Absence of tonic activity of the crossed olivocochlear bundle in determining compound action potential thresholds, amplitudes and masking phenomena in anaesthetised guinea pigs with normal hearing sensitivities.

In Nembutal- or Urethane-anaesthetised guinea pigs N1 audiograms and N1 input-output functions were measured as were compound action potential (CAP) tuning curves under forward masking and simultaneous masking conditions. Then the crossed olivocochlear bundle was lesioned at the floor of the fourth ventricle and the cochlear responses were re-measured. There were never any changes in the N1 audiograms, input-output functions, or the CAP tuning curves. Thus, the crossed efferent pathways do not appear to play any tonic role in determining cochlear threshold sensitivities, selectivities or masking phenomena in anaesthetised guinea pigs with normal hearing sensitivities.