Otoacoustic emissions: New discoveries lead to sophisticated applications

In this brief report, Sumitrajit Dhar, PhD, Professor at the Knowles Hearing Center of the Northwestern University at Evanston (USA), argues that a new understanding of the physiology of OAEs will open the doors to sophisticated clinical applications of OAEs – moving OAEs beyond the realm of screening into sophisticated differential diagnosis.

History

The discovery of otoacoustic emissions (OAEs), faint sounds generated by the outer hair cells, by David Kemp (Kemp 1978) has changed both hearing science and audiology in profound ways. For scientists interested in understanding the cochlea, OAEs are a convenient non-invasive tool to probe active mechanics of the cochlea. For the clinician, OAEs have become the mainstay for screening auditory function for patients of all ages. Perhaps most notably, the discovery and popularization of OAEs has a shared history with newborn hearing screening. Interestingly, thirty years since the discovery of OAEs, we continue to learn about the basic biological processes responsible for the generation and propagation of OAEs.

Although David Kemp was the first person to demonstrate that OAEs could be generated in the ear canal, knowledge of sounds being generated somewhere in the ear has existed for a couple of centuries. Giuseppe Tartini of Padua, a composer and musician in the eighteenth century, was perhaps the first to notice a third note when he played two other notes on his violin. Others in Germany and France also noted the same phenomenon around there same time. Composers began using these sounds generated in the ears in compositions. Psychoacoustic examination began early and peaked in the 1960s when prominent psychoacoustician Goldstein called these sounds in the ear a mark of the “essential nonlinearity” of the auditory periphery. While all this work was related to what we call distortion products (DPs) today, Thomas Gold predicted a nonlinear amplification mechanism in the cochlea based on his observation of the ear’s acute sensitivity and frequency selectivity in the 1940s. David Kemp himself became interested in exploring whether our ears generated sounds while working on psychophysical questions pertaining to variations in hearing thresholds and loudness estimates as a function of frequency.

Clinical Applications

Perhaps the most popular and effective clinical application using OAEs is in screening. OAEs can be used for screening all populations but is most popular in newborn and school age screenings. It is also not difficult to imagine that OAEs are a wonderful tool in differential diagnosis of conditions such as auditory neuropathy where normal OAEs are expected but neural responses such as auditory brainstem responses are affected. One of the clinical challenges has been in using DPOAEs to predict hearing thresholds. Various attempts to use DPOAE levels directly to predict hearing thresholds have met with limited success (Dhar and Shaffer 2004).

One could argue that threshold prediction is perhaps not the best use of OAEs as they are essentially limited to function of the auditory system up to and including the outer hair cells. Lesions beyond that point (as in auditory neuropathy) are not expected to be reflected in OAEs. However, if threshold prediction is important for a clinical application, it is perhaps best to use DPOAE input/output (I/O) functions with a transformation from sound pressure level to pressure as shown in Figure 3. The I/O function obtained at the frequency of interest is transformed such that DPOAE pressure rather than sound pressure level is represented. A trendline is then fitted with data points having adequate signal to noise ratio. The stimulus level for which this trend line crosses 0 μPa is considered the equivalent behavioral threshold (see Neely, Johnson et al. 2009 for details). OAEs may however be applied to more complex clinical problems than threshold estimation. There already is preliminary evidence that pre-exposure OAE levels may be predictive of an individual’s vulnerability to hearing loss following noise exposure (Lapsley Miller, Marshall et al. 2006). Similarly OAEs may provide the most convenient tool in evaluating the efferent auditory pathway (Deeter, Abel et al. 2009). As new discoveries about their physiological origin are made, the clinical utility of OAEs will grow and they will continue to be a reliable clinical tool for diverse applications.

Bibliography

  • Deeter, R., R. Abel, L. Calandruccio and S. Dhar (2009). "Contralateral acoustic stimulation alters the magnitude and phase of distortion product otoacoustic emissions." J Acoust Soc Am 126(5): 2413-2424.
  • Dhar, S. and L. A. Shaffer (2004). "Effects of a suppressor tone on distortion product otoacoustic emissions fine structure: why a universal suppressor level is not a practical solution to obtaining single-generator DP-grams." Ear Hear 25(6): 573-585.
  • Kemp, D. T. (1978). "Stimulated acoustic emissions from within the human auditory system." J Acoust Soc Am 64(5): 1386-1391.
  • Lapsley Miller, J. A., L. Marshall, L. M. Heller and L. M. Hughes (2006). "Low-level otoacoustic emissions may predict susceptibility to noise-induced hearing loss." J Acoust Soc Am 120(1): 280-296.
  • Neely, S. T., T. A. Johnson, J. Kopun, D. M. Dierking and M. P. Gorga (2009). "Distortionproduct otoacoustic emission input/output characteristics in normal-hearing and hearingimpaired human ears." J Acoust Soc Am 126(2): 728-738.

Read here the whole report on Audiology Infos Brazil (Portuguese).

Sumitrajit Dhar, PhD, Professor Fellow, Hugh Knowles Center, Nortwestern University School of Communication


Photo: ABA