Changing the speed and order of attentional selection in visual search.

Seminal event-related potential (ERP) studies of visual search reported that young adults serially inspect two singletons when searching for a target (serial search), but later results showed that the second singleton can be selected while the first singleton is still attended (partially parallel search). These contrasting results indicate that some yet-to-be identified factor can affect the speed of search. We hypothesized that single-target detection tasks promote serial inspection while dual-target comparison tasks promote parallel inspection. We recorded ERP activities associated with attentional selection (N2pc) and subsequent identification (SPCN) to track attentional processing of two singletons while healthy young adults participated in one of two detection tasks or a comparison task. One singleton was made to be more salient than the other to give it a "natural" selection advantage and thus promote some serial processing even in the comparison task. The timing of N2pc activities indicated that attention was deployed to the second singleton more quickly when participants compared the orientations of lines inside the two singletons than when they searched for one specific line that was more likely to be positioned inside one singleton or the other. Surprisingly, however, search was never fully serial, even in detection tasks that encouraged close inspection of individual items. Rather, in such detection tasks, items were selected serially but were processed for identification concurrently (as indexed by the SPCN). These findings are consistent with serial-parallel hybrid models of visual search.