Density effects in conjunction search: evidence for a coarse location mechanism of feature integration

Four experiments used the visual search paradigm to examine feature integration mechanisms. Reaction time to determine the presence or absence of a conjunctive target is relatively fast and exhaustive for low-density displays. Search rate is slow and self-terminating for high-density displays. Density effects do not arise when the target is defined by a unique feature. Two mechanisms are proposed for feature integration. A fast mechanism integrates features on the basis of coarse location information coded with the initial registration of the features. This coarse location mechanism requires that display items be spaced apart. A second, slower mechanism is used when objects are clumped together. The 2-mechanism hypothesis provides a resolution to conflicting findings in the visual search and illusory-conjunction literature. A possible interpretation of the findings with a single guided search mechanism for feature integration is also discussed.