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

Abstract

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 Pi 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.

Key Words:

• Auditory brainstem responses
• Generators
• Superior olivary complex
• Trapezoid body
• Guinea pig