On the origin of wave II of the auditory brain stem evoked response

The generators of the several components of the auditory brain stem evoked response (ABR) have yet to be completely identified. However, there is compelling evidence to suggest specific generators for waves I and II, namely the distal and proximal portions of the auditory nerve, respectively. Although there is no question concerning the origin of wave I, there remains some uncertainty as to whether or not wave II arises entirely from the auditory nerve. The purpose of this study was to examine the behavior of wave II, as recorded via vertical versus horizontal derivations, in an effort to determine if wave II in both derivations is equally affected by stimulus manipulations (i.e., as would be presumed from a single generator theory). ABR recordings were obtained from normal-hearing young adults using vertical (hairline-to-ipsilateral mastoid and hairline-to-contralateral mastoid) and horizontal (mastoid-to-mastoid) derivations of responses to clicks presented at a variety of stimulus levels (40-80 dB nHL) and rates (19.9-59.9/sec.). The results indicate a shorter latency for wave II recorded in the horizontal derivation (wave IIa), compared to the vertical derivation (wave IIb). Also, wave IIa was found to be more susceptible to increased stimulus rate and exhibited a different latency behavior than wave IIb. These findings seem difficult to reconcile on the basis of a single generator for waves IIa and IIb.