Lateralization and binaural discrimination of patients with pontine lesions

Lateralization and just-noticeable difference (jnd) measurements relative to the center were tested in a large group of patients with pontine lesions caused either by stroke or multiple sclerosis. Stimuli included binaural clicks, and low- and high-frequency narrow-band noise bursts. Two major types of abnormalities were revealed in the lateralization performances: perception of all stimuli, regardless of interaural differences (time and/or level) in the center of head (center-oriented), or lateralization of all stimuli to one side or the other of the head (side- oriented). The only significant correlation between jnd and lateralization performances was that an elevated jnd was always manifested in abnormal lateralization, while abnormality in lateralization did not necessarily indicate an abnormal jnd. Center-oriented lateralization was observed either for both interaural differences or only for one of them, and was found in both MS and stroke patients. All side-oriented stroke patients were similarly unable to center binaural stimuli for both time and level cues, whereas only one MS patient had this abnormality for interaural time differences, while his level performance was normal. More abnormalities were detected in the narrow band stimuli tests, although in some cases performance was more degraded for click stimuli. Lateralization tasks with high-frequency stimuli were more sensitive detectors of abnormality than jnd for any kind of stimulus, or lateralization tasks with low-frequency stimuli or America.