Generation of auditory brain stem responses (ABRs). II. Effects of surgical section of the trapezoid body on the ABR in guinea pigs and cat.

Auditory brain stem potentials were recorded between the skull and a non-cephalic reference electrode in guinea pig and cat before and after midline section of the trapezoid body from a ventral approach. The ABR after complete section was altered: N2 was delayed, P3 and N3 were lost, P4 and N4 were shortened in latency but only N4 was attenuated in amplitude. With partial section of the trapezoid body the amplitude of P3 was linearly related to the extent of the section. Recordings from the surface of the trapezoid body before and after section revealed a loss of activity contralateral to the ear stimulated and a preservation of activity ipsilateral to the section. Binaural interaction in P4 and N4 was attenuated in a linear manner as a function of the extent of trapezoid body section. This study suggests that auditory brain stem structures contralateral to the midline of the trapezoid body generate components N2, P3, N3, N4, whereas auditory structures ipsilateral to the midline generate components P1, N1, P2 and P4.