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.

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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.

Pitch is an essential cue that allows the auditory system to distinguish between sound sources. Pitch cues are less useful when listeners are not able to discriminate different pitches between the two ears, a problem encountered by listeners with hearing impairment (HI). Many listeners with HI will fuse the pitch of two dichotically presented tones over a larger range of interaural frequency disparities, i.e., have a broader fusion range, than listeners with normal hearing (NH). One potential explanation for broader fusion in listeners with HI is that hearing aids stimulate at high sound levels. The present study investigated effects of overall sound levels on pitch fusion in listeners with NH. It was hypothesized that if sound level increased, then fusion range would increase. Fusion ranges were measured by presenting a fixed frequency tone to a reference ear simultaneously with a variable frequency tone to the opposite ear and finding the range of frequencies that were fused with the reference frequency. No significant effects of sound level (comfortable level ± 15 decibels) on fusion range were found, even when tested within the range of levels where some listeners with HI show large fusion ranges. Results suggest that increased sound level does not explain increased fusion range in listeners with HI and imply that other factors associated with hearing loss might play a larger role.

Sound localization is an important aspect of auditory scene analysis, allowing listeners to group acoustic components from the same location into a single stream (Bregman 1990). Binaural pitch fusion, the fusion of different frequency tones across ears, can be thought of as one type of auditory streaming. Little is known about how sound localization cues affect binaural fusion. The goal of this study was to investigate the effects of interaural level differences (ILDs), one cue for sound localization, on binaural fusion of dichotic tones. Binaural pitch fusion was measured in adult normal-hearing (NH) listeners, using five ILD conditions: ILD = 0, 1.25, 2.5, 5, and 10 dB. Fusion ranges were measured by simultaneous presentation of reference and comparison stimuli in opposite ears, and varying the comparison stimulus to find the frequency range that fused with the reference stimulus. Preliminary results (5 NH; data collection is ongoing) show that small ILDs increase fusion ranges, while larger ILDs decrease fusion ranges. These findings suggest that ILDs can affect fusion range in NH listeners, and imply that simulated sound source location may provide a top-down grouping cue for binaural fusion. [This research was funded by a NIH-NIDCD grant R01 DC013307.]