Three-dimensional planar and vector analysis of brainstem auditory evoked potentials as a function of intensity in guinea pig.

Brainstem auditory evoked potentials (BAEPs) have been studied in guinea pigs as a function of intensity by means of the Three-channel Lissajous' trajectory (3-CLT) method. Data were quantified in terms of duration, orientation of planar segments and orientation of equivalent dipoles. The auditory stimulus was a click varying from 48 dB pSPL to 108 dB pSPL by steps of 10 dB pSPL. A semi-automated procedure was used to identify planar segments, by calculating the curvature (apex) and magnitude curves. Dipole analysis was made at the corresponding magnitude peaks and apices. Taking into account a symmetrical orientation of planar segments and equivalent dipoles, similar results were obtained after right and left stimulation. Six planar segments, A, B, C, D, E, F were analysed as were six equivalent dipoles corresponding roughly to P1, N1, P3, N3, P4 and N4 on a vertical lead. There was a parallel latency shift of the first and last point of planar segments as a function of intensity, accounting for the stable duration of a planar segment. Planar segment D (roughly corresponding to N3 on a vertical lead) disappeared as the intensity decreased. Only planar segments C and F and dipole N3 showed a significant change in their orientation. The stable duration of planar segments suggests that intensity coding in the auditory brainstem, in terms of a global function of a system, involved the same number of synapses in a serial circuit while the activated neurons increase in the in parallel circuitry, as the intensity increases. Similar changes in orientation of planar segments C (including P3 of guinea pig BAEPs) and F (including N4 of guinea pig BAEPs) suggest a possible common source with a depolarization followed by a repolarization.