Theeuwes reviews a long series of studies implementing the additional singleton paradigm in which search for a shape singleton is slowed by the presence of an irrelevant color singleton (e.g., Theeuwes, 1992
). Bacon and Egeth (1994)
proposed that perhaps participants weren’t really engaged in search for a specific shape; maybe they were looking for any target that differed markedly from its surrounding items. Such a processing mode would clearly leave participants vulnerable to a color singleton. We tested this by making efforts to prevent a singleton search strategy, and when we did, we found that a color singleton no longer captured attention.
This was meant as a modest proposal, with singleton detection mode best construed as an example of a top-down set (e.g., Folk, Remington, & Johnston, 1992
). Although the proposal has been invoked to help account for various experimental outcomes, Theeuwes (this issue)
expresses skepticism, going so far to state that “the concept of a search mode doesn’t explain much, if anything.”
Theeuwes questions why, if participants are capable of avoiding capture via feature search mode, do they not use it all the time. We have considered this issue, and we believe the answer could lie in the principle of satisficing. If we assume that establishing a feature-search mode requires the investment of effort, then participants are likely to settle for visual search strategies that yield adequate, if not optimal, performance (see Leber & Egeth, 2006
). Consider that the capture effects discussed here are on the order of 25 ms; in a typical 30-minute experimental session that includes 250 distractor trials, the cumulative cost of briefly attending irrelevant singletons is just a matter of a few seconds. Further, as Theeuwes describes, existing data have suggested an overall behavioral slowing when participants engage in feature-search mode (i.e., on distractor absent trials). Thus the degree to which overall speed, collapsed across distractor-present and distractor-absent trials, suffers as a result of adopting singleton detection mode could be negligible.
As for why the slowing occurs when feature-search mode is adopted, we have just begun to investigate this issue. Theeuwes’s perspective is that salience dominates attentional orienting early on, due to a totally stimulus-driven initial wave of processing. He further argues that only after this initial wave subsides can top-down control be implemented. According to this viewpoint, stimulus driven capture can be avoided only if observers delay their search until after the initial salience dominated processing subsides (van Zoest, Donk & Theeuwes, 2004). This line of reasoning is interesting, although we offer a speculative alternative: during periods of high goal-driven control, participants also adopt a more conservative responding strategy. That is, while greater cognitive control reduces vulnerability to distraction through greater attentional selectivity, it could be accompanied by a greater commitment to producing the correct response (akin to a speed-accuracy tradeoff). It will be important to test this alternative empirically.
Thus far we have discussed why observers might not mind interference from irrelevant stimuli, but to be clear, we do not wish to imply that singleton mode is the default mode of processing. Indeed, there is ample evidence that feature search mode is routinely engaged to reduce distraction from known singletons to some extent, even if not always optimally so. As Theeuwes noted, capture effects are much smaller when both the target and distractor are known in advance, suggesting that some degree of top-down control is present in these studies. Pinto, Olivers and Theeuwes (2005)
attributed at least some of the reduction in capture to priming, due to intertrial target-feature repetitions. However, we carried out a similar analysis and found that priming only accounts for part of the reduction in capture; the remaining difference is likely the result of some engagement of feature search (Lamy, Carmel, Egeth & Leber, 2006