Frequency dependence of binaural interaction in the auditory brainstem and middle latency responses.

Purpose: The primary purpose of this investigation was to determine the relative frequency representation of binaural function in the brainstem and cortex of adults. The secondary purpose was to compare adult responses to previously reported infant responses.

Methods: Simultaneous auditory brainstem responses and auditory middle responses were recorded monaurally and binaurally in 20 young women. The binaural (BIN) response was subtracted from the summed monaural waves (L+R) to obtain the binaural interaction components (BIC) from waves V (peak A) and Pa (BIC-Pa). Amplitude ratios were calculated as BIC/L+R. Repeated-measures analyses of variance evaluated responses to frequency (500 Hz vs. 4000 Hz), wave condition (L+R vs. BIN), and wave class (auditory brainstem response vs. auditory middle response).

Results: Waveforms were present for all conditions. The L+R responses were larger than the BIN responses, 500 Hz produced larger amplitudes than 4000 Hz, and Pa was larger than wave V. The largest response, overall, was the Pa(L+R) response to 500 Hz. For amplitude ratios, BIC-Pa/Pa(L+R) was larger than Peak A/[V(L+R)].

Conclusion: More neural resources are devoted to binaural function in the cortex than in the brainstem, and more resources are devoted to lower frequencies than to higher frequencies. The adult data confirm that previously recorded infant data reveal binaural immaturity. Longitudinal data should characterize developmental characteristics of binaural function.