TRANSPARENT MOTION AND OBJECT-BASED ATTENTION

The difficulty in processing two stimuli at once increases with their separation. Therefore to demonstrate constraints in dividing attention between objects, the effects of their spatial separation must be cont rolled. Duncan used superimposed objects to achieve this, and showed t hat judging two attributes is more accurate if they concern one object than if they concern two objects (Duncan, J. 1984. Journal of Experim ental Psychology: General, 113, 501-517). However, critics claim that differences in the spatial or spatial-frequency extent of attention ex ist between these conditions. We studied transparent motion defined by two sets of differently colored dots that were interspersed in the sa me region of space, and matched in spatial and spatial frequency prope rties. Each set moved in a distinct and randomly chosen direction. We found that simultaneous judgments of speed and direction were more acc urate when they concerned only one set than when they concerned differ ent sets. Furthermore, appraisal of the directions taken by two sets o f dots is more difficult than judging direction for only one set, a di fficulty that increases for briefer motion. We conclude that perceptua l grouping by common fate exerted a more powerful constraint than spat ial proximity, a result consistent with object-based attention. Eviden ce that this type of object-based attention operates at early stages o f vision is examined. (C) 1998 Elsevier Science B.V. All rights reserv ed.