DOI:10.1068/p2933 Distractor ratio influences patterns of eye movements during visual search

Abstract. We examined the flexibility of guidance in a conjunctive search task by manipulating the ratios between different types of distractors. Participants were asked to decide whether a target was present or absent among distractors sharing either colour or shape. Results indicated a strong effect of distractor ratio on search performance. Shorter latency to move, faster manual response, and fewer fixations per trial were observed at extreme distractor ratios. The distribution of saccadic endpoints also varied flexibly as a function of distractor ratio. When there were very few same-colour distractors, the saccadic selectivity was biased towards the colour dimension. In contrast, when most of the distractors shared colour with the target, the saccadic selectivity was biased towards the shape dimension. Results are discussed within the framework of the guided search model. How efficient are human observers in detecting a conjunctively defined search target, for example finding a red X among red Os and green Xs? This issue has been systematically investigated over the past two decades (see Treisman 1988; Wolfe 1998 for a review). Search efficiency has been related to a number of factors, including stimulus similarity (Duncan and Humphreys 1989), target eccentricity (Carrasco et al 1995; Scialf