Fine structure of the intracochlear potential field. II. Tone-evoked waveforms and cochlear microphonics

M. Zidanic and W. E. Brownell Department of Otolaryngology, Head and Neck Surgery, Johns Hopkins University, Baltimore, Maryland 21205-2195. 1. Extracellular evoked potentials to low-frequency pure-tone stimuli were recorded in the second cochlear turn of the anesthetized guinea pig. Spatial variations of the field potentials were characterized by advancing and withdrawing micropipettes along radial tracks in scala tympani (ST) and scala vestibuli (SV). Compound action potentials (CAPs) and cochlear microphonics (CM) are the major components of the evoked responses to 50- to 1,600- Hz stimuli. The relative contribution of CM and CAP to the evoked potentials varies with cochlear scala and location within the scala as well as with stimulus frequency and intensity. 2. In the 50- to 800-Hz frequency range, the largest CM in the second turn was recorded from scala media (SM). Below 500 Hz the CM in SV is larger than in ST, whereas above 500 Hz a larger CM is present in ST. The CM in SV is nearly in phase with the CM in SM, although it is smaller by a factor of two to four. The CM diminishes by another factor of two over a 100-microns depth range as an electrode is withdrawn out of SV through the spiral ligament. While the electrode is in SV or in the fluid outside the spiral ligament, the CM magnitude does not change by greater than 10%. The shape of the radial CM magnitude profile along tracks in SV shows little or no dependence on intensity in the 65- to 105-dB SPL range or on frequency in the 50- to 800-Hz range. 3. Unlike the CM profiles in SV, the shape of the CM magnitude and phase profiles in ST are a complex function of frequency and intensity. Below 500 Hz, the CM goes through a 140-180 degree radial phase shift over a 100-microns distance near the spiral ligament bordering ST. Concomitant with the large radial phase shift is a local minimum of the CM magnitude. The location of this "virtual ground point" can shift radially by as much as 100 microns over a 30-dB intensity range. The CM magnitude deep in ST is always larger than the CM outside the spiral ligament bordering ST. However, the ratio of the CM magnitudes at these two locations can vary from 0.1 to 0.8, the ratio tending to increase with intensity in the 200- to 800-Hz range.