Prediction of binaural click lateralization by brainstem auditory evoked potentials.

A previous study by Furst et al. (1985) has shown that in healthy subjects brainstem responses evoked by binaural auditory stimuli with interaural time difference (ITD) and interaural level difference (ILD) include information about the integration of data received by both ears. A correlation was found between the first major peak of the binaural difference waveform and perception of click lateralization and fusion. We have now tested whether a similar correlation exists in patients with multiple sclerosis (MS). The ability to lateralize dichotic clicks was tested in MS patients with normal audiograms. Two kinds of psychoacoustical experiments were employed: (1) A matching experiment in which the subject was asked to match the perceived positions of two click trains, one of which consisted of dichotic clicks with ILD and the other dichotic clicks with ITD; and (2) A positional JND experiment in which the subject was asked to determine the difference in perceived position of two successive click trains. Two reference positions were tested, the head center and the side of the head near the ear, while the control was either on ITD or on ILD. According to the psychoacoustical performances, three groups of patients were identified. Group I consisted of patients who performed normally in all the psychoacoustical experiments. Group II patients were able to lateralize binaural clicks but performed abnormally in the matching experiment and in the position discrimination experiment when the control was on ITD and the reference position was the head center. The patients in Group II performed normally in the discrimination experiments when the control was on ILD, and when the control was on ITD but the reference position was the head side. Group III consisted of those who were not able to perform either one of the psychoacoustical experiments. They perceived the same binaural clicks in different positions in different times. Brainstem auditory potentials evoked by dichotic clicks with different ILDs and ITDs were measured in all the MS patients, and the corresponding binaural difference (BD) waveforms were calculated. Whenever beta, the first major peak of BD, was identified it was used to obtain a physiological matching curve. It was derived by matching an ILD on the basis of similar beta latencies. For every patient, in either Group I or II, the physiological matching curve was very similar to his psychoacoustical matching curve.