Role of the efferent medial olivocochlear system in contralateral masking and binaural interactions: An electrophysiological study in guinea pigs

Contralateral broadband noise (BBN) elevates ipsilateral auditory threshold s (central masking) and reduces the amplitude of ipsilateral brainstem audi tory evoked potentials (BAEPs). Binaural interactions are complex psychophy sical phenomena, but binaural interaction components are easily extracted f rom BAEPs to monaural versus binaural click stimulation. However, contralat eral or binaural, acoustical stimulation is known to activate simultaneousl y the crossed and uncrossed medial olivocochlear (MOC) efferent systems and decrease activity in both cochleas. Particularly, contralateral BBN stimul ation suppresses in part ipsilateral peripheral activity. What is the role of such contralaterally induced peripheral suppression in the overall chang es in central BAEPs observed during contralateral masking or binaural stimu lation? Compound action potentials (CAPs) of the auditory nerve and BAEPs w ere recorded simultaneously in awake guinea pigs from electrodes chronicall y implanted on the round window of the cochlea and the surface of the brain . Peripheral and central measures of contralateral masking and binaural int eractions were obtained from responses to monaural or binaural clicks, with or without contralateral BBN, recorded before, during, and after the rever sible blockade of the MOC function following a single intramuscular injecti on of gentamicin. Contralateral BBN effectively reduced the amplitudes of C AP and of all BAEP peaks. CAP to ipsilateral click did not, however, change significantly from monaural to binaural click stimulation; still, normal b inaural interaction components developed in the BAEPs. When the medial effe rent function was blocked by gentamicin, the normal contralateral BBN suppr ession of CAP and of the earliest BAEP peak was lost; however, the later BA EP peaks were suppressed by contralateral BBN as before gentamicin, and the central binaural interaction components were unchanged. In these experimen tal conditions, the MOC efferent system seems to play little role in centra lly recorded contralateral masking and binaural interactions.