Effect of stimulus level and frequency on ABR and MLR binaural interaction in human neonates.

Auditory brainstem (ABR) and middle latency (MLR) responses were evoked by click and tone-burst stimuli from human neonates. Electrophysiologic evidence of binaural interaction was measured by subtracting waveforms obtained for binaural stimulus conditions from waveforms obtained for the sum of right ear monaural and left ear monaural stimulus conditions. The effects of stimulus level and stimulus frequency on binaural interaction were evaluated by measuring the number, latency and amplitude of components found in the derived binaural interaction waveform, that is, binaural interaction components (BIC). BICs were more prevalent in the latency range of ABRs than for MLRs. Click and tonal stimuli were equally effective for deriving ABR-BICs, while tone-bursts were somewhat less effective than clicks for deriving MLR-BICs. Stimulus-response dependencies for ABR and MLR component latencies were apparent in monaural, binaural and binaural interaction waveforms. Normalized amplitudes for BICs showed that low-frequency tone-burst stimuli resulted in the largest values compared to click and high-frequency tonal stimuli. Comparison of these results with published results from adults demonstrated immaturity of binaural interaction in neonates.