Effects of envelope bandwidth on importance functions for cochlear implant simulations

Abstract

Frequency-importance functions (FIFs) quantify intelligibility contributions of spectral regions of speech. In previous work, FIFs were considered as instruments for characterizing intelligibility contributions of individual cochlear implant electrode channels. Comparisons of FIFs for natural speech and vocoder-simulated implant processed speech showed that vocoding shifted peak importance regions downward in frequency by 0.5 octaves. These shifts were attributed to voicing cue changes, and may reflect increased reliance on low-frequency information (apart from periodicity cues) for correct voicing perception. The purpose of this study was to determine whether increasing channel envelope bandwidth would reverse these shifts by improving access to voicing and pitch cues. Importance functions were measured for 48 subjects with normal hearing, who listened to vowel-consonant-vowel tokens either as recorded or as output from five different vocoders that simulated implant processing. Envelopes were constructed using filters that either included or excluded pitch information. Results indicate that vocoding-based shifts are only partially counteracted by including pitch information; moreover, a substantial baseline shift is present even for vocoders with high spectral resolution. The results also suggest that vocoded speech intelligibility is most sensitive to a loss of spectral resolution in high-importance regions, a finding with possible implications for cochlear implant electrode mapping.

Individualized frequency importance functions for listeners with sensorineural hearing loss

“…This underscores the idea that such functions cannot be readily averaged across a population of listeners with hearing loss, and that techniques which measure frequency importance by utilizing data on a group of listeners may not be appropriate for these populations. Past techniques for deriving frequency importance required averaging data over several listeners (e.g., Studebaker and Sherbecoe, 1991;Healy et al, 2013), but more recent techniques have overcome this limitation (Bosen and Chatterjee, 2016;Shen and Kern 2018;Whitmal et al, 2015). The random combination strategy has allowed for examinations of frequency importance on an individual level for clinical populations such as cochlear implant users (i.e., Bosen and Chatterjee, 2016) or hearing aid users (the current study).…”

“…As an alternative, it may be more beneficial to base frequency importance functions on data from listeners with hearing loss, rather than modifying frequency importance functions from normal hearing listeners in an attempt to account for differences in speech perception between the two groups of listeners. To obtain the importance functions used in SII calculations, several techniques have been developed, including the standard technique that was utilized for the values in the SII (e.g., Studebaker and Sherbecoe, 1991), the correlational method (Doherty and Turner, 1996), the "hole" method (Kasturi et al, 2002), a method by Whitmal et al (2015), and the compound method (Apoux and Healy, 2012;Healy et al, 2013). Unfortunately, all of these a) Electronic mail: sarah.leopold@usu.edu techniques require averaging data across large numbers of listener participants.…”