EFFECTS OF CONTRALATERAL STIMULATION ON T RANSIENTLY EVOKED OTOACOUSTIC EMISSION

Background: In recent publications the influence of contralateral whit e noise on transient evoked otoacoustic emissions (TEOAE) is discussed with regard on contributions of the efferent auditory system. Methods : In the present study the effects have been investigated with regards to middle-ear muscles, efferents and cross hearing, TEOAE to monaural 40-80 dB SPL clicks were recorded in normal-hearing adults under simu ltaneous presentation of 20-60 dB SPL broadband noise to the contralat eral ear. Control runs were performed before, during a short break of, and after contralateral stimulation. The control run before contralat eral stimulation was used as a reference. Results: Decrease in TEOAE, and increase in accompanying noise floor, were found to follow the con tralateral stimulation. In particular a 1-3 dB decrease was found for contralateral noise levels of 40 and 60 dB SPL, even though the readin gs at 60 dB only were statistically significant (paired-samples t test , p=0.05). For both TEOAE and noise floor no systematic dependence on click intensity was seen. The control runs during temporary break and after contralateral noise revealed an increase in both TEOAE and noise floor. As a rule, the TEOAE adapted to the reference within 2-3 min f ollowing the cessation of contralateral stimulation, whereas the incre ased noise floor level was still noted after 10 min. Conclusions: Trad itionally, suppressing effects of contralateral stimulation on TEOAE h ave been attributed to cochlear efferents (CEs). Occasionally, the mid dle-ear muscle and cross hearing involvement have been considered as w ell. Substantially, the present results and findings of other workers are inconsistent with the basic knowledge of CE functioning: (i) The d ecrease in TEOAE under contralateral stimulation is in conflict with a n increase in cochlear microphonics and summating potentials observed during activation of CEs; (ii) contralateral suppression of TEOAE exhi bited no significant dependence on the test-stimulus level while the C Es are known to be efficient in the range of the low signal intensitie s only, and (iii) acoustic activation of the CEs can hardly be expecte d to reach levels sufficient to influence the TEOAE mechanism. The pre sent findings, i.e. decrease in TEOAE and increase in noise floor leve l, can more reasonably be explained as being mainly attributable to ac tivation of the middle-ear muscles.