STUDY ON TEMPORAL MODIFICATIONS OF EVOKED OTOACOUSTIC EMISSIONS DURING CONTRALATERAL ACOUSTIC STIMULATION IN THE NORMAL HUMAN EAR

In 32 subjects, the medial olivo-cochlear efferent system was activate d by a contralateral acoustic stimulation (CAS) which is able to mimic the inhibitory effects of electrical stimulation of the crossed olivo -cochlear bundle. A white noise CAS of 35dB SL induced temporal altera tions in otoacoustic emissions evoked by clicks of 63dB SPL, called ph ase shift effect (PSE). The PSE appeared as an advance of signals in 8 0% of cases and occurred after a latency of about 8ms. Two different s ignal processing methods (in time and frequency domains), showed that the phase shift was mainly located on lower frequencies with a maximum around 1.5kHz and a lack around 4kHz. When contralateral stimulation intensity was varying from 25 to 45 dB SPL, the PSE, increased linearl y ; the shape of the curve allowed to rule out a middle ear reflex. On the contrary, when ipsilateral stimulation intensity was varying from 57 to 69 dB SPL, the PSE decreased linearly, indicating that PSE orig inated in inner ear mechanisms. Lastly, a significant correlation had been evidenced between the amplitude of the suppressive (inhibitory) e ffect of CAS, and the amplitude of the phase shift effect, allowing on e to suggest that both effects may have a common physiological origin.