Event monitoring: Can we detect more than one event at a time?

A prior study by Wu and Wolfe found that the capacity for event monitoring (e.g. did an item change its state?) is more limited than for classic multiple object tracking. That limited capacity, K, could arise from either of two situations. It could be that people can detect K events simultaneously or it could be that they can successfully detect just one event at a time while monitoring K out of a total of N items. In the three different experiments of the present study, observers were asked to monitor a set of moving objects while watching for two critical events occurring in that set. Observers' performance can be well described by a model that includes an ability to detect two changes at once. Our results suggest that the capacity for event monitoring is further limited when tracking an additional event, but within the monitored set, people can detect at least two events simultaneously.