Generation of auditory brain stem responses (ABRs). I. Effects of injection of a local anesthetic (procaine HCI) into the trapezoid body of guinea pigs and cat.

Auditory brain stem potentials were recorded between the skull and a non-cephalic reference electrode in anesthetized guinea pigs before and after the injection of a local anesthetic agent (procaine HCI) into the trapezoid body from a ventral approach. All components except P1, N1 and P2 were affected; N2 was delayed; P3 and N3 were lost; P4 was both broadened in duration and shortened in latency; N4 was attenuated in amplitude. All of these changes were temporary and recovery of the components occurred. Identification of the altered components was aided by their latency and amplitude changes as a function of both stimulus intensity and rate. This study implicates the trapezoid body as contributing to the generation of auditory brain stem components beginning with N2.