Visual motion assists in social cognition

Recent evidence suggests a link between visual motion processing and social cognition. When person A watches person B, the brain of A apparently generates a fictitious, subthreshold motion signal streaming from B to the object of B’s attention. These previous studies, being correlative, were unable to establish any functional role for the false motion signals. Here, we directly tested whether subthreshold motion processing plays a role in judging the attention of others. We asked, if we contaminate people’s visual input with a subthreshold motion signal streaming from an agent to an object, can we manipulate people’s judgments about that agent’s attention? Participants viewed a display including faces, objects, and a subthreshold motion hidden in the background. Participants’ judgments of the attentional state of the faces was significantly altered by the hidden motion signal. Faces from which subthreshold motion was streaming toward an object were judged as paying more attention to the object. Control experiments showed the effect was specific to the agent-to-object motion direction and to judging attention, not action or spatial orientation. These results suggest that when the brain models other minds, it uses a subthreshold motion signal, streaming from an individual to an object, to help represent attentional state. This type of social-cognitive model, tapping perceptual mechanisms that evolved to process physical events in the real world, may help to explain the extraordinary cultural persistence of beliefs in mind processes having physical manifestation. These findings, therefore, may have larger implications for human psychology and cultural belief.

A recent series of reports suggests a link between visual motion processing and social cognition. The human brain appears to fabricate a subtle, false motion signal when looking at another person. The fictitious motion streams in a beam from the other person toward the object of that person’s attention (1–3). The signal can be observed directly with functional MRI in motion-processing cortical areas, and it can be observed indirectly by how it causes a motion aftereffect in the area of a scene between a face and the object of the face’s attention. The signal, however, is perceptually subthreshold—people are not explicitly aware of it. The functional purpose, if any, of this subthreshold false motion signal is not known, although we speculated it is part of the social toolkit for modeling the attention of others. Because previous studies were correlative—showing a correlation between social cognition and an internally generated motion signal—the causal relationship is not known (4, 5). To establish this new subfield of study in which social cognition taps into preexisting perceptual machinery to model the mind states of others, a direct causal experiment is needed. Here, we provide that test. We asked, if we contaminate a participant’s visual world with a subthreshold motion that streams from another person toward an object, can we manipulate the participant’s perception of that other person’s attention? The results demonstrated a behaviorally meaningful impact of subthreshold motion on social judgments. It explains why the human brain fabricates a motion signal during social cognition. Modeling the attention state of others is a crucial part of social cognition (6–9), and recruiting the motion-processing system evidently contributes to that model. It may have proved adaptive to co-opt the brain’s existing motion-processing mechanism to encode sources and targets of attention, in essence drawing a quick visual sketch with moving arrows to help keep track of who is attending to what in a complex environment. We suggest that the beam of motion represents, instantaneously, the relationship between an agent and the target of its attention. In this interpretation, the subthreshold motion signal balances two adaptive pressures: it is strong enough to influence social cognition in a meaningful direction while at the same time not so strong that it materially interferes with the normal motion perception of real objects. This type of social-cognitive model, borrowing low-level perceptual mechanisms that evolved to process physical events in the real world, may help to explain the extraordinary cultural persistence of beliefs in mind processes having physical manifestation. It is a common belief across time and cultures that attentive gaze comes with a palpable outward flow and that other properties of the mind are linked to specific physical auras and flows. The present findings, therefore, may have larger implications for human psychology and cultural belief.