The importance of phase of stimulus and the reference recording electrode in brain stem auditory evoked potentials.

In normal subjects the phase of the stimulus and the reference electrode of the brain stem auditory evoked potential (BAEP) were investigated. Compared to condensation (C), rarefaction (R) clicks produced longer latencies, especially for peaks IN, IIIN and IV and lower amplitudes, mainly for IIN. The amplitude of the initial phase of R clicks was lower in the acoustical wave form (with piezoelectric phones) than the succeeding higher amplitude C phase, but damping characteristics varied with recording conditions. In comparing Ipsi vs. Contra ear reference the Ipsi showed a shorter latency for II and IIN, a longer latency for III, IIIN and IV and a higher amplitude, except for II. The relative contribution of the central vertex (Cz) and of each ear to the standard Cz-ear linkage was determined by referring each to an indifferent electrode. The activity at the Ipsi ear was out of phase with Cz up to peak IV, especially I, IN and III (but not II) with a resulting high amplitude in the Cz-ear linkage; in-phase relationships were noted between Cz and the Contra ear resulting in a relatively low amplitude in the Cz-Contra ear linkage. Discussion of the latter points included the complex, but significant changes of the amplitude and latency of Cz activity by an ear reference.