Binaural interaction in auditory brain-stem responses: effects of masking

Binaural interaction (BI) in auditory brain-stem responses (ABRs) can be shown by comparing a binaurally elicited ABR to the algebraic sum of monaurally elicited ABRs. Subtracting the summed monaural ABR from binaural ABR yields a wave form assumed to contain response components attributable to BI. It has been suggested that acoustic crosstalk accounts for some of the 'BI' seen with this technique and that contralateral masking should be used during monaural stimulation to eliminate crosstalk. However, this practice might in itself confound the results, even if the masking noise were not intense enough to affect the opposite ear, by 'central masking' of brain-stem neural activity. We studied the effects of contralateral wide-band masking on BI/ABR in 10 normal adult subjects. Clicks were presented at levels from 55 to 115 dB peSPL, at 10 dB intervals. Masking was presented at 73 dB SPL (47 dB effective masking level); based on pilot studies of interaural attenuation, this was a level expected to be at, or just below, the threshold of audibility in the contralateral ear. BI/ABR wave forms were not noticeably affected by the addition of contralateral noise in the monaural trials. In addition, BI/ABR was seen (as previously reported) at levels well below any possibility of crossover artifact. Thus, BI/ABR is not simply attributable to crossover. Contralateral masking is not necessary in recording BI/ABR, within the limits of the stimuli and transducers used in this study.