The capacity of multiple-target search

Can individuals look for multiple objects at the same time? A simple question, but answering it has proven difficult. In this review, we describe possible cognitive architectures and their predictions about the capacity of visual search. We broadly distinguish three stages at which limitations may occur: (1) preparation (establishing and maintaining a mental representation of a search target), (2) selection (using this mental representation to extract candidate targets from the visual input), and (3) post-selection processing (verifying that the selected information actually is a target). We then review the empirical evidence from various paradigms, together with their strengths and pitfalls. The emerging picture is that multiple target search comes with costs, but the magnitude of this cost differs depending on the processing stage. Selection appears strongly limited, while preparation of multiple search target representations in anticipation of a search is possible with relatively small costs. Finally, there is currently not sufficient information to determine the capacity limitations of post-selection processing. We hope that our review contributes to better targeted research into the mechanisms of multiple-target search. A better understanding of multiple-target search will also contribute to better design of real-life multiple-target search problems, reducing the risk of detrimental search failures.