Tone decay for hearing-impaired listeners with and without dead regions in the cochlea

For people with normal hearing, a sustained tone with a frequency within the standard audiometric range remains audible when presented at a level well above threshold. However, for a pure tone with frequency close to the upper limit of hearing (well above 8 kHz), the loudness may decrease within seconds and the tone may decay to inaudibility, even when presented at a level between 20 and 40 dB SL. Scharf [in Hearing Research and Theory, edited by J. V. Tobias and E. D. Schubert (Academic, New York, 1983), Vol. 2, pp. 1-53] suggested that marked loudness adaptation only occurs when the excitation pattern evoked by a tone is spatially limited. The upper limit of hearing may be comparable to the boundary of a "dead region," which is a region with a complete loss of inner hair cell (IHC) and/or neural function. The present study investigated the perceived decay of pure tones for 9 normal-hearing subjects and 12 subjects with moderate to severe sensorineural hearing loss, using a wide range of frequencies (0.125-12 kHz). A dead region was diagnosed for 8 of the 12 subjects. No consistent association was found between the degree of tone decay and the presence of a dead region. Subjects with dead regions did not experience significantly more tone decay than subjects with comparable absolute thresholds but without a dead region, even when the frequency of the tone fell within or close to the edge of a dead region. For severely hearing-impaired subjects, spatial restriction of the excitation pattern was neither necessary nor sufficient to lead to tone decay. The prevalence of tone decay was not well predicted by the audiometric threshold at the test frequency. It is proposed that tone decay depends on the physiological condition of the place in the cochlea where the tone is detected, which, in a case involving a dead region, is the place adjacent to the dead region. The prevalence of tone decay increased when the audiometric threshold was above 50 dB HL in the frequency region where the tone was detected.