[Lateral efferent system and evoked auditory potentials. Response in tinnitus].

The effects of ipsi- or contralateral masking have been studied in 45 normal hearing subjects and in 49 tinnitus patients. With normal subjects, ipsilateral masking during auditory stimulation involves--in opposition to contralateral masking which does not alter any latencies--a significant lengthening of all the latencies in the brainstem, except that of the first wave (right and left), and a significant decreasing of the amplitudes of the waves I L and R. It was important to know whether the lengthening of the latencies in the brainstem--which we suppose to be due to the stimulation of the lateral efferent system--was not in fact due to a preferential stimulation of the auditory cells from the basis of the cochlea. Yet experiments have shown that continuous masking of 40 dB at 750 Hz or 4000 Hz added to stimulation, involved no significant modification of the latencies in normal subjects. Therefore the lengthening of the latencies obtained with white noise masking, belonged to an other sphere. The second problem concerned the responses of tinnitus patients. Ipsilateral masking in white noises does not alter any latencies in the patients' brainstem. They seem to have lost the possibility of reacting to any additional noise coming from the outside. As we have already noticed, their latencies before masking were different from those of normal subjects. With masking the patients, latencies remained unchanged whereas the latencies of normal subjects lengthened and caught up the patients. To conclude, we suppose the lateral efferent system can be explored by the adjunction of white noise masking, ipsilateral to the stimulation. The expected response is a lengthening of interpeak I-V latency, without any modification of the latency of the first wave. Tinnitus patients generally do not have that type of response to white noise masking. Their lateral efferent system may not be operating.