Sound lateralization and interaural discrimination. Effects of brainstem infarcts and multiple sclerosis lesions

Subjects with brainstem lesions due to either an infarct or multiple sclero sis (MS) underwent two types of binaural testing (lateralization testing an d interaural discrimination) for three types of sounds (clicks and high and low frequency narrow-band noise) with two kinds of interaural differences (level and time). Two major types of abnormalities were revealed in the lat eralization performances: perception of all stimuli, regardless of interaur al differences (time and/or level) in the center of the head (center-orient ed), or lateralization of all stimuli to one side or the other of the head (side-oriented). Similar patterns of abnormal lateralization (center-orient ed and side-oriented) occurred for MS and stroke patients. A subject's patt ern of abnormal lateralization testing was the same regardless of the type of stimulus or type of interaural disparity. Lateralization testing was a m ore sensitive test than interaural discrimination testing for both types of subjects. Magnetic resonance image (MRI) scanning in three orthogonal plan es of the brainstem was used to detect lesions. A semi-automated algorithm superimposed the auditory pathway onto each MRI section. Whenever a lesion overlapped the auditory pathway, some binaural performance was abnormal and vice versa. Given a lateralization test abnormality, whether the pattern w as center-oriented or side-oriented was mainly determined by lesion site. C enter-oriented performance was principally associated with caudal pontine l esions and side-oriented performance with lesions rostral to the superior o livary complex. For lesions restricted to the lateral lemniscus and/or infe rior colliculus, whether unilateral or bilateral, just noticeable differenc es (JNDs) were nearly always abnormal, but for caudal pontine lesions JNDs could be normal or abnormal. MS subjects were more sensitive to interaural time delays than interaural level differences particularly for caudal ponti ne lesions, while stroke patients showed no differential sensitivity to the two kinds of interaural differences. These results suggest that neural pro cessing of binaural stimuli is multilevel and begins with independent inter aural time and level analyzers in the caudal pens. (C) 2000 Elsevier Scienc e B.V. All rights reserved.