Age and the human cochlear traveling wave delay

Cochlear physiologic changes that occur with aging also may affect the mechanics and filtering properties of the cochlea. In studying aging effects, it is also necessary to consider the effects of hearing threshold losses because thresholds generally increase with age, and threshold losses also affect the filtering properties of the cochlea. A distortion product emission (DPE)-based measurement of human cochlear traveling wave delays allows an indirect assessment of the mechanics of the traveling wave and how it might change with age.

The cochlear traveling wave delay has been derived from DPE phase measurements using an f1 sweep paradigm (Kimberley, Brown, & Eggermont, 1993). In the present study, traveling wave delay was estimated from DPE phase measurements taken from the left ears of 91 subjects (22 to 78 yr) with auditory thresholds ranging from normal hearing (< or = 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 kHz. Pure-tone thresholds were determined at eight frequencies through a 4-interval, forced-choice, adaptive-level psychophysical test. DPE phase and amplitude measurements were made at these eight frequencies using CUBeDIS-related software (AT&T Bell Labs). Round-trip traveling wave estimates were calculated from the DPE phase measurements for each of the eight f2 frequencies (places).

Traveling wave delays estimated using DPE phase were not significantly affected by mild hearing losses. Traveling wave delay, however, does increase slightly with age. Optimal f2/f1 ratio and associated DPE amplitude also were tested for age or threshold effects. The optimal f2/f1 ratio remained unaffected by age or hearing loss but decreased with increasing frequency. The maximum DPE amplitude decreased with both advancing age and increasing pure-tone threshold, so it was impossible to isolate the contribution of each individual factor.

Mild hearing losses do not affect the round-trip traveling wave delay. There is a slight age dependence, however. This may suggest that there are age-related but threshold-independent factors that may alter such properties as basilar membrane stiffness or cochlear fluid composition. The relationship between maximum DPE amplitude and threshold, at the frequencies tested, is a better predictor of hearing thresholds than the DPE amplitude as obtained from a fixed ratio DPE amplitude measurement.