Investigating the status of biological stimuli as objects of attention in multiple object tracking

Abstract

Background: Humans are able to track multiple simultaneously moving objects. A number of factors have been identified that can influence the ease with which objects can be attended and tracked. Here, we explored the possibility that object tracking abilities may be specialized for tracking biological targets such as people.

Methodology/principal findings: We used the Multiple Object Tracking (MOT) paradigm to explore whether the high-level biological status of the targets affects the efficiency of attentional selection and tracking. In Experiment 1, we assessed the tracking of point-light biological motion figures. As controls, we used either the same stimuli or point-light letters, presented in upright, inverted or scrambled configurations. While scrambling significantly affected performance for both letters and point-light figures, there was an effect of inversion restricted to biological motion, inverted figures being harder to track. In Experiment 2, we found that tracking performance was equivalent for natural point-light walkers and 'moon-walkers', whose implied direction was incongruent with their actual direction of motion. In Experiment 3, we found higher tracking accuracy for inverted faces compared with upright faces. Thus, there was a double dissociation between inversion effects for biological motion and faces, with no inversion effect for our non-biological stimuli (letters, houses).

Conclusions/significance: MOT is sensitive to some, but not all naturalistic aspects of biological stimuli. There does not appear to be a highly specialized role for tracking people. However, MOT appears constrained by principles of object segmentation and grouping, where effectively grouped, coherent objects, but not necessarily biological objects, are tracked most successfully.

Figure 1. Frames from the biological (left panel) and non-biological (right panel) point-light stimuli in the upright, inverted and scrambled conditions.

Participants were presented these displays at the end of each trial and were prompted to report the numbers corresponding to the four targets they had been tracking (see Methods and Materials: Experiment 1). During the trial, when the targets and distracters were moving around the screen, all points appeared in the same colour (white), apart from the first 30 frames in which the four targets flashed in red.

Figure 2. Accuracy (percentage correct) for tracking the upright, inverted and scrambled presentation of biological and letter targets.

Error bars depict standard error. Scrambled stimuli were tracked less successfully compared with upright stimuli. In addition, inverted biological motion was tracked less accurately compared with upright biological motion (see Results: Experiment 1).

Figure 3. Accuracy for upright and inverted and face and house stimuli.

Error bars are standard error. Inverted faces were tracked significantly more accurately compared with upright faces (see Results: Experiment 3).