Spatial resolution and object segmentation efficiency constrain grouping effects in attentive tracking

Previous studies of multiple object tracking suggested that spatiotemporal features (e.g., speed, direction, location) and surface features (e.g., color, shape, size) can guide perceptual grouping. However, it is still unclear how target-distractor distinctiveness and target-background similarity affect grouping effects in attentive tracking. To address these two questions, three experiments have been carried out in the current study. In Experiment 1, we manipulated the target-distractor distinctiveness and found that tracking performance logarithmically improved when the target-distractor distinctiveness linearly increased. In Experiment 2a and 2b, we varied the target-background similarity and found that too high or too low target-background similarity damaged the tracking performance, while only the middle target-background similarity resulted in the best tracking performance. These findings reveal that not only target-distractor distinctiveness but also target-background similarity plays a vital role in guiding the attention of perceptual grouping in attentive tracking. The guidance induced by target-distractor distinctiveness is constrained by the spatial resolution, while the guidance induced by target-background similarity is constrained by the efficiency of object segmentation. Additionally, our results showed that tracking capacity varied with the target-distractor distinctiveness and the target-background similarity, even though the number of targets being tracked was fixed. It suggests that there may be a trade-off between the difficulty of tracking and the number of targets that can be tracked. Thus, tracking capacity is more likely to be limited by the flexible attention resources rather than the number of fixed slots.