Attributing intentions to random motion engages the posterior superior temporal sulcus

Abstract

The right posterior superior temporal sulcus (pSTS) is a neural region involved in assessing the goals and intentions underlying the motion of social agents. Recent research has identified visual cues, such as chasing, that trigger animacy detection and intention attribution. When readily available in a visual display, these cues reliably activate the pSTS. Here, using functional magnetic resonance imaging, we examined if attributing intentions to random motion would likewise engage the pSTS. Participants viewed displays of four moving circles and were instructed to search for chasing or mirror-correlated motion. On chasing trials, one circle chased another circle, invoking the percept of an intentional agent; while on correlated motion trials, one circle's motion was mirror reflected by another. On the remaining trials, all circles moved randomly. As expected, pSTS activation was greater when participants searched for chasing vs correlated motion when these cues were present in the displays. Of critical importance, pSTS activation was also greater when participants searched for chasing compared to mirror-correlated motion when the displays in both search conditions were statistically identical random motion. We conclude that pSTS activity associated with intention attribution can be invoked by top-down processes in the absence of reliable visual cues for intentionality.

Keywords: biological motion; fMRI; intention attribution; posterior superior temporal sulcus; social perception.

Fig. 1

Illustration of the three types of movies used, each consisting of four dynamic, colored (red, blue, green and yellow) circles, adapted from Gao et al. (2009). In all movies, the red circle and two other circles moved independently of one another in a haphazard manner (depicted by the curved arrows). (A) In chase motion movies, the fourth circle (here, the yellow circle) consistently moved toward the red circle, with a maximum deviation of 30° from the direct path between the fourth circle and the red circle. (B) In mirror motion movies, the fourth circle (here, the yellow circle) moved as a mirror image of the red circle, with the center of the display as the point of reflection. (C) In random motion movies, all circles had independent trajectories.

Fig. 2

Schematic illustration of one chase trial and one mirror trial. Participants were presented with a question for 2 s, followed by a 10 s movie of moving circles, and finally a response screen for 2 s. Trials were separated by a 12 s fixation inter-trial interval.

Fig. 3

(A) Activation map from the chase detect > mirror detect contrast, displayed on a 3D rendered brain using AFNI’s surface mapping tool (SUMA, http://afni.nimh.nih.gov/afni/suma). (B) Activation map from the chase project > mirror project contrast. In both images, the color bar ranges from Z = 2.3 (dark orange) to Z = 6.3 (bright yellow). The right pSTS is highlighted with a green circle.

Fig. 4

(A) Lateral view of the right hemisphere showing the intersection of voxels that reached significance at the cluster-corrected threshold in the chase detect > mirror detect, chase project > mirror project and biological motion > scrambled motion contrasts. The pSTS is highlighted in green. (B) Bar graphs plotting the average percent signal changes for the pSTS region highlighted in green from (A) for each of the four experimental conditions.