Academic journal article Canadian Journal of Experimental Psychology

Guidance of Eye Movements during Conjunctive Visual Search: The Distractor-Ratio Effect

Academic journal article Canadian Journal of Experimental Psychology

Guidance of Eye Movements during Conjunctive Visual Search: The Distractor-Ratio Effect

Article excerpt

Abstract The distractor-ratio effect refers to the finding that search performance in a conjunctive visual search task depends on the relative frequency of two types or subsets of distractors when the total number of items in a display is fixed. Previously, Shen, Reingold, and Pomplun (2000) examined participants' patterns of eye movements in a distractor-ratio paradigm and demonstrated that on any given trial saccadic endpoints were biased towards the smaller subset of distractors and participants flexibly switched between different subsets across trials. The current study explored the boundary conditions of this tendency to flexibly search through a smaller subset of distractors by examining the influence of several manipulations known to modulate search efficiency, including stimulus discriminability (Experiment 1), within-dimension versus cross-dimension conjunction search and distractor heterogeneity (Experiment 2). The results indicated that the flexibility of visual guidance and saccadic bias exemplified by the distractor-ratio effect is a robust phenomenon that mediates search efficiency by adapting to changes in the relative informativeness of stimulus dimensions and features.

Visual search is one of the dominant paradigms used for investigating visual attention. In a typical conjunctive search task, participants have to decide whether a prespecified search target is embedded in an array of distractors (nontarget elements), each of which shares one or multiple features with the target item. Each trial contains an equal number of distractors from each type whereas the total number of items within a search display (display size) is manipulated. Search efficiency is examined by the change in response time and/or error rate as a function of display size (see Treisman, 1988; Wolfe, 1998 for a review). Based on the response time and error rate data, several models of visual search have been proposed to explain search efficiency across a variety of search tasks (e.g., the original feature-integration theory, Treisman, 1988; Treisman & Gelade, 1980; the attentional-engagement theory by Duncan & Humphreys, 1989; the guided-search model by Wolfe, 1994; Wolfe, Cave, & Franzel, 1989; see also the revised feature-integration theory by Treisman & Sato, 1990).

Several previous studies have shown that search performance in a conjunctive search task is also sensitive to the distribution of distractor types, even when the total number of items in a display remains constant (e.g., Bacon & Egeth, 1997; Egeth, Virzi, & Garbart, 1984; Kaptein, Theeuwes, & van der Heijden, 1995; Poisson & Wilkinson, 1992; Zohary & Hochstein, 1989). For example, Zohary and Hochstein (1989) adopted a Colour x Orientation conjunction search task and asked participants to decide whether a red horizontal bar was present among an array of red vertical (same-colour distractors) and green horizontal (same-orientation distractors) bars. The search display was presented very briefly (50 ms) and then, after a variable interval (stimulus onset asynchrony, SOA), masked. One critical manipulation in this study was the ratio between the two types of distractors (same-colour vs. same-orientation) presented in a given array. Zohary and Hochstein found that the SOA required to reach a 70% correct response rate was a quadratic function of the number of distractors sharing colour with the search target. Specifically, detection was relatively easy for displays with extreme distractor ratios (i.e., either the same-colour or same-orientation distractors were rare) but relatively difficult for displays in which the two types of distractors were equally represented. The finding that visual-search efficiency in a conjunctive search task depends on the relative frequency of the two types of distractors has been referred to as the distractor-ratio effect (Bacon & Egeth, 1997).

Different interpretations have been proposed to explain the distractor-ratio effect. …

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