John J. Guinan, Jr., Ph.D.
Professor of Otology and Laryngology, Harvard Medical School
Eaton-Peabody Laboratories, Massachusetts Eye and Ear Infirmary
There are several reflexes which allow the central nervous system to change the behavior of the peripheral auditory system: the middle-ear muscles, and the olivocochlear efferents. One focus of my work has been the acoustic stapedius reflex, the middle-ear muscle that responds to sound in humans. My students and I have demonstrated the brainstem locations, axon pathways, and sound response properties of stapedius motoneurons, their organization in the brainstem and the stapedius muscle, and the mechanisms by which stapedius contractions reduce masking by low-frequency sounds. Our work shows that stapedius motor organization is different from most muscles and may be a model for muscles which pull against fixed loads (e.g. eye, facial and laryngeal muscles).
A second focus is the medial olivocochlear (MOC) efferents. To understand the role of MOC efferents in hearing, we are measuring MOC effects in humans using otoacoustic emissions (OAEs), sounds produced within cochlea that can be measured in the ear canal. We are working to make measurements of OAEs in humans provide tests of efferent effects that may be useful for predicting susceptibility to damage from loud sounds and to provide insight into why some people have trouble understanding speech in noisy places.
Our study of the effects of MOC efferents has enabled us to learn important new things about the mechanical mechanisms that make mammalian hearing sensitive and frequency selective. MOC efferents synapse on outer hair cells, the motors that amplify sound within the cochlea. By studying the effects of MOC efferents on the output of the cochlea, the firing of auditory-nerve fibers, we have been able to show ways in which the cochlea works in the low-frequency region that is important for understanding speech.
1. Guinan JJ, Jr., Lin T, Cheng H. Medial-olivocochlear-efferent inhibition of the first peak of auditory-nerve responses: Evidence for a new motion within the cochlea. J Acoust Soc Am. 2005;118:2421-2433.
2. Guinan JJ, Jr (2006) Olivocochlear Efferents: Anatomy, Physiology, Function, and the Measurement of Efferent Effects in Humans. Ear Hear 27:589-607.
3. Backus BC, Guinan JJ, Jr. Measurement of the distribution of medial olivocochlear acoustic reflex strengths across normal-hearing individuals via otoacoustic emissions. J Assoc Res Otolaryngol. 2007;8:484-96.
4. Guinan, J.J., Jr., Cooper, N.P. 2008. Medial olivocochlear efferent inhibition of basilar-membrane responses to clicks: evidence for two modes of cochlear mechanical excitation. J Acoust Soc Am 124, 1080-92.