Abstract
Cochlear implants (CIs) convey the amplitude envelope of speech by modulating high-rate pulse trains. However, not all of the envelope may be necessary to perceive amplitude modulations (AMs); the effective envelope depth may be limited by forward and backward masking from the envelope peaks. Three experiments used modulated pulse trains to measure which portions of the envelope can be effectively processed by CI users as a function of AM frequency. Experiment 1 used a three-interval forced-choice task to test the ability of CI users to discriminate less-modulated pulse trains from a fully modulated standard, without controlling for loudness. The stimuli in experiment 2 were identical, but a two-interval task was used in which participants were required to choose the less-modulated interval, ignoring loudness. Catch trials, in which judgements based on level or modulation depth would give opposing answers, were included. Experiment 3 employed novel stimuli whose modulation envelope could be modified below a variable point in the dynamic range, without changing the loudness of the stimulus. Overall, results showed that substantial portions of the envelope are not accurately encoded by CI users. In experiment 1, where loudness cues were available, participants on average were insensitive to changes in the bottom 30% of their dynamic range. In experiment 2, where loudness was controlled, participants appeared insensitive to changes in the bottom 50% of the dynamic range. In experiment 3, participants were insensitive to changes in the bottom 80% of the dynamic range. We discuss potential reasons for this insensitivity and implications for CI speech-processing strategies.
Highlights
In most contemporary processing strategies, cochlear implants (CIs) convey information about the sound-energy envelope in each frequency channel by modulating the amplitudes of high-rate pulse trains
A similar standard was employed in assessing modulation depth discrimination, but with phase duration sinusoidally modulated by 50% around a duration of 100 μs, and the Gomersall et al (2016) presented contiguous frequency bands of noise, each of which was amplitude modulated stochastically, both to normal hearing (NH) listeners and via the auxiliary input of the MedEL device to CI listeners
The present results show that, for single-channel CI stimulation, CI listeners are insensitive to stimulus differences that occur over a substantial portion of the envelope, below that occurring at the peaks
Summary
In most contemporary processing strategies, cochlear implants (CIs) convey information about the sound-energy envelope in each frequency channel by modulating the amplitudes of high-rate pulse trains. Variation in the MDT as a function of modulation rate defines the temporal modulation transfer function, which is a measure commonly used to describe the limitations on temporal processing by CI users Performance on this task is often very good, and, at least at high levels and for slow modulations, often corresponds to only 1–2% of the subject’s dynamic range, defined on a decibel scale (Chatterjee and Oberzut 2011; Fraser and McKay 2012; Green et al 2012; Shannon 1992). These small MDTs suggest that CI listeners can accurately encode amplitude modulations near the peaks of the amplitude envelope
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