1 Department of Speech and Hearing Sciences University of Arizona, Tucson
Otoacoustic emissions are low-intensity sounds that are produced in the cochlea and transmitted through the middle ear apparatus to the ear canal. They can be detected and extracted from the background noise in the ear canal through the use of a sensitive microphone and selective filtering or averaging techniques. The technical aspects of emission recording are very similar to those associated with the detection and capture of auditory evoked potentials. Emissions provide an acoustic link to a physiological window through which we can view the auditory periphery using frequency-specific stimuli that are presented at low and moderate intensities. The window provides an opportunity to examine cochlear activity that occurs prior to stimulation of the nervous system.
Tonal emissions occur spontaneously in approximately 40% of people who have normal thresholds for pure-tone stimuli. SOAE and other types of emissions may be influenced by both ipsilateral and contralateral stimuli. One form of interaction results in suppression of the emission, and the tuning patterns associated with suppression of emissions by ipsilateral stimuli have characteristics that are similar to tuning patterns associated with single cochlear hair cells and individual neurons of the auditory nerve. These findings and other lines of evidence support the conclusion that an emission having tonal characteristics is produced from a very restricted region of the cochlear partition.
Emissions may be evoked by brief click or tonal stimuli, and by continuous tonal stimuli, in virtually all individuals who have normal pure-tone thresholds and uncompromised middle ear systems. The EOAE are compromised by conditions that compromise the function of the cochlea, and they hold promise as tools that might be employed in screening for hearing loss. Preliminary findings suggest that screening employing TEOAE produces a yield that is similar to that produced by screening programs based on auditory brainstem responses. Emissions may offer advantages over current screening methods because of the ease with which they can be recorded and their apparent independence from neurological influence.
Many questions regarding the origin and nature of emissions remain unanswered, but they appear to offer great sensitivity to the status of the auditory periphery. DPOAE provide an opportunity to scan the cochlear partition from base to apex with frequency-specific stimuli, and give the examiner a detailed view of the status of the end organ. The study of DPOAE holds great promise in refinement of site of lesion identification. It is exciting to witness the development of a tool to help clinical examiners probe the function of the previously inaccessible cochlea.
Key Words: emissions, distortion products, screening, hearing loss
Submitted on July 3, 1991
Accepted on August 9, 1991
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