Spatiotemporal responses of the visual system in demyelinating diseases

Previous studies have shown that, in patients with lesions of the visual pathway, contrast sensitivity (CS) measured with stationary sine-wave gratings can demonstrate deficits, that is, anomalies in pattern detection. The purpose of this study was to investigate whether temporal processing can be affected in demyelinating lesions of the visual pathway. CS was measured for eight spatial frequencies (SF), using stationary and temporally modulated stimuli in a group of 10 patients with multiple sclerosis or optic neuritis. A control group was composed of 48 normal eyes. With stationary stimuli, CS losses were found in 17 eyes of patients; 11 of these eyes were 'unaffected'. The importance of the CS measurements in detecting early subclinical visual damage is emphasized. The effect of temporal modulation in patients with CS deficits was different from that observed in normals: CS deficits were modified by temporal variation in three distinct manners. In 7 eyes (type 1 effect), temporal modulation reduced the deficit at all SFs. In type 2 (5 eyes), the effect was reversed at low and high SFs: the deficit was reduced at low SFs (below 2-3 cycles/deg) and aggravated at high SFs. In type 3 (4 eyes), CS deficit was aggravated over most SFs. Thus these patients exhibited anomalies of the spatiotemporal responses which could not be predicted by the CS to stationary stimuli. Some theoretical models of spatiotemporal processing proposed in normal vision might account for our results. Possible explanations of these findings are proposed and discussed.