Click lateralization by multiple sclerosis patients

Abstract

Lateralization of dichotic clicks can be tested with gradually increasing interaural level differences (ILD) and interaural time differences (ITD). However, the lateralization of dichotic clicks with ITD is limited to small ITDS only (< 1 ms). The ability to lateralize dichotic clicks was tested in multiple sclerosis (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, where one included dichotic clicks with ILD and the other included dichotic clicks with ITD; (2) a positional jnd experiment in which the subject was asked to determine the difference in position of two successive click trains. Two reference positions were tested, the head center (ITD = 0 and ILD = 0) and midway between the center and the ear (ITD = 0.8 ms and ILD = 0 or ILD = 20 dB and ITD = 0). For each reference the experiment was performed first with control on ITD and then with control on ILD. From a group of 15 MS patients, seven performed poorly in the positional jnd experiment when the control was on ITD, and normally when the control was on ILD. Those subjects also reported that the position of each dichotic click with ITD < 1 ms was perceived at the head center, and, therefore, they had difficulties in performing the matching experiment. Brainstem auditory potentials (BAP) evoked by dichotic clicks with different ILDs and ITDs were measured in all the above MS patients. Principal component analysis of the measured waveforms indicates an abnormal behavior as a function of ITD in patients who performed abnormally in the psychoacoustical experiments. The anaylsis of the waveforms of all patients as a function of ILD resembles the normal behavior.