The effect of brainstem lesions on brainstem auditory evoked potentials in the cat

Brainstem auditory evoked potentials (BAEPs) were recorded before and after cuts were made in either the midline trapezoid body (TB), the lateral lemniscus (LL), or the combined dorsal and intermediate acoustic striae (DAS/IAS) in 23 anesthetized cats. Monaural and binaural rarefaction clicks were presented at a rate of 10 per s, and the potentials recorded from a vertex electrode referenced to either earbar or to the neck. The potentials were filtered so that fast and slow components could be examined separately and special efforts were exerted to obtain stable conditions so that small changes in waveforms could be significant. Lesions of the DAS/IAS produced negligible changes in either the fast or slow waves. Lesions of the midline TB reduced the amplitudes of peaks P3 through P5, while greatly reducing the amplitude of the slow wave. Complete lesions of the LL always reduced the amplitude of the slow wave. Lesions of the ventral part of the LL were more likely to reduce the amplitude of P4-P5. Our interpretations of these lesion experiments are based on the idea that individual fast peaks of the BAEP represent compound action potentials of fiber pathways. According to this view, only synchronized activity generated in populations of neurons that are both favorably oriented in space and significant in number, will contribute to the fast peak.