2010 Presentations (Communicative Events)

Subharmonics and Auditory Nerve Tuning Curves in Gerbil

Huang, Stanley; Dong, Wei; Olson, Elizabeth S.

In the literature, an auditory nerve tuning curve obtained from single unit recording typically has a tail and a tip. Responses beyond the tip in the well supra-CF frequency region had never been documented. In a study designed to explore that region specifically, we found plateau responses in the supra-CF region of several auditory nerve tuning curves at very high sound pressure levels (~120dB SPL). (Huang & Olson, ARO 2009, poster #623) However, a complicating issue at high sound pressure levels is the generation of subharmonics, which are likely generated in the eardrum (Dallos & Linnell 1966 JASA 40(3):561-564). Indeed, we found subharmonics in a subset of ear canal pressure we measured in gerbils, and they might have contributed to the supra-CF responses. Here we present auditory nerve tuning curves in which supra-CF neural responses were present while subharmonics were beneath the noise floor in the ear canal pressure. Thus these detections of supra-CF neural responses did not seem to suffer from subharmonic "contamination." To probe this further, we delivered loud tones and compared ear canal pressure and intracochlear pressure at subharmonics frequencies, and the quantitative relationship between the two reinforced that subharmonics were probably not responsible for the supra-CF neural responses we measured. It is conceivable that this subharmonic finding has at least two clinical implications. First, eardrum produced subharmonics might occur with very high power hearing aids. This could pose a problem to patients with band limited or high frequency hearing loss, since subharmonics that are within their normal-hearing frequencies could be perceived to be "louder" than the fundamental. Second, we found that the eardrum (the putative source of the subharmonics) was a reasonably effective sound radiator. Thus hearing aids that drive the eardrum directly might produce feedback.