The effect of spectral smearing on the identification of pure F0 intonation contours in vocoder simulations of cochlear implants

Abstract

Objectives: Performance of cochlear implant (CI) users on linguistic intonation recognition is poorer than that of normal-hearing listeners, due to the limited spectral detail provided by the implant. A higher spectral resolution is provided by narrow rather than by broad filter slopes. The corresponding effect of the filter slope on the identification of linguistic intonation conveyed by pitch movements alone was tested using vocoder simulations.

Methods: Re-synthesized intonation variants of naturally produced phrases were processed by a 15-channel noise vocoder using a narrow (40 dB/octave) and a broad (20 dB/octave) filter slope. There were three different intonation patterns (rise/fall/rise-fall), differentiated purely by pitch and each associated to a different meaning. In both slope conditions as well as a condition with unprocessed stimuli, 24 normally hearing Dutch adults listened to a phrase, indicating which of two meanings was associated to it (i.e. a counterbalanced selection of two of the three contours).

Results: As expected, performance for the unprocessed stimuli was better than for the vocoded stimuli. No overall difference between the filter conditions was found.

Discussion and conclusions: These results are taken to indicate that neither the narrow (20 dB/octave) nor the shallow (40 dB/octave) slope provide enough spectral detail to identify pure F(0) intonation contours. For users of a certain class of CIs, results could imply that their intonation perception would not benefit from steeper slopes. For them, perception of pitch movements in language requires more extreme filter slopes, more electrodes, and/or additional (phonetic/contextual) cues.

Keywords: Cochlear implants; Intonation; Spectral smearing; Speech perception; Vocoders.