Binaural-cue Weighting and Training-Induced Reweighting Across Frequencies

During sound lateralization, the information provided by interaural differences in time (ITD) and level (ILD) is weighted, with ITDs and ILDs dominating for low and high frequencies, respectively. For mid frequencies, the weighting between these binaural cues can be changed via training. The present study investigated whether binaural-cue weights change gradually with increasing frequency region, whether they can be changed in various frequency regions, and whether such binaural-cue reweighting generalizes to untrained frequencies. In two experiments, a total of 39 participants lateralized 500-ms, 1/3-octave-wide noise bursts containing various ITD/ILD combinations in a virtual audio-visual environment. Binaural-cue weights were measured before and after a 2-session training in which, depending on the group, either ITDs or ILDs were visually reinforced. In experiment 1, four frequency bands (centered at 1000, 1587, 2520, and 4000 Hz) and a multiband stimulus comprising all four bands were presented during weight measurements. During training, only the 1000-, 2520-, and 4000-Hz bands were presented. In experiment 2, the weight measurements only included the two mid-frequency bands, while the training only included the 1587-Hz band. ILD weights increased gradually from low- to high-frequency bands. When ILDs were reinforced during training, they increased for the 4000- (experiment 1) and 2520-Hz band (experiment 2). When ITDs were reinforced, ITD weights increased only for the 1587-Hz band (at specific azimuths). This suggests that ILD reweighting requires high, and ITD reweighting requires low frequencies without including frequency regions providing fine-structure ITD cues. The changes in binaural-cue weights were independent of the trained bands, suggesting some generalization of binaural-cue reweighting.