Otoacoustic emissions are sounds produced by the cochlea (inner ear) upon stimulation, and were first observed by Kemp in 1978. Distortion product otoacoustic emissions (DPOAEs) are a special class of OAEs that arise when two sinusoidal stimuli are simultaneously introduced to the ear. Distortion products are defined as outputs that do not reside at the input frequencies to a system; the presence of distortion products in general implies nonlinear response. The most prominent DPOAE observed is the cubic product, located at 2f1 – f2. This DPOAE frequency is referred to as fd.

DPOAE’s are presumed to arise from the action of outer hair cells (OHCs). OHCs are mechanically active cells in the cochlea that are able to change length based on their transmembrane voltage. They can be thought of frequency-specific amplifiers that help ‘sharpen the notch’ of the cochlear tuning curve, giving us adequate frequency resolution to interpret speech. DPOAEs vary in intensity according to both absolute and relative levels of the fundamental frequencies f1 and f2, and also vary in intensity according to the ratio of f2/f1. These variations have been empirically defined over the past several decades.

In clinical use, the absence of DPOAE activity is considered an indicator of cochlear pathology. The ability to predict DPOAE response is an active area of research due to its potential clinical utility. As an example, Kujawa et. al. [1] observed an exponential decay in the distortion product (fd) amplitude that is dependent on an intact olivocochlear (OC) bundle, and propose their assay as a reliable metric of OC reflex strength. Our project attempts to replicate this assay for future application to investigate how various pathologic conditions affect the OC reflex.