Emer O'Connor, Tom C. A. Freeman, Tom H. Margrain; Sensitivity to retinal and extra-retinal motion signals as a function of age. Journal of Vision 2008;8(6):669. doi: 10.1167/8.6.669.
During smooth pursuit, object motion is often judged by combining estimates of retinal motion with extra-retinal estimates of eye velocity. The latter are thought to be based on motor commands emanating from the eye movement system. Retinal motion sensitivity is known to decline in older observers but little is known about the precision of extra-retinal signals. We therefore measured retinal speed discrimination thresholds using a standard 2AFC technique and compared results to a pursuit condition in which the same stimulus was tracked by eye. The stimulus consisted of dim dots on a black background in a dark room. Using a sample of older (mean age = 67.9) and younger participants (mean age = 22.5), speed discrimination was assessed for standard speeds of 4.8, 9.6, 19.2 deg/s. In the 'fixation' condition, a static fixation point was displayed and participants were asked to judge the motion of the moving stimulus presented for on average 200ms. In the 'pursuit' condition, participants judged the same stimulus motion whilst tracking a pursuit target moving at the same speed as the dot pattern. Experiment 1 replicated previous studies by showing a significant age-related decline in discrimination performance in the fixation condition. Interestingly, we found no difference in the pursuit condition. However, the lack of effect could have resulted from participants using the pursuit target (prior to the dot pattern) as an additional source of information about motion not available in the fixation condition. Experiment 2 resolved this problem by equating duration across the two conditions and also controlling for possible use of retinal slip that could occur before pursuit was initiated. With these controls a similar pattern of results was observed, with the exception that both age groups performed worse in the pursuit condition. These results imply that extra-retinal signals show more resistance to age-related decline.