Abstract
Last year, we showed that targets in Multiple Object Tracking (MOT) can be indexed voluntarily as well as involuntarily. However, whereas involuntary indexing is automatic and simultaneous, voluntary indexing requires that each target object be visited serially. This year we present further evidence to support this claim.
In experiment 1, the observer was asked to track 3 out of 8 gray, disk-shaped, objects on a black screen. The targets were either the objects on which a vertical line flashed (flash condition), or the objects on which the vertical line did not flash (nonflash condition). We varied the amount of time available for indexing by having the vertical line flash once (300 ms) or three times (900 ms). We hypothesized that if serial visitation was necessary, then we should see better performance with the longer indexing interval. As expected, for the flash condition, the results showed no difference in tracking performance across indexing intervals. For the nonflash condition, tracking improved with the longer indexing intervals. This is consistent with the serial visitation hypothesis. However, a similar result could also be obtained if the automatically assigned indexes first had to be detached from the flashed objects and re-applied to the nonflashed target objects, a process which could itself explain the additional time it takes to index nonflashed items. Experiment 2 controlled for automatic indexing by having horizontal and vertical lines flash on all 8 objects simultaneously, instead of requiring observers to track nonflashed objects. Observers had to track the 3 objects that were cued with either horizontal lines or vertical lines. The results of this experiment show the same pattern of performance as the nonflash condition, suggesting serial visitation. This study provides further evidence in support of a serial visitation explanation of voluntary indexing.