Positional noise in Landolt-C stimuli reduces spatial resolution: a study with younger and older observers.

In the present study we examined the effect of positional noise on spatial resolution in younger and older observers. We used a yes/no discrimination task in which observers indicated whether the size of two gaps in a Landolt-C-like contour was the same or not. The proportion of trials observers perceived one gap larger was measured when gaps-position was fixed (low positional noise) and random (high positional noise). Specifically, we compared, across conditions and groups, the values of threshold, lower and upper asymptote of the psychometric function. In the younger group, noise does not prevent detection of gap-size difference although sensitivity is lower, as revealed by higher threshold and lower upper asymptote, i.e., the proportion of responses "I see a larger gap" at the largest gap-size difference (asymptotic performance). In the older group detection is prevented, as revealed by threshold, lower and upper asymptote data. This may be because, at stimulus onset, high positional noise has associated coarse filter analysers averaging across the two gaps, which cannot be switched off.