Diminished spatial summation contributes to the age deficit in the discrimination of low-contrast vernier oscillation

Purpose. This study sought to determine whether a decline in spatial summat ion contributes to age-related deficits on oscillatory displacement thresho lds (ODT's). A secondary goal was to evaluate the extent of spatial summati on on a dynamic version of vernier hyperacuity. Methods. The ODT's and cont rast sensitivity functions (CSF's) of optimally corrected young and old obs ervers were compared as a function of vernier target length (4, 8, or 32 mi n), contrast level (5 or 30%), and oscillation rate (2 or 8 Hz). Results. A ge deficits on ODT's were related directly to rate of oscillation, but not target contrast. No age difference was seen in ODT's for short low-contrast targets; as target length increased thresholds improved more rapidly for y oung than old observers; this pattern was reversed at high contrast. ODT's were related strongly and consistently to contrast sensitivity for old but not young observers. Conclusions. Diminished spatial summation appears to c ontribute to the aging visual system's loss of temporal discrimination for low contrast oscillating targets. Spatial summation contributes more import antly to the displacement discrimination of oscillating than to static vern ier targets.