Current understanding suggests that there are two different mechanisms by which otoacoustic
emissions (OAEs) are generated in the cochlea. These mechanisms include a nonlinear-distortion
mechanism and a coherent-reflection mechanism. Distortion product OAEs (DPOAEs) are
believed to include contributions from both mechanisms, while stimulus frequency OAEs
(SFOAES), at least at low and moderate levels, are believed to be generated primarily
by the coherent-reflection mechanism. In the case of DPOAEs, the interaction of the
two mechanisms produces a series of alternating peaks and valleys in the response
level when recorded in small frequency increments. This pattern of peaks and valleys
typically is referred to as fine structure. There has been much speculation that the
interaction of the two mechanisms and the resulting fine structure limits the clinical
test performance of DPOAEs. There are few data to address this speculation. Here,
we review the literature that describes the cochlear source mechanisms and their potential
relationship to clinical applications. We then present results for preliminary data
collected in a group of 10 normal-hearing subjects where we explore the influence
of common approaches to setting DPOAE stimulus parameters on the resulting fine structure.
These preliminary results suggest that, at the moderate stimulus levels used in clinical
applications, each of the different stimulus parameters results in a similar amount
of fine structure and, therefore, fine structure cannot be eliminated through manipulation
of stimulus parameters. We also review the results of some preliminary efforts to
identify stimulus parameters that can be used to record SFOAEs (OAEs generated by
the reflection mechanism). The potential clinical applications of SFOAEs have received
little attention in the literature. By identifying stimulus parameters producing robust
responses in normal-hearing ears, it may be possible to more fully evaluate clinical
applications of SFOAEs.
Key Words
Fine structure - otoacoustic emissions - stimulus parameters
