ATTENTIONAL SELECTION BY DISTRACTOR SUPPRESSION

Selective attention was studied in displays containing singletons popp ing out for their odd form or color, The target was defined as the for m-singleton, the distracter as the color-singleton. The task was to di scriminate the length of a longer line inside the target, Target-distr actor similarity was controlled using a threshold measurement as depen dent variable in experiments in which distracter presence vs absence, bottom-up vs top-down selection (through knowledge of target features) , and target-distracter distance were manipulated, The results in the bottom-up condition showed that length threshold was elevated when a d istracter was present and that this elevation progressively increased as the number of distracters was increased from one to two, This set-s ize effect was not accounted by the hypothesis that selective attentio n intervenes only at the stage of decision before response, Selective attention produced a suppressive surround in which discriminability of neighboring objects was strongly reduced, and a larger surround in wh ich discriminability was reduced by an approximately constant amount, Different results were found in the top-down condition in which target discriminability was unaffected by distracter presence and no effect of target-distracter distance was found, On the other hand, response t imes in both bottom-up and top-down conditions were slower the shorter the target-distracter distance was, On the basis of the experimental results, selective attention is a parallel process of spatial filterin g at an intermediate processing level operating after objects have bee n segmented, This filtering stage explores high level interactions bet ween objects taking control on combinatorial explosion by operating ov er only a limited spatial extent: it picks out a selected object and i nhibits the neighboring objects; then, non-selected objects are suppre ssed across the overall image, When no feature-based selection is avai lable in the current behavior, this filtering influences perception in decreasing discriminability of non-selected objects, When feature-bas ed selection is available, spatial interactions are set before stimulu s arrival, hence only the unmatching objects have their discriminabili ty diminished, (C) 1998 Elsevier Science Ltd, All rights reserved.