Effects of low-frequency biasing on auditory-nerve activity

Abstract

Intensity functions of single fibers in the auditory nerve of gerbils were obtained to tone bursts alone and tone bursts superimposed on a low‐frequency biasing signal. The bias was a triangular sound‐pressure waveform with a period of 100 ms and was usually presented at 90 dB SPL. Cochlear microphonics (CM) recorded in scala media of the basal and second turns of the cochlea were trapezoidal in shape in response to the triangular waveform of sound pressure. Assuming the CM waveform reflects basilar‐membrane displacement at very low frequencies, it is concluded that the input impedance of the gerbil cochlea is substantially resistive at frequencies as low as 10 Hz. Intensity functions of single fibers with characteristic frequencies (CFs) below about 12 kHz exhibited enhanced thresholds to CF tones associated with basilar‐membrane displacement toward scala tympani; conversely, thresholds to CF tones were suppressed during displacement toward scala vestibuli. Tone‐alone thresholds were usually between the two biased conditions. The bias had little effect on responses of tones placed below or above CF and on the activity of fibers with CFs greater than about 12 kHz. These physiological results are compatible with corresponding psychophysicalmeasures obtained from human observers.