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Spontaneous Otoacoustic Emissions Reveal an Efficient Auditory Efferent Network.

Purpose
Understanding speech often involves processing input from multiple modalities. The availability of visual information may make auditory input less critical for comprehension. This study examines whether the auditory system is sensitive to the presence of complementary sources of input when exerting top-down control over the amplification of speech stimuli.


Method
Auditory gain in the cochlea was assessed by monitoring spontaneous otoacoustic emissions (SOAEs), which are by-products of the amplification process. SOAEs were recorded while 32 participants (23 women, nine men; Mage = 21.13) identified speech sounds such as "ba" and "ga." The speech sounds were presented either alone or with complementary visual input, as well as in quiet or with 6-talker babble.


Results
Analyses revealed that there was a greater reduction in the amplification of noisy auditory stimuli compared with quiet. This reduced amplification may aid in the perception of speech by improving the signal-to-noise ratio. Critically, there was a greater reduction in amplification when speech sounds were presented bimodally with visual information relative to when they were presented unimodally. This effect was evidenced by greater changes in SOAE levels from baseline to stimuli presentation in audiovisual trials relative to audio-only trials.


Conclusions
The results suggest that even the earliest stages of speech comprehension are modulated by top-down influences, resulting in changes to SOAEs depending on the presence of bimodal or unimodal input. Neural processes responsible for changes in cochlear function are sensitive to redundancy across auditory and visual input channels and coordinate activity to maximize efficiency in the auditory periphery.