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.

Spatial release from masking was evaluated as a function of target-masker spatial separation in listeners with normal hearing and with chronic asymmetric hearing loss. The target was one of 20 vocoded words spoken by a female talker. The masker was a stream of vocoded nonsense sentences spoken by two other female talkers. Subjects listened in an anechoic chamber with loudspeakers at 1.9 m distance. The target was always presented from directly in front while the masker was presented from the front or from left or right at varying angular separations. In some conditions a copy of the masker with a delay of 4 ms was introduced at the front loudspeaker, simulating a reflection. Subjects detected the target in an adaptive 4AFC task. Results showed strong indications of informational masking in the co-located condition and spatial release that increased with masker angular separation, reaching almost 27 dB at wide separations in normal hearing listeners, and 22 dB even with the reflection. Listeners with asymmetric thresholds showed high across-listener variability in the spatial conditions but often smaller and strongly asymmetric release, presumably due to head shadow. The reflection reduced this head shadow advantage, and in one subject almost completely obliterated spatial release.