CONTRALATERAL ACOUSTIC STIMULATION INDUCES A PHASE ADVANCE IN EVOKED OTOACOUSTIC EMISSIONS IN HUMANS

In 28 normal-hearing human subjects, the medial olivocochlear efferent system was activated by contralateral acoustic stimulation which is a bl to mimic the inhibitory effects of electrical stimulation of the cr ossed olivocochlear bundle. A first experiment on 16 subjects demonstr ated that a contralateral white noise of 35 dB SL was able to induce t emporal changes on transiently evoked otoacoustic emissions in respons e to clicks of 63 dB SPL. These temporal changes consisted of an advan ce of click-evoked otoacoustic signals in 87% of cases and is referred to as phase-shift effect. The phase advance, quantified using two sig nal processing methods in both time and frequency domains, was found t o be mainly associated with lower frequencies, with a maximal effect a t 1.5 kHz and minimal effects around 3.5 and 4 kHz. In a second experi ment, carried out on 12 subjects, a negative relationship was found to exist between the ipsilateral stimulation level (level of clicks rang ing from 57 to 69 dB SPL) and the phase-shift effect (PSE). Specifical ly in the range of levels tested (25-45 dB SL), a linear relationship presenting non obvious saturation effect was observed between the cont ralateral level and the PSE. The PSE was examined in 6 additional subj ects exhibiting pathological symptoms; 2 of 3 individuals, who had no contraleteral stapedial reflexes unilaterally, showed the PSE whereas this response was reduced or absent in 3 other subjects in the ear wit h severed efferents associated with a vestibular neurotomy. The integr ity of olivocochlear efferents was, therefore, necessary to obtain a f ull effect but the absence of stapedial reflex did not prevent the eff ect from occurring.