Binaural fusion with ecologically valid stimuli

Abstract

Binaural fusion, or the perception of a single auditory “image” can arise from simultaneous inputs across ears. Fusion is fundamental to binaural abilities, including sound source localization. One potent determiner of degree of fusion is the interaural correlation of the binaurally presented waveforms. Noise waveforms can be used to manipulate the interaural correlation, but the normalized correlation of the envelopes of such noises is limited to between 1.0 and 0.785 (π/4). As a result of differing properties between noises and speech, pairs of speech sounds can be generated having normalized interaural envelope correlations well below 0.785. We measured cochlear implant users’ perceived degree of binaural fusion while varying the interaural correlation of speech-like signals from 0.4 to 1.0. Stimuli were generated using speech-based envelopes from multi-channel vocoders that modulated 1000 pulse-per-second pulse trains. Envelopes derived from different vocoder channels were extracted to yield interaural correlations that varied over a wider range than would be possible using noises. Preliminary results suggest that the speech-based stimuli, while differing qualitatively from noises and being more ecologically valid, yield similar effects of interaural correlation on binaural fusion, while allowing a larger range of interaural correlations to be employed.