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What are otoacoustic emissions (OAEs)?
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Otoacoustic emissions are sounds that are produced by healthy ears in response to acoustic stimulation. They are considered to be epiphenomena, and byproducts of the activity of the outer hair cells in the cochlea.
The human auditory system has a remarkable ability to discriminate sounds that differ by no more than a few cycles per second or by a few decibels. George von Bekesy viewed the role of the human cochlea as purely passive and suggested that tuning of the response that is necessary to achieve pitch and intensity discrimination occurs in the central portion of the auditory pathway. Alternatively, Gold in 1948 suggested that the cochlea is an active participant in tuning the auditory response. He speculated that the cochlea has a positive feedback system that would produce spontaneous emissions.
Kemp first reported Otoacoustic emissions in 1978. Although his observations were greeted with skepticism, OAEs have since been reliably confirmed. They appear to be generated in the cochlear outer hair cells.
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How are OAEs measured? |
OAEs are measured by presenting a series of very brief acoustic stimuli, clicks, to the ear through a probe that is inserted in the outer third of the ear canal. The probe contains a loudspeaker that generates clicks and a microphone that measures the resulting OAEs that are produced in the cochlea and are then reflected back through the middle ear into the outer ear canal. The resulting sound that is picked up by the microphone is digitized and processed by specially designed hardware and software. The very low-level OAEs are seperated by the software from both the background noise and from the contamination of the evoking clicks.
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What is the clinical use of OAEs? |
The creation of OAEs by the cochlea and the re-emission of this energy as sound from the ear serve no important physiological purpose that can be determined. Their clinical significance is that they are evidence of a vital sensory process arising in the cochlea. OAEs only occur in a normal cochlea with normal hearing. If there is damage to the outer hair cells producing mild hearing loss, then OAEs are not evoked. A good rule of thumb is that OAEs are present if hearing is 35 dB or better. Because OAEs are evoked by signals that have a wide frequency response, a broad region of the cochlea responds, providing information on the frequency range from 1000 Hz to 4000 Hz.
OAEs were approved by the FDA for clinical use on July 14, 1989. They are most appropriate for use in difficult-to-test patients: newborn infants, young children, and developmentally delayed populations. OAEs only provide information about the activity of the cochlea, and do not assess the status of the auditory pathway, which is tested by the brainstem auditory evoked response (BAER).
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