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 summation contributes to age‐related deficits on oscillatory displacement thresholds (ODT's). A secondary goal was to evaluate the extent of spatial summation on a dynamic version of vernier hyperacuity. Methods. The ODT's and contrast sensitivity functions (CSF's) of optimally corrected young and old observers were compared as a function of vernier target length (4, 8, or 32 min), contrast level (5 or 30%), and oscillation rate (2 or 8 Hz). Results. Age 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 young 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 contribute to the aging visual system's loss of temporal discrimination for low contrast oscillating targets. Spatial summation contributes more importantly to the displacement discrimination of oscillating than to static vernier targets.