The relationship between interaural coherence, interaural level differences, and binaural fusion

Binaural fusion is the perception of a single auditory image when signals are presented to the two ears. Decreasing the interaural coherence of the signals often degrades binaural fusion. However, perception of interaural level differences (ILDs) is minimally affected by interaural coherence changes. This study aims to determine if there is an interaction between ILDs and binaural fusion in the presence of reduced interaural coherence. In Experiment 1, vocoded stimuli with varying envelope interaural coherences were presented containing a range of ILDs. Participants indicated the perceived size and lateralization of the auditory image they perceived. Stimuli with reduced interaural coherence were binaurally fused when there was a large ILD, even when participants indicated that the same stimulus was unfused when there was a 0 dB ILD. This suggests that signals with low interaural coherence can become binaurally fused when the stimulus has large ILDs. To determine if participants could choose to attend to only one ear for stimuli that were reported to be fused in the presence of ILDs in Experiment 1 (i.e. if complete binaural fusion occurred), participants indicated the perceived loudness in their right ear for stimuli with varying right-ear intensities in Experiment 2. When the stimulus had a left-biased ILD, participants had a reduced ability to report changes in right ear signal intensity, suggesting that largely complete binaural fusion was fostered by the presence of large ILDs.