Abstract
Both multiple-object visual tracking (MVT) and inefficient visual search are held to demand visual attention. However, we have previously shown (ARVO 2000) that both tasks can be performed concurrently within a single trial with minimal performance loss on either task. How is this possible? Here we test the hypothesis that observers switch back and forth between MVT and search during a trial. Since our original search task was presented briefly during an extended MVT episode, observers may have stopped tracking long enough to search, then returned attention to the MVT items. Previously (VSS 2001), we showed that tracked items can disappear for up to 370 ms during a trial and be recovered with minimal loss of accuracy. In the present series of experiments, we show that observers can perform a difficult search task during this blank interval and still maintain tracking performance (Exp 1). Nevertheless, observers might be performing MVT and search simultaneously with separate resources. In Exp 2, we measured reaction time (RT) to a search task performed alone or concurrently with MVT. The RT × set size slope was unchanged by the addition of MVT, but the intercept increased by 500 ms, consistent with the task-switching hypothesis rather than complete independence. If observers can switch readily between MVT and search, can they perform two MVT tasks simultaneously? In Exp 3, we measured trade-offs between two simultaneous MVT tasks. We observed perfect trade-offs between these tasks. Observers cannot perform two MVT tasks (or two search tasks) at the same time. We propose that search and tracking can coexist because they can trade control over attentional resources. Information about the state of an MVT task appears to be maintained in a representation that enables attention to be deployed to the search task, and subsequently successfully redeployed back to the tracked items if the search episode is not too long.