Effects of real-time cochlear implant simulation on speech production

Abstract

Investigations using normal-hearing subjects listening to simulations of cochlear implant (CI) acoustic processing have provided substantial information about the impact of these distorted listening conditions on the accuracy of auditory perception, but extensions of this method to the domain of speech production have been limited. In the present study, a portable, real-time vocoder was used to simulate conditions of CI auditory feedback during speech production in NH subjects. Acoustic-phonetic characteristics of sibilant fricatives, aspirated stops, and F1/F2 vowel qualities were analyzed for changes as a result of CI simulation of acoustic speech feedback. Significant changes specific to F1 were observed; speakers reduced their phonological vowel height contrast, typically via talker-specific raising of the low vowels [æ] and [ɑ] or lowering of high vowels [i] and [u]. Comparisons to the results of both localized feedback perturbation procedures and investigations of speech production in deaf adults with CIs are discussed.

FIG. 1.

(Color online) F1/F2 average [i, æ, ɑ, u] qualities for speakers in the experimental group (n = 9) across recording epochs. Baseline, unperturbed productions are shown in black with a solid interpolation mapping the boundaries of their vowel space. Vowels produced under initial CI-simulation of acoustic feedback (epoch 2) have a large-dash interpolation, and those produced after increased experience with the real-time CI simulation (epoch 3) have a small-dash vowel space boundary.

FIG. 2.

(Color online) Average F1/F2 vowel productions for speakers in the control group. No feedback perturbation was present in any of the recording epochs; materials were repeated three times under identical conditions. First repetition is shown in black (solid line), second and third repetitions are in dark and light gray, respectively (dashed lines). A global phonological lowering (increase in F1 frequencies) was observed.

FIG. 3.

(Color online) Mean F1/F2 vowel qualities for [i, æ, ɑ, u] produced by two experimental talkers with different realizations of individual significant vowel × epoch interactions in F1 (p's ≤ 0.005; see Table III). S1 (top panel) shows significant isolated raising of low vowels in CI-simulated (dashed lines) versus unperturbed speech, while S7 (bottom panel) shows similar isolated lowering of high vowels.

FIG. 4.

Average F1 frequencies of the low front vowel [æ] for talkers in the experimental group (n = 9) across recording epochs. Those talkers whose within-subject data showed significant global changes in F1 or significant vowel-specific changes involving [æ] are plotted with solid lines; those without significant changes related to [æ] are dashed. Channel boundaries in the spectral processing of the CI simulator are also indicated. No straightforward relationship is apparent between a talker's F1 at baseline relative to the CI-simulator channels and the speaker's subsequent production behavior.