Timbre and Speech Perception in Bimodal and Bilateral Cochlear-Implant Listeners

Objectives: This study was designed to evaluate the contribution of temporal and spectral cues for timbre perception in listeners with a cochlear implant (CI) in one ear and low-frequency residual hearing in the contralateral ear (bimodal hearing), and listeners with two CIs. Specifically, it examined the relationship between timbre and speech perception in these two groups of listeners. It was hypothesized that, similar to speech recognition, temporal-envelope cues are dominant cues for timbre perception, and the reliance of spectral cues was reduced in both bimodal and bilateral CI users compared with that in normal-hearing listeners. It was further hypothesized that the patterns of results with regard to combined benefit would be similar between timbre and speech perception. Design: Seven bimodal and five bilateral CI users participated. Sixteen stimuli that synthesized western musical instruments were used for the timbre-perception task. Sixteen consonants in the /aCa/ context and nine monophthongs in the /hVd/ context were used for the phoneme-recognition task. Each subject was tested on three listening conditions-individual device alone (single CI, or hearing aid [HA] alone) and combined use of devices (CI + HA, or 2CIs). For the timbre-perception task, each listener made judgments of dissimilarity between stimulus pairs. Multidimensional scaling analysis was performed to derive the coordinates of the dimensions that best fit the data. Correlational analyses were performed to relate the coordinates of each dimension and the temporal-envelope (impulsiveness) and spectral-envelope (spectral-centroid) features of the stimuli. For phoneme-recognition task, each listener identified the phoneme he or she heard by choosing an answer displayed on the computer screen. Overall percent correct phoneme-identification scores and percent information transmission for consonant and vowel features were calculated. Results: There were strong correlations between impulsiveness and the first dimension (Dim 1) of the timbre space, but correlations between spectral centroid and the second dimension (Dim 2) were weak for all listening conditions for both groups of listeners. As a group, the combined use of devices did not significantly improve listeners' ability to perceive differences in musical timbre compared with the better single-device condition. Some of the bimodal and bilateral CI users showed a considerably strengthened correlation between spectral centroid and Dim 2 in the combined condition compared with a single CI or an HA. There was a lack of relationship between percent correct phoneme recognition and timbre perception for all listening conditions. However, there was a consistent pattern regarding the combined benefit between timbre perception and vowel recognition. In general, listeners who demonstrated combined benefit for vowel recognition also showed a considerable increase in correlation between spectral centroid and Dim 2 with the combined use of devices compared with the single-device conditions. Improved correlation was not evident for those who did not demonstrate combined benefit for vowel recognition. Conclusions: Similar to speech recognition, temporal envelope was a dominant cue for timbre perception in bimodal and bilateral CI users. In addition, there was a close relationship between timbre perception and vowel recognition with regard to combined benefit. The present findings suggest that speech recognition and timbre perception could be enhanced when listeners received different spectral cues from individual devices.