A new auditory threshold estimation technique for low frequencies: proof of concept

Presently available nonbehavioral methods to estimate auditory thresholds perform less well at frequencies below 1 kHz than at 1 kHz and above. For many uses, such as providing accurate infant hearing aid amplification for low-frequency vowels, an accurate nonbehavioral method to estimate low-frequency thresholds is needed. A novel technique was developed to estimate low-frequency cochlear thresholds based on the use of a previously reported waveform. It was determined how well the method worked by comparing the resulting thresholds to thresholds from onset-response compound action potentials (CAPs) and single-auditory-nerve (AN)- fibers in cats. A long-term goal is to translate this technique for use in humans.

An electrode near the cochlea records a combination of cochlear microphonic (CM) and neural responses. In response to low-frequency, near threshold-level tones, the CM is almost sinusoidal whereas the neural responses occur preferentially at one phase of the tone. If the tone is presented again but with its polarity reversed, the neural response keeps the same shape, but shifts ½ cycle in time. Averaging responses to tones presented separately at opposite polarities overlaps and interleaves the neural responses and yields a waveform in which the CM is canceled and the neural response appears twice each tone cycle, that is, the resulting neural response is mostly at twice the tone frequency. The resultant waveform is referred to as "the auditory nerve overlapped waveform" (ANOW). In this study, ANOW level functions were measured in anesthetized cats from 10 to 80 dB SPL in 10 dB steps using tones between 0.3 and 1 kHz. As a response metric, the magnitude of the ANOW component was calculated at twice the tone frequency (ANOW2f). The ANOW threshold was the sound level where the interpolated ANOW2f crossed a statistical criterion that was higher than 95% of the noise floor distribution. ANOW thresholds were compared with onset-CAP thresholds from the same recordings and single-AN-fiber thresholds from the same animals.

ANOW and onset-CAP level functions were obtained for 0.3 to 1 kHz tones, and single-AN-fiber responses from cats. Except at 1 kHz, typical ANOW thresholds were mostly 10 to 20 dB more sensitive than onset-CAP thresholds and 10 to 20 dB less sensitive than the most sensitive single-AN-fiber thresholds.

ANOW provides frequency-specific estimates of cochlear neural thresholds over a frequency range that is important for hearing but is not well accessed by nonbehavioral, objective methods. Results suggest that with further targeted development, the ANOW low-frequency threshold estimation technique can be useful both clinically in humans and in basic-science animal experiments.