Limits to the role of a common fundamental frequency in the fusion of two sounds with different spatial cues

Two experiments establish constraints on the ability of a common fundamental frequency (F0) to perceptually fuse low-pass filtered and complementary high-pass filtered speech presented to different ears. In experiment 1 the filter cut-off is set at 1 kHz. When the filters are sharp, giving little overlap in frequency between the two sounds, listeners report hearing two sounds even when the sounds at the two ears are on the same F0. Shallower filters give more fusion. In experiment 2, the filters' cut-off frequency is varied together with their slope. Fusion becomes more frequent when the signals at the two ears share low-frequency components. This constraint mirrors the natural filtering by head-shadow of sound sources presented to one side. The mechanisms underlying perceptual fusion may thus be similar to those underlying auditory localization.