EFFECTS OF MULTIPLE-SCLEROSIS BRAIN-STEM LESIONS ON SOUND LATERALIZATION AND BRAIN-STEM AUDITORY-EVOKED POTENTIALS

Magnetic resonance (MR) imaging, brainstem auditory evoked potentials (BAEPs), and tests of interaural time and level discrimination were pe rformed on sixteen subjects with multiple sclerosis (MS). Objective cr iteria were used to define MR lesions. Of the eleven subjects in whom no pontine lesions were detected and the one subject who had pontine l esions that did not encroach upon the auditory pathways, all had norma l BAEPs and interaural level discrimination, although a few had abnorm al interaural time discrimination. Of four subjects with lesions invol ving the pontine auditory pathway, all had both abnormal BAEPs and abn ormal interaural time discrimination; one also had abnormal interaural level discrimination. Analysis of the data suggest the following: wav es I and II are generated peripheral to the middle of the ventral acou stic stria (VAS); wave III is generated ipsilaterally in the region of the rostral VAS, caudal superior olivary complex (SOC) and trapezoid body (TB); and waves V and L are generated contralaterally, rostral to the SOC-TB. The region of the ipsilateral rostral SOC-TB is implicate d as part of the pathway involved in the generation of waves V and L. Interaural time discrimination of both high and low frequency stimuli were affected by all brainstem lesions that encroached on auditory pat hways. A unilateral lesion in the region of the LL affected interaural time discrimination for low-frequency stimuli less severely than bila teral lesions of the LL or a unilateral lesion of the VAS. The only in teraural level discrimination abnormality occurred for a subject with a unilateral lesion involving the entire rostral VAS. It appears that detailed analysis of lesion locations coupled with electrophysiologica l and psychophysical data holds promise for testing hypotheses concern ing the function of various human auditory brainstem structures.