in the vocoded ear, or both ears, expressed as percentage-point gain. Error bars indicate ±1 standard error of the mean.

FIG. 4.

Comparsion of performance for one condition assessed in experiment 2 (nfV) and two conditions assessed in experiment 1 (gV + nfLPHC, gV + cLPHC). Error bars indicate ±1 standard error of the mean.