Binaural pitch fusion is the fusion of dichotically presented tones that evoke different pitches between the ears. In normal-hearing (NH) listeners, the frequency range over which binaural pitch fusion occurs is usually <0.2 octaves. Recently, broad fusion ranges of 1-4 octaves were demonstrated in bimodal cochlear implant users. In the current study, it was hypothesized that hearing aid (HA) users would also exhibit broad fusion. Fusion ranges were measured in both NH and hearing-impaired (HI) listeners with hearing losses ranging from mild-moderate to severe-profound, and relationships of fusion range with demographic factors and with diplacusis were examined. Fusion ranges of NH and HI listeners averaged 0.17 ± 0.13 octaves and 1.7 ± 1.5 octaves, respectively. In HI listeners, fusion ranges were positively correlated with a principal component measure of the covarying factors of young age, early age of hearing loss onset, and long durations of hearing loss and HA use, but not with hearing threshold, amplification level, or diplacusis. In NH listeners, no correlations were observed with age, hearing threshold, or diplacusis. The association of broad fusion with early onset, long duration of hearing loss suggests a possible role of long-term experience with hearing loss and amplification in the development of broad fusion.

Bilateral CI listeners have abnormally broad fusion, similar to hearing aid and bimodal CI listeners. This broad fusion may explain the variability of binaural benefits for speech perception in quiet and in noise in bilateral CI users.

words)In the normal auditory system, central auditory neurons are sharply tuned to the same frequency ranges for each ear. This precise tuning is mirrored behaviorally as the binaural fusion of tones evoking similar pitches across ears. In contrast, hearing-impaired listeners exhibit abnormally broad tuning of binaural pitch fusion, fusing sounds with pitches differing by up to 3-4 octaves across ears into a single object. Here we present evidence that such broad fusion may similarly impair the segregation and recognition of speech based on voice pitch differences in a 'cocktail party' environment. Speech recognition performance in a multi-talker environment was measured in four groups of adult subjects: normal-hearing (NH) listeners and hearing-impaired listeners with bilateral hearing aids (HAs), bimodal cochlear implant (CI) worn with a contralateral HA, or bilateral CIs. Performance was measured as the threshold target-to-masker ratio needed to understand a target talker in the presence of masker talkers either co-located or symmetrically spatially separated from the target. Binaural pitch fusion was also measured.Voice pitch differences between target and masker talkers improved speech recognition performance for the NH, bilateral HA, and bimodal CI groups, but not the bilateral CI group. Spatial separation only improved performance for the NH group, indicating an inability of the hearing-impaired groups to benefit from spatial release from masking. A moderate to strong negative correlation was observed between the benefit from voice pitch differences and the breadth of binaural pitch fusion in all groups except the bilateral CI group in the co-located spatial condition. Hence, tuning of binaural pitch fusion predicts the ability to segregate voices based on pitch when acoustic cues are available. The findings suggest that obligatory binaural 3 fusion, with a concomitant loss of information from individual streams, may occur at a level of processing before auditory object formation and segregation.

Objectives: Binaural pitch fusion is the perceptual integration of stimuli that evoke different pitches between the ears into a single auditory image. Adults who use hearing aids (HAs) or cochlear implants (CIs) often experience abnormally broad binaural pitch fusion, such that sounds differing in pitch by as much as 3 to 4 octaves are fused across ears, leading to spectral averaging and speech perception interference. The main goal of this study was to measure binaural pitch fusion in children with different hearing device combinations and compare results across groups and with adults. A second goal was to examine the relationship of binaural pitch fusion to interaural pitch differences or pitch match range, a measure of sequential pitch discriminability. Design: Binaural pitch fusion was measured in children between the ages of 6.1 and 11.1 years with bilateral HAs (n = 9), bimodal CI (n = 10), bilateral CIs (n = 17), as well as normal-hearing (NH) children (n = 21). Depending on device combination, stimuli were pure tones or electric pulse trains delivered to individual electrodes. Fusion ranges were measured using simultaneous, dichotic presentation of reference and comparison stimuli in opposite ears, and varying the comparison stimulus to find the range that fused with the reference stimulus. Interaural pitch match functions were measured using sequential presentation of reference and comparison stimuli, and varying the comparison stimulus to find the pitch match center and range. Results: Children with bilateral HAs had significantly broader binaural pitch fusion than children with NH, bimodal CI, or bilateral CIs. Children with NH and bilateral HAs, but not children with bimodal or bilateral CIs, had significantly broader fusion than adults with the same hearing status and device configuration. In children with bilateral CIs, fusion range was correlated with several variables that were also correlated with each other: pure-tone average in the second implanted ear before CI, and duration of prior bilateral HA, bimodal CI, or bilateral CI experience. No relationship was observed between fusion range and pitch match differences or range. Conclusions: The findings suggest that binaural pitch fusion is still developing in this age range and depends on hearing device combination but not on interaural pitch differences or discriminability.

Spatial and voice gender separation of target from masking speech leads to substantial release from masking in normal-hearing listeners. However, binaural pitch fusion is often broad in cochlear implant (CI) listeners, such that dichotic stimuli with pitches differing by up to 3-4 octaves are fused (Reiss et al., 2014). We hypothesized that broad binaural fusion could reduce a listener’s ability to separate competing speech streams with different voice pitches, and thus reduce the voice gender as well as spatial benefit for speech perception in noise. Speech reception thresholds were measured in both bilateral and bimodal CI users, using male and female target talkers at two spatial configurations (co-location and 60-degrees of target-masker separation). Binaural pitch fusion was also measured. Different-gender maskers improved target detection performance in bimodal CI users, and performance was better with female than male targets in bilateral CI users. No spatial benefit was seen in either CI group. As hypothesized, voice gender masking release was strongly correlated with binaural fusion range in bimodal CI users. These results suggest that sharp binaural fusion is necessary for maximal speech perception in noise in bimodal CI users, but does not benefit bilateral CI users. [Work supported by NIH-NIDCD grant R01 DC01337.]