Foveal interocular time thresholds and latency differences in multiple sclerosis

Nineteen patients with suspected multiple sclerosis (MS) and 28 control subjects were foveally stimulated by a small cross formed by rectangular red-light-emitting diodes. By means of crossed polarizers one eye was exposed only to the horizontal bar of the cross, the other to the vertical. Stimulus onset asynchrony ranged from 0 to +/- 300 ms (the horizontal bar preceding or following the vertical). The task was to indicate whether the horizontal or vertical bar had appeared first. Compared with normal subjects, MS patients exhibited much higher time thresholds (ranging from -150 to +130 ms) and had considerable interocular latency differences (up to 29 ms), indicating unilateral or asymmetrical impairment of the visual pathways. The psychophysical latency differences of the patients were compared to monocular latencies and interocular latency differences of the visually evoked cortical potential (VEP) obtained by foveal stimulation. Under the stimulus conditions chosen in this study, the diagnostic value of the psychophysical measurements was equal to or, for McAlpine's classes I and II of definite and probable MS patients, better than that based on VEP recordings.