Use of an adaptive-bandwidth protocol to measure importance functions for simulated cochlear implant frequency channels

Abstract

The Articulation Index and Speech Intelligibility Index predict intelligibility scores from measurements of speech and hearing parameters. One component in the prediction is the frequencyimportance function, a weighting function that characterizes contributions of particular spectral regions of speech to speech intelligibility. The purpose of this study was to determine whether such importance functions could similarly characterize contributions of electrode channels in cochlear implant systems. Thirty-eight subjects with normal hearing listened to vowel-consonant-vowel tokens, either as recorded or as output from vocoders that simulated aspects of cochlear implant processing. Importance functions were measured using the method of Whitmal and DeRoy [J. Acoust. Soc. Am. 130, 4032-4043 (2011)], in which signal bandwidths were varied adaptively to produce specified token recognition scores in accordance with the transformed up-down rules of Levitt [J. Acoust. Soc. Am. 49, 467-477 (1971)]. Psychometric functions constructed from recognition scores were subsequently converted into importance functions. Comparisons of the resulting importance functions indicate that vocoder processing causes peak importance regions to shift downward in frequency. This shift is attributed to changes in strategy and capability for detecting voicing in speech, and is consistent with previously measured data.

Effects of envelope bandwidth on importance functions for cochlear implant simulations

“…The present study also extends the previous approach of Whitmal and DeRoy (2012) for use in applications involving low-fidelity speech. The Whitmal/DeRoy protocol measured recognition scores for highly intelligible speech at five fixed performance levels between 15.9%-and 84.1%-correct.…”

“…The signal processing methods utilized in this experiment were used in previous studies and consist of (optional) tone vocoding and adaptive-bandwidth filtering as prescribed by Whitmal and DeRoy (2012). These processes are described below.…”

“…Stimuli consisted of 23 consonants /b d g p t k f Õ v ð h s S z Z tS dZ m n w l r j/ recorded in /a/C/a/ format (Whitmal et al, 2007;Whitmal and DeRoy, 2012). The stimuli were spoken by a female talker with an American English dialect and digitally recorded in a sound-treated booth (IAC 1604) with 16-bit resolution at a 22 050 Hz sampling rate.…”

“…Peak FIF values for meaningful words and sentences are located at lower frequencies (Hirsh et al, 1954;Studebaker et al, 1987;Pavlovic, 1994;DePaolis et al, 1996;Eisenberg et al, 1998). Whitmal and DeRoy (2012) speculated that FIFs could be used to characterize CI electrode channel contributions to intelligibility. They measured intelligibility scores for filtered vowel-consonant-vowel (VCV) words in CI simulations with 16-and 28-channel vocoders, and used adaptivebandwidth filters to derive performance-bandwidth psychometric-functions for each vocoder from those scores.…”

Optimizing frequency-to-electrode allocation for individual cochlear implant users

“…According to the SII model, third octave bands with 23 center frequencies between 160 and 8000 Hz all contribute to speech intelligibility, 24 suggesting that an optimal frequency map will include these frequencies, although the 25 most important third octave bands are those with center frequencies of 1.6, 2 and 2.5 1 kHz. A study with normal-hearing participants listening to cochlear implant 2 simulations found that the peak in the relative band importance function was 3 approximately half an octave lower for cochlear implant simulations than for 4 unprocessed speech (Whitmal and DeRoy, 2012). This suggests that for CI users, 5 lower frequency sounds are relatively more important for speech intelligibility than 6 higher frequency sounds, when compared to normal-hearing listeners.…”