Attentional modulation of visual search depends on the relation between eccentricity and spatial interactions

We investigated whether the improvement by attention in visual search is due to the exclusion of distractors, change of decisional criteria, or signal enhancement. Observers searched for the presence of a target line with small (10°) or large (20°) orientation differences from 2, 4, or 8 background elements that were either close or sparse, and either presented at large (~5 deg) or small (~1.5 deg) eccentricity. Results show that element separation affects target sensitivity in opposite directions: d' is higher at small spacing for less eccentric elements, and is higher at large spacing for more eccentric elements. Cueing relevant elements in displays with small spacing and a fixed number of eight elements improves visual search only in the periphery, where the difference in sensitivity due to orientation differences and the reduction in performance at small spacing both disappear. These results make it possible to distinguish a general increase of sensitivity for the target in the periphery (signal enhancement) from a more specific modulation of spatial interaction by attention, with the consequent reduction of crowding (distractor exclusion).