Evaluation of FFP in kangaroo rat: Apical or basal?

Abstract

Davis and Hirsh [Audiology 15, 181–195 (1976)] argue that the intensity required (40 dB SL) to evoke the 500-Hz frequency-following potential (FFP) in human subjects is sufficient to stimulate the basal cochlea, driving high-frequency neurons at the tails of their tuning curves. We investigated this question by comparing the threshold of the vertex FFP with the phase-locking thresholds of low and high best frequency neurons to 500-Hz tone bursts in auditory nerve and cochlear nuclei of kangaroo rats. The effects of broadband and high-pass noise on the FFP and phase-locked neuronal responses were also compared. Only neurons with best frequencies around 500 Hz locked at FFP threshold levels. Neurons with best frequencies up to 1.5 kHz locked when the sound was increased 20 dB, and a 40-dB increase produced locking in 2.0–2.5-kHz neurons. The masking data are in agreement with the conclusions that within 10 dB of FFP threshold only a restricted region of the cochlea is activated, and a greater region is involved as intensity is increased. However, with appropriate high-pass masking, the 500-Hz region of the cochlea can be selectively evaluated. [Supported by NIH and AFOSR.]