Conscious perception relies on sensory input processed by the nuclei in the nervous system. Neuronal adaptation allows the nuclei to provide useful input affecting also our auditory perception. For instance, sound localization that relies on the binaural cues can be biased by an adaptor carrying specific localization information. Here, we investigated how the auditory system adapts to the binaural cues, i.e. to interaural time (ITD) and level (ILD) differences, using wide-band stimuli containing the natural ITD- and/or ILD cues that exist in free-field listening conditions.
To this end, HRTF-, ITD-, and ILD targets were created using white-noise signals filtered with (modified) non-individual head-related transfer functions (HRTFs; horizontal angles between ±15° in 5° spacing) so that the ITD- and ILD targets contained the phase- and magnitude responses of the HRTFs, respectively. Psychometric functions for laterality were obtained in a spatial discrimination paradigm before and after exposure to an adaptor sequence. Three adaptation conditions were employed following Phillips & Hall (2005): One asymmetric with an ILD adaptor on one side and an ITD adaptor on the other, and two symmetric conditions with either ITD or ILD adaptors on both sides of the midline. The adaptors were presented in alternating sequences with directional cues corresponding to ±60°.
Each condition had a specific effect on the psychometric functions. The asymmetric condition shifted the thresholds for all targets, potentially because the ILD adaptor affected the ILD channel tuned to that side. Both symmetric conditions reduced the slopes of the functions: ITD adaptors reduced the slope of the ITD target and this effect was pronounced with ILD adaptors and –targets. Although monaural effects may have contributed to the shift caused by the asymmetric condition, the results imply that both ITD- and ILD processing are prone to binaural adaptation, the latter perhaps more than the former.