Binaural phase effects in the auditory brainstem response.

This study investigated the possibility of a correlate of the masking level difference (MLD) in the auditory brainstem response (ABR), given that behavioral data indicate the brainstem origination of the MLD. Previous studies have reported MLD characteristics in the late auditory evoked potentials. In the present study, stimuli were presented in S(o)N(o) and S pi N(o) to female adults with normal hearing. In experiment 1, a binaural phasic difference potential (BPD) was derived by subtracting the suprathreshold ABRs to S(o)N(o) from S pi N(o) for tone pips of 500 to 4000 Hz. The magnitude of the BPD was inversely related to frequency. In experiment 2, thresholds to 500-Hz tone pips in S(o)N(o) and S pi N(o) were equal. In a case study of a subject with no behavioral MLD, the 500-Hz BPD was abnormal on one side, suggesting unilateral interruption of phase synchrony in the brain stem. Thus, the results of this study, in conjunction with previous studies on late potential correlates of the MLD, suggest that the ability to maintain phase information through the brain stem is necessary but not sufficient to generate the MLD.