ACCESS TO GLOBAL AND LOCAL PROPERTIES IN VISUAL-SEARCH FOR COMPOUND STIMULI

The question of whether attention is drawn more easily to global or local aspects of a stimulus has been debated for more than 100 years. We examined it anew, using the visual search task, which distinguishes sensory from attentional effects. Subjects searched for a target feature (e.g., triangle vs. square), which was equally likely to occur in the local elements of a compound search item, in its global structure, or in both. Element size and spacing were used to manipulate whether search was generally easier for local or global targets (e.g., small size and dense spacing favor global detection). The novel result was that these factors had very little influence on search slopes for local targets, whereas they had large effects on search slopes for global targets. This result suggests that a qualitatively different process underlies detection at the global level in traditional compound stimuli. Our proposal that an attention-demanding grouping stage is involved was confirmed in a final experiment in which grouping was made selectively difficult at the local level.