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Perceptual correlates of neural plasticity related to spontaneous otoacoustic emissions? A. Norena, C. Micheyl, J.D. Durrant, S. Chéry-Croze, and L. Collet Hearing Research, Volume 171, Issues 1-2, September 2002, Pages 66-71 Neurosciences and Sensory System Laboratory, CNRS UMR 5020, Claude Bernard University (Lyon 1), Edouard Herriot Hospital, Place d'Arsonval, 69437, Lyon Cedex 03, France Abstract: The inner ear contains receptor cells that oscillate spontaneously, generating
waves that propagate backward in the cochlea, ultimately causing sound
to be radiated into the ear canal - the spontaneous otoacoustic emissions
(SOAEs). Except in rare conditions, these internally generated signals
appear to go unheard. The intensity of SOAEs admittedly hovers near the
threshold of detection, but they are essentially continuous and perhaps
last a lifetime. The hypothesis is tested that the frequency difference
limen (DLF) is affected by SOAEs. The results show that the DLF systematically
improves near SOAE frequencies, determined ipsi- or contralaterally
to the SOAEs, arguing for a central effect. The results are
discussed in the context of central plasticity. Comment: Permanent inner-ear tones, the so-called spontaneous otoacoustic emissions (SOAEs), are measured in the ear canal of the majority of healthy human ears. They are caused by mechanical oscillations of frequency-tuned sensory cells in the inner ear, the so-called outer hair cells. Norena and others have now found that SOAEs change hearing even in the contralateral ear. They measured frequency discrimination at and around the frequency of the SOAE, both in the SOAE ear and in the other ear. In five of the six tested subjects they found improved frequency discrimination at the SOAE frequency in the SOAE ear, and in all six subjects in the other ear at the same frequency. In the flanking frequency ranges below and above the SOAE, the improved hearing acuity of both ears declined gradually with the distance from the SOAE frequency. Simulations of SOAEs by low-level tones via headphone in a control group could not elicit the contralateral effect. The authors concluded that SOAEs are apparently related to long-term alterations in the central auditory system. These new results agree with earlier findings of a bilateral interdependence of SOAEs, and with earlier indications that the neural relay stations between the SOAEs of both ears are the interconnected central nuclei of the two colliculi inferiores in the upper brainstem (auditory midbrain). The results of Norena and others show once again that permanent tones in our ears (SOAEs) are much more than accidental epiphenomena of inner ear mechanics. (Comment Martin Braun) |