Visual search tasks, in which observers look for a target embedded among distractors, have long been used to study attentional deployment (for reviews, see Eckstein,
2011; Nakayama & Martini,
2011). There are two main categories of search. In feature search, the target differs from the distractors in a single dimension (e.g., color, shape, orientation). In this case, typically reaction time and accuracy are independent of the set size—that is, the number of distractors in the search array (near-zero slope). In conjunction search, the target is a combination of at least two features of different dimensions (e.g., color and shape). In such a case, typically the more stimuli, the longer the time required to find the target (positive slope) and the lower the accuracy (negative slope). The original view for these findings relied on an explanation based on processing speed: Feature search is preattentive and parallel, whereas conjunction search is attentive and sequential (Treisman & Gelade,
1980; Wolfe, Cave, & Franzel,
1989). However, many findings have questioned this two-stage assumption. For instance, in multiple studies conjunction searches have not yielded serial functions (e.g., Carrasco, Evert, Chang, & Katz,
1995; Egeth, Virzi, & Garbart,
1984; McLeod, Driver, & Crisp,
1988; Nakayama & Silverman,
1986); in conjunction tasks, search slopes have been shown to progress from steep (serial) to flat (parallel) within an experimental session (Carrasco, Ponte, Rechea, & Sampedro,
1998b); in feature searches with increasing set size, performance has diminished (as assessed by speed–accuracy trade-off methods; e.g., Carrasco, Giordano, & McElree,
2006; Carrasco & McElree,
2001; Dosher, Han, & Lu,
2004; McElree & Carrasco,
1999) and reaction times have decreased (e.g., Bravo & Nakayama,
1992; Buetti, Cronin, Madison, Wang, & Lleras,
2016; Carrasco et al.,
1995; Carrasco & Chang,
1995). Even though these studies—and others, such as a recent one showing that slopes differ as a function of response method (yes/no detection vs. go/no-go)—provide evidence against this two-stage assumption, it is still very influential (Kristjánsson,
2015).