Motion perception and ageing at scotopic light levels

Abstract

We investigated how elderly observers perceive scotopic motion. Full-screen drifting sinusoidal gratings generated by CRS VSG-2/3 were presented on a 21-inch Sony monitor for 500 ms. Scotopic conditions were simulated using neutral density filters. Following 30 min dark adaptation, motion detection and speed discrimination thresholds were measured in two groups of young (20–30 years) and older (50–75years) subjects. Motion detection threshold was assessed for two spatial frequencies at both ends of the visible range. Grating speed varied starting from a very low value, according to the method of constant stimuli involving forced choice direction discrimination task. Speed discrimination performance for the same spatial frequencies was studied using the method of single stimuli. Subjects indicated if the grating was moving faster or slower than the mean drift rate of the range. Slow (2.6 deg/s) and faster (9.2 deg/s) drift rates were used. Older subjects, unable to detect slow moving gratings, had higher motion detection threshold than younger subjects especially at low spatial frequency. Speed discrimination worsened for older subjects, more so at the low drift rate. Scotopic CSF did not correlate with the loss demonstrated by older subjects. Our results indicate visual motion processing impairments with age under scotopic conditions.