BINAURAL AUDITORY PROCESSING IN MULTIPLE-SCLEROSIS SUBJECTS

In order to relate human auditory processing to physiological and anat omical experimental animal data, we have examined the interrelation-sh ips between behavioral, elecirophysiological and anatomical data obtai ned from human subjects with focal brainstem lesions. Thirty-eight sub jects with multiple sclerosis were studied with tests of interaural ti me and level discrimination (just noticeable differences or jnds), bra instem auditory evoked potentials and magnetic resonance (MR) imaging. Interaural testing used two types of stimuli, high-pass ( > 4000 Hz) and low-pass ( < 1000 Hz) noise bursts. Abnormal time jnds (Tjnd) were far more common than abnormal level jnds (70% vs 11%); especially for the high-pass (Hp) noise (70% abnormal vs 40% abnormal for low-pass ( Lp) noise). The HpTjnd could be abnormal with no other abnormalities; however, whenever the BAEPs, LpTjnd and/or level jnds were abnormal Hp Tjnd was always abnormal. Abnormal wave III amplitude was associated w ith abnormalities in both time jnds, but abnormal wave III latency wit h only abnormal HpTjnds. Abnormal wave V amplitude, when unilateral, w as associated with a major HpTjnd abnormality, and, when bilateral, wi th both HpTjnd and LpTjnd major abnormalities. Sixteen of the subjects had their MR scans obtained with a uniform protocol and could be anal yzed with objective criteria. In all four subjects with lesions involv ing the pontine auditory pathway, the BAEPs and both time jnds were ab normal. Of the twelve subjects with no lesions involving the pontine a uditory pathway, all had normal BAEPs and level jnds, ten had normal L pTjnds, but only five had normal HpTjnds. We conclude that interaural time discrimination is closely related to the BAEPs and is dependent u pon the stimulus spectrum. Redundant encoding of low-frequency sounds in the discharge patterns of auditory neurons, may explain why the HpT jnd is a better indicator of neural desynchrony than the LpTjnd. Encro achment of MS lesions upon the pontine auditory pathway always is asso ciated with abnormal BAEPs and abnormal interaural time discrimination but may have normal interaural level discrimination. Our data provide one of the most direct demonstrations in humans of relationships amon g auditory performance, evoked potentials and anatomy. We present a mo del showing that many of these interrelationships can be readily inter preted using ideas developed from work on animals, even though these r elationships could not have been predicted with confidence beforehand. This work provides a clear advance in our understanding of human audi tory processing and should serve as a basis for future studies.