The effects of aging on directionally selective masking.

Motion perception is degraded in older adults. Previous studies suggest that this effect of aging may be due in part to an increase in the bandwidth of directionally selective mechanisms. We tested this idea by measuring directional masking in younger and older adults. Experiments 1-3 measured the contrast needed to discriminate the direction of coherently moving signal dots embedded in high-contrast mask dots. The distribution of mask dot directions was varied with notch filters, and directional selectivity was indexed by the slope of the threshold-versus-notch function. Thresholds were higher and directional selectivity of masking was lower in older compared to younger adults. However, age differences were eliminated when signal and mask contrast were expressed as multiples of discrimination thresholds for unmasked motion. Experiments 4-5 measured direction discrimination thresholds by varying the proportion of coherently moving dots embedded in a mask consisting of dots whose directions varied across conditions. All dots were high contrast, so age differences in masking are unlikely to be caused by differences in contrast sensitivity in these conditions. Nevertheless, Experiments 4-5 also found higher discrimination thresholds and reduced directional selectivity in older adults. The results are consistent with the hypothesis that directionally selective mechanisms become more broadly tuned during senescence.