Temporal correspondence of intracranial, cochlear and scalp-recorded human auditory nerve action potentials

Abstract

Conventional, vertex-ipsilateral ear records (‘A’), as well as 3-channel Lissajous' trajectories (3-CLTs) of auditory brain-stem evoked potentials (ABEPs) were recorded from the scalp simultaneously with tympanic membrane electrocochleograms (‘TME’) and auditory nerve compound action potentials (‘8-AP’) recorded intracranially using a wick electrode on the auditory nerve between the internal auditory meatus and the brain-stem. The recordings were made during surgical procedures exposing the auditory nerve. The peak latency recorded from ‘TME’ corresponded to trajectory amplitude peak ‘a’ of 3-LLT and to peak ‘I’ of the ‘A’ channel ABEP. Peak latency of ‘8-AP’ was slightly longer than the latency of peak ‘II’ of ‘A’ when ‘8-AP’ was recorded from the root entry zone and the same or shorter when recorded from the nerve trunk. ‘8-AP’ peak latency was shorter than trajectory amplitude peak ‘b’ of 3-CLT regardless of where the wick electrode was along the nerve. Peak latencies from all recordings sites clustered into two distinct groups—those that included N1 from ‘TME’, peak ‘I’ of the ‘A’ record and trajectory amplitude peak ‘a’ of 3-CLT, and those that included the negative peak of ‘8-AP’ and trajectory amplitude peak ‘b’ of 3-CLT, as well as peak ‘II’ of the ‘A’ record, when present. In one case, the latency of peak ‘II’ and trajectory amplitude peak ‘b’ was manipulated by changing the conductive properties of the medium surrounding the auditory nerve. These results are consistent with other evidence proposing: (1) the most distal (cochlear) portion of the auditory nerve is the generator of the first ABEP component (‘I’, ‘a’); (2) the proximal auditory nerve is the major contributor to the ‘A’ channel ABEP component ‘II’; (3) in addition to the auditory nerve, more central structures participate in the generation of the 3-CLT ‘b’ component.