Academic journal article Perception and Psychophysics

Distractor Familiarity Leads to More Efficient Visual Search for Complex Stimuli

Academic journal article Perception and Psychophysics

Distractor Familiarity Leads to More Efficient Visual Search for Complex Stimuli

Article excerpt

Previous reports suggest that distractor familiarity plays an important role in determining visual search efficiency. However, the specific tasks used in those studies limit the extension of their findings to real-world situations and everyday images. In the present study, subjects engaged in a prolonged period of search experience as a control of their level of familiarity with a large set of target and distractor images. Reaction times and search slopes decreased dramatically over this period, especially for trials with a large target eccentricity and many distractors. Following extended practice, search among familiar distractors was more efficient than search among unfamiliar distractors. Furthermore, we found that familiar targets were located more efficiently than unfamiliar targets and that subjects were faster at locating targets that they had experienced in the majority of the search trials. These results show that prolonged visual experience facilitates processing of both target and distractor items during search.

Efficient search is central to real-world vision. Animals in the wild, for example, must constantly be on the lookout for potential predators, which appear in many different forms and contexts. Prior experience can play an important role in this search process. Some targets, for example, may appear more frequently than others, and search strategies can be adjusted to match these probabilities. At the same time, the surrounding environment may contain regularities that can be incorporated into search heuristics. Finally, the act of search itself may become more refined with experience. In other words, search efficiency can be enhanced by knowing what to look for, what not to look for, and how to look.

It is known that visual search efficiency in laboratory settings is determined by certain properties of the image array used. For example, if a search target contains particular physical attributes that are absent in the set of distractors, such as a "Q" among "O"s, search is highly efficient (Treisman & Souther, 1985). Such highly efficient search conditions are commonly referred to as pop-out conditions because the target is located rapidly, at a rate nearly independent of the number of distractors in the search array. Other aspects of the visual display are known to make search less efficient. For example, heterogeneity of the distractor set generally increases search times (Duncan & Humphreys, 1989).

This article is concerned primarily with the effect of image familiarity on visual search. Frith (1974) first noted that the level of familiarity with target and distractors influences search efficiency. She found that it is more difficult to visually scan for letters among their mirror images than vice versa. Similarly, Reicher, Snyder, and Richards (1976) reported that search among familiar distractors was more efficient than search among unfamiliar ones. They attributed this difference to the segmentation stage of search rather than to poor classification of unfamiliar distractors (Richards & Reicher, 1978).

More recently, Wang, Cavanagh, and Green (1994) observed pop-out search conditions (search slope < 8 msec/item, by their definition) when unfamiliar targets were searched among familiar distractors, but not when other target-distractor combinations were used. In other words, highly efficient search required a familiarity difference between targets and distractors, with the distractors being familiar. Wang et al. concluded that familiar items are processed rapidly and easily but unfamiliar ones require more attention. An alternative interpretation is that familiarity serves as a primitive feature whereas novelty represents a departure from that standard (Treisman & Souther, 1985). In either case, novel items can pop out of a familiar background.

Using stimulus sets with feature difference controlled between targets and distractors, Malinowski and Hiibner (2001) and Shen and Reingold (2001) demonstrated that what is necessary for efficient search is familiarity with the distractors, not a familiarity difference between targets and distractors. …

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