Effects of electrolytic lesions of the superior olivary complex and trapezoid body on brainstem auditory-evoked potentials in the guinea pig. II. Three-channel Lissajous' trajectory analysis.

Brainstem auditory-evoked potentials (BAEP) were studied both with vertex (positive) - tragus (negative) derivation and three-channel Lissajous' trajectory (3-CLT) planar analysis in guinea pigs (GP) after destruction of the ipsilateral superior olivary complex (SOC) (6 GP), contralateral SOC (8 GP) referred to the stimulation side, and trapezoid body (TB) in the sagittal line (8 GP). The size of the lesion was evaluated on histological examination. The right ear was locally destroyed with aminoside instillation. The left ear was stimulated with unfiltered clicks and tone bursts of different frequencies at 100 dB SPL. Three questions were the basis of this study: (i) What are the effects of brainstem lesions on 3-CLT? (ii) Is there a relationship between planar segments and underlying sources? (iii) What is the generator of planar segment C (P3) in GP? 3-CLT analysis was made on plane A (corresponding to P1 on a vertex-tragus channel), plane B (N1-P2) and plane C (P3.) Plane C disappeared after TB lesions and remained after ipsi- and contralateral SOC destructions. However, plane C disappeared after extensive lesions to the contralateral SOC affecting the TB. Planes A and B were always observed whatever the lesion, with sometimes a modification of their orientation. A localized brainstem lesion induced a local modification of the three-dimensional trajectory. These data are in agreement with the assumption that some planes of 3-CLT reflect the activity of underlying generators. However, the loop which contains the planar segment is a better indicator of the global electrical activity of the source. Plane C (P3) would have a contralateral source which is probably not the SOC but the afferent curved fibers of the TB close to the contralateral SOC.