Effect of Interaural Intensity Differences on Binaural Summation of Brainstem Auditory-evoked Potentials

With binaural stimulation the two specific parameters that can modify the neural responses substantially are the intensity differences and the time of arrival of the stimulus at the two ears; these events on electrophysiological evidence, being mediated at the superior olivary complex and at the inferior colliculus, the origins of Waves III and V respectively. Comparison of the brainstem auditory-evoked potentials to monaural stimulation with that due to binaural stimulation reveals a significant increase only in the amplitude of Wave V. The amount of increase is reduced from 68.7% with equal intensities at the two ears to 44% with a 10-dB interaural intensity difference. With a 20-dB interaural intensity difference the amplitude of Wave V reverts to that due to monaural stimulation. The 68.7% greater amplitude for Wave V was not due to effective increase in loudness sensation on binaural stimulation (which is equivalent to a monaural increase in intensity of 3 dB) but to binaural summation. In contrast to Wave V, Wave III decreased in amplitude below that resulting from monaural stimulation with intensity differences of 10 and 20 dB at the two ears. This supports the suggestion of inhibitory processes at that level of the brainstem. As only Wave V shows binaural summation the region of inferior colliculus is suggested as the site for the occurrence of this phenomenon. Binaural interaction can also be demonstrated within a limited range with intensity differences at the two ears.