Capacity Limits for Spatial Discrimination

(abstract) When observers are searching for a single target amongst multiple distracters, accuracy usually decreases with the number of distractors (the set-size effect). Set-size effects have been measured to seek evidence for a limit on the capacity for spatial discriminations. Experiments with unmasked displays have found no evidence for a sensory bottleneck. On the other hand, postmasked displays produce set-size effects too large to be explained by uncertainty alone. When mask-onset asynchronies are short (eg 100 ms), orientationdefined targets frequently seem to be adjacent to their true positions and tilt thresholds increase with the square root of the number of cued items. These results suggest that the mean of an ensemble can be computed when there is too little time to evaluate each item individually. Precues can improve discrimination even in the absence of distracters, but since multiple precues are almost as effective as single ones, most of this enhancement is pre-attentive.