Several studies have investigated the effects of aging on motion perception,
1,3,6 although the studies are inconclusive as to what extent aging affects motion sensitivity, or what the underlying cause is. The lowest signal-to-noise dot ratio (motion coherence threshold) in random dot kinematograms is often used to examine how aging affects motion sensitivity.
2–4,15–17 Some studies report that motion coherence thresholds are impaired in older observers,
1,3,6 whereas others have found no difference with age,
2,5,18 or that the difference is manifest only at low stimulus contrast,
1 or depends on speed. At moderate speeds (4–8°/s), several groups have found a deficit in motion coherence sensitivity in older observers,
1,3,6 although others have found no difference with age
2,5,18 or the difference may be manifest only at low stimulus contrasts.
1 Sensitivity deficits in older observers have been reported at high (>16°/s) speeds
3,18 and low (<1°/s) speeds,
2 whereas other groups have found no change as a function of age.
5 In principle, any loss of motion sensitivity could be caused by at least two classes of underlying deficits: there could be increased uncertainty about the direction of individual dots; or there could be a reduction in the proportion of dots that are processed. It is possible that these two processes could change at different rates with speed and aging or that deficits in one process could be offset by improvements in another with age. Motion coherence paradigms are not able to identify how or which of these two parameters are affected. In the present study, we used an equivalent noise (EN) paradigm to provide a direct estimate of these two factors.
19,20 The essential difference between the direction threshold stimulus we used in this study and the motion coherence stimuli used in other studies, is that in the EN stimulus, all dots were signal elements, and gave information about the total direction.
19 Many studies investigating motion sensitivity with age have used contrast noise in their stimuli.
8,21–23 Stimuli with low contrast can be a disadvantage when testing the elderly since any loss in motion sensitivity may be due to age-related reduction in contrast sensitivity
24,25 rather than an explicit motion processing deficit. To avoid problems with visibility of the stimuli, we use dots whose contrast was well above detection threshold and to examine speed-dependent effects, we examine motion perception at low and high speeds.