Distortion product otoacoustic emissions (DPOAEs) measured in the ear canal are composed of OAEs generated by at least two mechanisms coming from different places in the cochlea. Otoacoustic emission (OAE) models hypothesize that reduction of cochlear gain will differentially impact the components. The purpose of the current experiment was to provide preliminary data about DPOAE components in adults with hearing loss in relation to OAE models and explore whether evaluation of the relative amplitudes of generator and reflection components can enhance identification of hearing loss.
DPOAEs were measured from 45 adult ears; 21 had normal hearing (≤15 dB HL) and 24 with mild-to-severe sensorineural hearing loss (>15 dB HL). The higher frequency primary (f2) was swept logarithmically between 1500 and 6000 Hz, and f2/f1 was 1.22. The two equal-level primaries varied from 55 to 75 dB SPL in 5 dB steps. The swept primary procedure permitted the measurement of the amplitude and phase of the DPOAE fine structure and the extraction of the two major components (generator and reflection) by varying the predicted delays of the analysis windows.
DPOAE fine structure was reduced or absent in ears with hearing loss. DPOAE generator and reflection components were lower in ears with hearing loss than those with normal hearing, especially for the reflection component. Significant correlations were found between the generator component and hearing threshold but not between reflection levels and hearing threshold. Most ears with normal hearing had both components, but only a small number of ears with hearing loss had both components.
The reflection component is not recordable or low in level in ears with hearing loss, explaining the reduced or absent DPOAE fine structure. DPOAE generator components are also lower in level in ears with hearing loss than in ears without hearing loss. In ears that had both measurable generator and reflection components, the relationship between the two did not depend on the presence or absence of hearing loss. Because reflection components are not measurable in many ears with hearing thresholds >15 dB HL, stimuli that evoke other types of reflection emissions, such as stimulus-frequency or long-latency transient-evoked emissions, should be explored in conjunction with DPOAE generator components.