Graduation Year

2016

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Communication Sciences and Disorders

Major Professor

Gail S. Donaldson, Ph.D.

Committee Member

Jean Krause, Ph.D.

Committee Member

Jennifer Lister, Ph.D.

Committee Member

Catherine Rogers, Ph.D.

Keywords

Cochlear Implant, Dynamic, Formant Resolution, Spectral Acuity, Spectral Resolution, Static

Abstract

For cochlear implant (CI) listeners, poorer than normal speech recognition abilities are typically attributed to degraded spectral acuity. However, estimates of spectral acuity have most often been obtained using simple (tonal) stimuli, presented directly to the implanted electrodes, rather than through the speech processor as occurs in everyday listening. Further, little is known about spectral acuity for dynamic stimuli, as compared to static stimuli, even though the perception of dynamic spectral cues is important for speech perception.

The primary goal of the current study was to examine spectral acuity in CI listeners, and a comparison group of normal hearing (NH) listeners, for both static and dynamic stimuli presented through the speech processor. In addition to measuring static and dynamic spectral acuity for simple stimuli (pure tones) in Experiment 1, spectral acuity was measured for complex stimuli (synthetic vowels) in Experiment 2, because measures obtained with speech-like stimuli are more likely to reflect listeners’ ability to make use of spectral cues in naturally-produced speech. Sixteen postlingually-deaf, adult CI users and sixteen NH listeners served as subjects in both experiments.

In Experiment 1, frequency discrimination limens (FDLs) were obtained for 1.5 kHz reference tones, and frequency glide discrimination limens (FGDLs) were obtained for pure-tone frequency glides centered on 1.5 kHz. Glide direction identification thresholds (GDITs) were also measured, in order to determine the amount of frequency change required to identify glide direction. All three measures were obtained for stimuli having both longer (150 ms) and shorter (50 ms) durations.

Spectral acuity for dynamic stimuli (FGDLs, GDITs) was poorer than spectral acuity for static stimuli (FDLs) for both listener groups at both stimulus durations. Stimulus duration had a significant effect on thresholds in NH listeners, for all three measures, but had no significant effect on thresholds in CI listeners for any measure. Regression analyses revealed no systematic relationship between FDLs and FGDLs in NH listeners at either stimulus duration. For CI listeners, the relationship between FDLs and FGDLs was significant at both stimulus durations, suggesting that, for tonal signals, the factors that determine spectral acuity for static stimuli also largely determine spectral acuity for dynamic stimuli.

In Experiment 2, estimates of static and dynamic spectral acuity were obtained using three-formant synthetic vowels, modeled after the vowel /^/. Formant discrimination thresholds (FDTs) were measured for changes in static F2 frequency, whereas formant transition discrimination thresholds (FTDTs) were measured for stimuli that varied in the extent of F2 frequency change. FDTs were measured with 150-ms stimuli, and FTDTs were measured with both 150-ms and 50-ms stimuli. For both listener groups, FTDTs were similar for the longer and shorter stimulus durations, and FTDTs were larger than FDTs at the common duration of 150 ms. Measures from Experiment 2 were compared to analogous measures from Experiment 1 in order to examine the effect of stimulus context (simple versus complex) on estimates of spectral acuity. For NH listeners, measures obtained with complex stimuli (FDTs, FTDTs) were consistently larger than the corresponding measures obtained with simple stimuli (FDLs, FGDLs). For CI listeners, the relationship between simple and complex measures differed across two subgroups of subjects. For one subgroup, thresholds obtained with complex stimuli were smaller than those obtained with simple stimuli; for another subgroup the pattern was reversed. On the basis of these findings, it was concluded that estimates of spectral acuity obtained with simple stimuli cannot accurately predict estimates of spectral acuity obtained with complex (speech-like) stimuli in CI listeners. However, a significant relationship was observed between FDTs and FTDTs. Thus, similar to the measures obtained with pure-tone stimuli in Experiment 1 (FDLs and FGDLs), estimates of static spectral acuity (FDTs) appear to predict estimates of dynamic spectral acuity (FTDTs) when both measures are obtained with stimuli of similar complexity in CI listeners.

Taken together, findings from Experiments 1 and 2 support the following conclusions: (1) Dynamic spectral acuity is poorer than static spectral acuity for both simple and complex stimuli. This outcome was true for both NH and CI listeners, despite the fact that absolute thresholds were substantially larger, on average, for the CI group. (2) For stimuli having the same level of complexity (i.e., tonal or speech-like), dynamic spectral acuity in CI listeners appears to be determined by the same factors that determine spectral acuity for static stimuli. (3) For CI listeners, no systematic relationship was observed between analogous measures of spectral acuity obtained with simple, as compared to complex, stimuli. (4) It is expected that measures of spectral acuity based on complex stimuli would provide a better indication of CI users’ ability to make use of spectral cues in speech; therefore, it may be advisable for studies attempting to examine the relationship between spectral acuity and speech perception in this population to measure spectral acuity using complex, rather than simple, stimuli. (5) Findings from the current study are consistent with recent vowel identification studies suggesting that some poorer-performing CI users have little or no access to dynamic spectral cues, while access to such cues may be relatively good in some better-performing CI users. However, additional research is needed to examine relationship between estimates of spectral acuity obtained here for speech-like stimuli (FDTs, FTDTs) and individual CI users’ perception of static and dynamic spectral cues in naturally-produced speech.

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