Covert neural and autonomic signatures of shared perception.

Humans often co-perceive stimuli with others, yet the neurocognitive effects of such shared perceptual contexts are underexplored. We tested whether awareness that a visual stimulus is simultaneously available to another person, without interaction, modulates behavioral performance and neurophysiological signatures of perceptual decision-making. Thirty-three participants completed 640 trials of a Random Dot Kinematogram motion discrimination task while EEG and pupillometry data were recorded. A confederate was present, with a divider ensuring that, on each trial, the stimulus was either jointly visible to both or privately visible to the participant. Participants received no feedback and engaged in no interaction, isolating the effect of joint visibility. Behavioral performance was unaffected by social context, but EEG analysis revealed context-specific neural patterns emerging after cue onset and before stimulus presentation, suggesting proactive encoding of the social context. Additionally, pupil size was significantly greater during public visibility trials, indicating heightened arousal associated with social vigilance. These findings suggest that co-perception induces covert social vigilance-anticipatory arousal and neural readiness in response to co-visibility, even without interaction. Such covert markers could serve as biomarkers for altered social salience processing in clinical populations, such as those with social anxiety disorder or autism.