Objective measures of binaural interaction using interaural phase differences

Binaural interaction in the auditory system occurs when a sound is presented to both ears and the nervous system generates activity in response to this stimulation. In the present study, the neural responses generated by interaural phase differences (IPDs) were investigated using auditory evoked potentials (AEPs). The AEP paradigms were designed to observe the neural responses from subcortical (Experiment 1) and cortical (Experiment 2) regions. In addition to the physiological measures, behavioral (psychophysical) responses were obtained using comparable stimuli to reveal a possible perceptual correlate. The first experiment focused on subcortical responses. Two onset-dependent responses (auditory brainstem response & middle latency response) and two steady-state responses (fine-structure and envelope frequency following response) were observed in multiple IPD conditions. The results of the first experiment indicated that the amplitude of the responses decreased modestly as the IPD increased. However, compared to the in-phase condition, only the maximal IPD achieved statistical significance. The second experiment concentrated on the cortical response. Specifically, the P1-N1-P2 responses [acoustic change complex (ACC)] were analyzed. The amplitude of the ACC increased as the IPD increased. Additionally, the ACC amplitudes showed a significant positive correlation with the psychophysical measures. Taken together, the two experiments suggest that the cortical response, and the ACC in particular, may be used as a biomarker for binaural interaction.