Effect of olivocochlear bundle section on evoked otoacoustic emissions recorded using maximum length sequences

Area of Science:

• Audiology
• Neuroscience
• Otoacoustic Emissions

Background:

• Transient evoked otoacoustic emissions (TEOAEs) amplitude decreases at high stimulation rates, a phenomenon termed the 'rate effect'.
• The involvement of ipsilateral efferent activity has been hypothesized as a potential explanation for this rate effect.

Purpose of the Study:

• To investigate the role of efferent activity in the MLS rate effect on TEOAEs.
• To determine if efferent innervation influences TEOAE amplitude reduction at high click rates.

Main Methods:

• TEOAEs were recorded from normal-hearing adults and patients with unilateral vestibular nerve section (affecting efferent pathways).
• Stimulation rates varied from conventional (40 clicks/s) to high (5000 clicks/s) using maximum length sequences (MLS).
• Amplitude changes in TEOAEs were compared between ipsilateral and contralateral ears relative to the vestibular nerve section.

Main Results:

• Increasing click rates from 40 to 5000 clicks/s reduced TEOAE amplitude.
• This amplitude reduction was comparable in ears with and without efferent innervation (ipsilateral vs. contralateral to the surgery).
• The reduction also matched that observed in normal-hearing individuals.

Conclusions:

• The hypothesis that efferent activity is the primary mechanism behind the MLS rate effect is rejected.
• Intracochlear processes are proposed as the more likely underlying mechanism for the observed TEOAE amplitude reduction at high stimulation rates.