High Frequency Plateau in Gerbil Auditory Nerve Tuning Curves

Ruggero et al. (PNAS 97(22) p.11744, 2000) compared threshold tuning curves of chinchilla basilar membrane (BM) vibrations and auditory nerve (AN) fibers. They noted that the AN tuning curves lacked the supra-CF frequency plateaus that are present in BM responses and suggested that BM vibrations do not translate into AN responses in the supra-CF frequency region. This observation is relevant to the relationship between BM motion (macromechanics) and hair cell stimulation (micromechanics). To further investigate the discrepancy, we recorded AN responses from the gerbil, concentrating on the supra-CF region of single AN fibers. We observed a supra-CF frequency plateau in AN responses at very high sound pressure levels (> 100dB SPL). This AN plateau was at least 10 to 15dB higher than what is predicted from BM plateaus. However, at high sound pressure levels, we also recorded subharmonics in the acoustic signal in the ear canal. The subharmonics were not produced by the speaker and appeared to be produced in the auditory mechanics. Dallos and Linnell (JASA 40(3) p.561, 1966) studied similar subharmonics, and concluded that they were produced in the ear drum and in the cochlea. We still need to determine whether the subharmonics are responsible for the supra-CF frequency plateaus in the AN responses or if the AN responses are due to the motion of the BM at the fundamental frequency