Prof. Dr. Pim van Dijk
Department of Otorhinolaryngology, Head & Neck Surgery
University of Groningen, University Medical Center Groningen, The Netherlands
Probing internal noise in the cochlea via spontaneous otoacoustic emissions
Spontaneous otoacoustic emissions (SOAEs) are often cited as the primary evidence for active amplification in the inner ear. Yet, it is unclear how SOAEs improve auditory perception. The active amplification is presumably incorporated in a feedback loop, which enhances sound-induced vibrations in the inner ear. However, active feedback has two potential consequences which are not necessarily positive: (1) the feedback loop can become instable, which leads to spontaneous generation of sound (e.g. SOAEs), (2) in addition to boosting a sound stimulus, also any internal noise in the ear might be amplified, which would lead to a detriment of hearing thresholds. Interestingly, spontaneous otoacoustic emission themselves provide a window to study inner ear noise when studying their interaction with near-threshold tones. Previously published data and new experiments will be review, showing: (1) at a SOAE frequency, the threshold of hearing is relatively low as compared to neighboring frequencies, (2) near threshold tones at an SOAE frequency cause a nearly deterministic response of the emission, showing that internal noise in the inner-ear feedback loop plays no role in tone-detection at these frequencies, and (3) tones slightly above or below an SOAE frequency cause a stochastic response of the SOAE due to internal noise in the emission generator, which helps their perception. Thus, the inner ear functions in a way where noise either plays no role in tone detection or is helpful for tone detection. As such, the active feedback appears to function in a way that optimally supports auditory sensitivity.