AGE AND THE HUMAN COCHLEAR TRAVELING-WAVE DELAY

Objective: Cochlear physiologic changes that occur with aging also may affect the mechanics and filtering properties of the cochlea. In stud ying aging effects, it is also necessary to consider the effects of he aring threshold losses because thresholds generally increase with age, and threshold losses also affect the filtering properties of the coch lea. A distortion product emission (DPE)-based measurement of human co chlear traveling wave delays allows an indirect assessment of the mech anics of the traveling wave and how it might change with age. Design: The cochlear traveling wave delay has been derived from DPE phase meas urements using an f(1) sweep paradigm (Kimberley, Brown, & Eggermont, 1993). In the present study, traveling wave delay was estimated from D PE phase measurements taken from the left ears of 91 subjects (22 to 7 8 yr) with auditory thresholds ranging from normal hearing (less than or equal to 25 dB SPL, ANSI 1969) to mild cochlear hearing damage. No one under the age of 65 had hearing losses exceeding 40 dB SPL to 8 kH z. Pure-tone thresholds were determined at eight frequencies through a 4-interval, forced-choice, adaptive-level psychophysical test. DPE ph ase and amplitude measurements were made at these eight frequencies us ing CUBeDIS-reIated software (AT&T Bell Labs). Round-trip traveling wa ve estimates were calculated from the DPE phase measurements for each of the eight f(2) frequencies (places). Results: Traveling wave delays estimated using DPE phase were not significantly affected by mild hea ring losses. Traveling wave delay, however, does increase slightly wit h age. Optimal f(2)/f(1) ratio and associated DPE amplitude also were tested for age or threshold effects. The optimal f(2)/f(1) ratio remai ned unaffected by age or hearing loss but decreased with increasing fr equency. The maximum DPE amplitude decreased with both advancing age a nd increasing pure-tone threshold, so it was impossible to isolate the contribution of each individual factor. Conclusions: Mild hearing los ses do not affect the round-trip traveling wave delay. There is a slig ht age dependence, however. This may suggest that there are age-relate d but threshold-independent factors that may alter such properties as basilar membrane stiffness or cochlear fluid composition. The relation ship between maximum DPE amplitude and threshold, at the frequencies t ested, is a better predictor of hearing thresholds than the DPE amplit ude as obtained from a fixed ratio DPE amplitude measurement.