The effects of interaural correlation and interaural asymmetry on binaural fusion

Binaural fusion, or the ability to combine signals from the two ears into a unitary percept, can be challenging for bilateral cochlear implant users. Both interaural correlation (i.e., the extent to which the signal in both ears are correlated) and interaural asymmetry (i.e., the deviation from the stimulation of matched locations across ears) can affect binaural fusion. The goal of this study was to investigate the interaction between these factors. Bilateral cochlear implant users were tested on a binaural fusion task. They were presented with signals where the interaural correlation of the envelopes and the interaural asymmetry were manipulated independently. Participants indicated the sound they heard by moving a dial to control an image that varied in terms of the size of the image, the number of images (one or two), and the location of the images in the participants' head. Both interaural correlation and interaural asymmetry were found to affect binaural fusion, although interaural correlation needed to be substantially degraded to affect binaural fusion. The results suggest that both factors affect binaural fusion but, in real-world situations, where there are relatively small degradations of interaural correlation, interaural asymmetry may be the dominant cue affecting binaural fusion.