Visual search for targets defined by combinations of color, shape, and size: An examination of the task constraints on feature and conjunction searches

Abstract

A series of experiments are reported in which the comparative constraints on single-feature and conjunction searches were examined. The first three tested the idea that the critical differences between these searches reflect the number of stimulus attributes that subjects must extract to make a response, that is, one in the feature condition, two in the conjunction condition. Targets were defined by possible pairwise combinations of a color, a size, and a shape. In another condition, subjects searched for two simultaneously present feature targets. Search for these targets did not differ qualitatively from that for a single-feature target (i.e., search remained parallel), but there was a constant increment to feature search functions. The final experiment examined the possibility that the number of relevant shared features between targets and distractors may also be critical. The results showed that the number of relevant shared features affected the rate of search but not its nature. Thus, differences between feature and conjunction searches do not reflect the number of relevant stimulus attributes that must be detected, or the number of relevant features shared between targets and distractors. Nevertheless, evidence for attentional involvement was found when subjects searched for two simultaneously present features. This goes against the claim that differences between the two types of search occur because feature searches can be carried out preattentively, while only conjunction searches involve the serial application of focused attention. An account of visual search performance that emphasizes the fidelity of coding of target information seems a more adequate proposal. © 1987 Psychonomic Society, Inc.