Measuring Preattentive Processes: When is Pop-out Not Enough?

Three behavioural tasks (standard visual search, multiple target search, and numberdiscrimination)weretestedwithavariety of stimuli relevanttotwo-stage theories of vision. One purpose was to compare the two versions of the search task; a second was to explore the relations between processes of search and enumeration. The two search tasks were quite consistent overall, with strong correlations between searchRT slope and multiple target RT gain across stimuli. Analysis of RT slopes for the 1–3 range in the number discrimination task revealed four categories of stimuli: Small slopes (5–40ms/item); moderately large slopes (100–200ms/item); very large slopes (400ms/item or more), and negative slopes (–214ms/item). Although the correlations between number discrimination and the two search tasks were positive, there were several interesting discrepancies, including stimuli with large search slopes and small number discrimination slopes, and those with small search slopes and large number discrimination slopes. These patterns suggest that, at least for some stimuli, subjects may be able to enumerate targets while they are also searching for them, and that search processes sometimes interfere with the processes required for enumeration.