Remote Extracochlear versus Intracochlear Recordings in the Guinea Pig

Abstract

A comparison was made of the whole-nerve action potential recorded from a wire electrode placed on the tympanic membrane (TM) (remote extracochlear recording) and that recorded from intracochlear electrodes in the first and third turn scala tympani of the guinea pig. Cochlear microphonics and summating potentials were also studied. The expected decreased sensitivity at the TM site was observed in all responses, although the microphonic (CM) was diminished more than the action potential (AP) as compared to the responses monitored via the ST₁ electrode. Nevertheless, the TM-recorded cochlear potentials largely mimicked the ST₁-recorded potentials in a qualitative manner, i.e., similar configurations of the CM pseudo-threshold functions. It was consistently observed that the N₂ of the AP is proportionally larger (relative to N₁) in the TM recording than in the ST₁ recording, These phenomena were attributed to the complex spread of the bioelectrical potentials in the nonhomogeneous volume conductor formed by the tissues of the temporal bone. It was suggested that the observed change in the N₁/N₂ ratio at the different sites of recording is due to the different contributions of second-order neuronal discharges to the compound AP. In effect, the TM electrode “sees” proportionally more activity from second-order fibers than does the ST₁ electrode, and the ST₃ electrode, less.