Preemption Effects in Visual Search: Evidence for Low-Level Grouping
Ronald A. Rensink, Department of Psychology, Harvard University, Cambridge MA, USA.
James T. Enns, Department of Psychology, University of British Columbia, Vancouver BC, Canada.

Psychological Review, 102: 101-130. 1995.   [pdf]


Experiments are presented showing that visual search for Mueller-Lyer (ML) stimuli is based on complete configurations rather than component segments. Segments easily detected in isolation were difficult to detect when embedded in a configuration, indicating preemption by low-level groups. This preemption--which caused stimulus components to become inaccessible to rapid search--was an "all or nothing" effect, and so could serve as a powerful test of grouping. It is shown that these effects are unlikely to be due to blurring by simple spatial filters at early visual levels. It is proposed instead that they are due to more sophisticated processes that rapidly bind contour fragments into spatially-extended assemblies. These results support the view that rapid visual search cannot access the primitives formed at the earliest stages of visual processing; rather, it can access only higher level, more ecologically relevant structures.

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