Effects of Stimulus Level on Speech Perception with Cochlear Prostheses

Abstract

This study is one of a series that examines stimulus features important for cochlear implant function. Here, we examine effects of stimulus level. In subjects with cochlear implants, a number of psychophysical tests of temporal discrimination (pulse rate discrimination, gap detection, etc.) show marked improvement as a function of stimulus level through most or all of the dynamic range, while electrode-place discrimination can improve or degrade as a function of level. In this study, effects of these combined potential influences were studied by examining the effects of stimulus level on syllable identification. We tested two hypotheses: that syllable identification varies as a function of stimulus level and that level and electrode configuration interact in affecting syllable identification. We examined vowel and consonant identification as a function of stimulus level for bipolar and monopolar electrode configurations. We used experimental processor maps where upper and lower stimulation limits of each electrode pair were equated to eliminate confounding effects of dynamic range, which varies across subjects and electrodes. For each channel, stimulation amplitude was set to a fixed percentage of its dynamic range. Eight adult subjects with Nucleus CI24M implants were tested using the SPEAK processing strategy. With each electrode configuration, stimulus levels were tested from 0% to 90% of the dynamic range in nine steps. The effects on consonant and vowel identification were similar. Phoneme identification was usually better for monopolar than for bipolar stimulation. In the lower half of the dynamic range, syllable identification usually increased as a function of stimulus level. In the upper half of the dynamic range, syllable identification continued to increase as a function of level to 90% of the dynamic range for some subjects, while for others there was no appreciable change or a decrease as a function of level. Decreases in performance at high levels were more common with monopolar than bipolar stimulation. These results suggest that if speech processors are programmed to optimize level for each individual, speech perception performance could be improved.

Effects of Threshold Adjustment on Speech Perception in Nucleus Cochlear Implant Recipients

“…The impact of varying the T-level below or above hearing threshold has been investigated in several studies (Skinner et al 1999;Zeng & Galvin 1999;Franck et al 2003;Spahr & Dorman 2005;Boyd 2006;Dawson et al 2007;Zhou & Pfingst 2014;Boyle & Moore 2015). However, as the signal processing differs across different CI devices (Vaerenberg et al 2014), the impact of these different T-level settings cannot be easily generalized from one device to another.…”

Psychophysical Metrics and Speech Recognition in Cochlear Implant Users

“…In the study reported here, we examined suprathreshold measures: maximum comfortable loudness levels (C levels) and dynamic ranges (DRs). Because most speech information delivered by cochlear implants is at suprathreshold levels and speech recognition generally improves as a function of level [Franck et al, 2002], we reasoned that psychophysical responses to suprathreshold stimuli might be more strongly correlated with speech recognition than measures taken just at the threshold level.…”

Interactive evolution for cochlear implants fitting

“…-Better results might be obtained by decreasing the dynamic range [18]. -Only using a subset of electrodes might improve speech recognition [42].…”