Effects of electrolytic lesions of the superior olivary complex and trapezoid body on brainstem auditory-evoked potentials in the guinea pig. I. Vertex-tragus recordings.

Brainstem auditory-evoked potentials (BAEP) were recorded between vertex and tragus in 22 guinea pigs, after destruction of the contralateral ear in order to produce monaural stimulation. Stimuli employed were 100 dB SPL clicks and tone bursts. Normative recordings showed five positive peaks, P1-P5 and four negative peaks, N1-N4. Electrolytic lesions were then made to the superior olivary complex (SOC) ipsilateral to the stimulation (6 animals), contralateral to the stimulation (8 animals), and trapezoid body (TB) (8 animals). In each group a statistical analysis of the results (amplitudes and latencies) was made (paired t test). After lesions to the ipsilateral SOC, there was an increase in the amplitude of P1 and a decrease in P3. After lesions to the contralateral SOC, there was a larger decrease in P3, which disappeared in 2 guinea pigs which had a large lesion involving many TB fibers. After TB lesions, there was a decrease in the P2-N2 composite wave, disappearance of P3, a decrease in P4, and an increase in the latency of P1 and P1-P2 and P2-P4 intervals. After ipsilateral SOC lesions, which certainly involved the uncrossed olivocochlear efferent tract, the increase in P1 suggested a disinhibition of the efferent fibers. The large decrease or the loss of P3 after TB lesions and SOC lesions involving many TB fibers suggested that P3 could be generated by the contralateral part of the TB fibers, in the vicinity of the contralateral SOC. These data are in agreement with the predominance of fiber tracts in the generation of BAEP.