CONTRALATERAL AND IPSILATERAL SUPPRESSION OF EVOKED OTOACOUSTIC EMISSIONS AT HIGH STIMULATION RATES

A previous study has shown that the amplitude of otoacoustic emissions decreases with increase in stimulus rate. Furthermore, these changes are similar to the changes initialed by contralateral suppression of the click-evoked otoacoustic emission. Further studies have been carried out which showed that contralateral suppression, due to a 60 dB SL white-noise suppressor, has a magnitude of about 2.7 dB for the normal subjects used in this study. The equivalent ipsilateral 'suppression' is approximately 8 dB al a stimulus rate of 2000 clicks/s. The two effects have an equivalent magnitude at a stimulus rate of approximately 300 clicks/s. In acoustic neuroma patients, where the nerve pathway is impeded by the tumour, the ipsilateral 'suppression' effect is virtually absent. These findings suggest that an ipsilateral 'suppression' similar in form but larger in magnitude than the more usual contralateral suppression can be created by increasing the stimulus rate using maximum length sequence (MLS) techniques.