Splitting versus sharing focal attention: comment on Castiello and Umiltà (1992).

A popular metaphor for visual attention is that of a spotlight that enhances perceptual processing within its beam. Many studies on the orienting of visual attention have addressed whether the beam is a unified structure or whether it can be split between noncontiguous locations in space. Although most of the evidence favors the unified model, U. Castiello and C. Umiltà (1992) claimed recently to have results that could most easily be accounted for by a model of visual attention in which resources can be allocated flexibly to independent locations in space. It is argued that Castiello and Umiltà used only indirect empirical evidence to support their position and that their results are not inconsistent with the unified model. Two studies are reported in which important aspects of Castiello and Umiltà's experiments were maintained and a probe procedure was implemented to assess directly if attention was split between 2 spatial locations or if a unified focus of attention was expanded to incorporate the 2 locations. The results clearly supported the latter position.