Binaural interaction effects on the auditory brainstem response of the cat and kitten

The auditory brainstem response (ABR) is a series of volume conducted potentials that can be recorded from the scalp within 10 ms following auditory stimulation. Differences between ABRs evoked with binaural stimulation and those constructed by summing equivalent numbers of monaural stimulation to each ear indicate the presence of binaural interaction for some ABR potentials but not for others. In the present study, ABR binaural interactions were studied in both cats and kittens. Binaural interactions were not seen for waves 1-3 but were present at the latencies of waves 4, 5 and, in most cases, 6. The sound intensities used were selected to insure that acoustic cross-over, i.e. sound presented monaurally to one ear stimulating the other, did not influence binaural interaction effects. Experiments with cats that had been monaurally deafened confirmed that the effects observed were not due to acoustic cross-over. Systematic manipulation of stimulus rate and intensity produced marked changes in the level of binaural interaction. Increases in stimulation rate from 10 to 100 clicks/s reduced binaural interaction for wave 4 and reversed the direction of binaural interaction for wave 5. Wave 6 was not generally present at rates above 10 clicks/s. Reduction of stimulus intensity reduced binaural interaction for wave 4. Binaural interaction effects were at adult levels in kittens of 20 days for waves 4 and 5. Wave 6 was not present until 30 days of age. These data suggest a possible model of the physiological processes producing binaural interaction which is based on occlusion as seen in other areas of the nervous system. In such a model, convergent input becomes more important in driving the generators of some ABR potentials when the system is stressed (as, for example, by increased stimulus rate), than it is when the system is not stressed.