Abstract: In Multiple Object Tracking task, participants can successfully track 4-5 targets even though all the stimuli (typically, 8 objects) disappeared for several hundred milliseconds during the tracking time. One theory by Horowitz, (other authors) states that the relevant information of targets was stored into a so-called off-line memory when the stimuli disappeared. The successful tracking of re-appeared targets is then based on the comparison between the information obtained from the present display and the information that was stored. According to my knowledge, there are few studies to investigate what information was stored. The goal of the present study was to tap into this question by examining the effect of target location and trajectory on the target tracking.
Eighteen undergraduates in Beijing Normal University participated in this study. They were 18-24 years old. They all reported to have normal or correct-to-normal vision and normal color perception. Two dependent variables, Target Number and Re-appearance Position were manipulated in a within-subject design (see details in the second part of the paper).The experimental procedure was controlled by Multiple Object Tracking program (developed in the lab). Participants pressed one of the two keys to judge the presented stimuli as target or non-target. Response accuracy was recorded by computer. The principle analysis was 4×5 analysis of variances (ANOVAs).
The results showed a significant main effect of Target Number. Accuracy decreased with the increase of target number. The main effect of Re-appearance Position was also significant. The targets re-appeared in the same location as where it disappeared were easier to track than the targets in the different location were. No significant interaction between Target Number and Target Location was observed.
The findings of the present study indicated that at least three types of information (target location, target trajectory, and target number) were encoded into the off-line memory. The re-appearance of the stimuli serves as a strong cue to retrieve the stored information. Targets were distinguished from the distractors based on this information then they were successfully tracked after they are re-presented. Furthermore, this information only can be retrieved efficiently when there are less than 2 targets. With the increase of target number, the effects of target location and trajectory became smaller. Further research should be focused on how the target number interacts with target location and trajectory

Key words: Object Tracking, Target Recovery, Task Switching Hypothesis, Location Information, Trajectory Information