Comparing behavioral and otoacoustic emission fine structures

Behavioral and otoacoustic emission (OAE)-based measures of cochlear function often exhibit "fine structure," quasiperiodic fluctuations in magnitude (and/or phase) as a function of frequency. For pure-tone detection thresholds and OAE responses to low-level clicks or tones, in particular, this fine structure is thought to arise via multiple intracochlear reflections of the stimulus-evoked OAE between its site of origin and the middle ear boundary. Cochlear resonances occur at frequencies for which the reflections sum in phase, thus enhancing behavioral sensitivity and OAE responses. Here we explore predictions of this theoretical framework via detailed measurements of behavioral thresholds and OAEs evoked by single tones, i.e., stimulus-frequency OAEs (SFOAEs). Consistent with the cochlear resonance framework, we find that the fine structure observed in hearing thresholds is nearly identical to the fine structure of both SFOAE amplitudes and delays. In addition, the frequency separation between adjacent threshold minima is associated with one cycle of SFOAE phase accumulation. However, the relationship between SFOAE amplitude and fine structure magnitude is less clear, perhaps due to variability in the strength of the reflection mechanism at the middle ear boundary.