Automatic neural mechanisms of social synchrony: pupil and Blink responses in adults with ADHD symptoms

Human social interactions involve both conscious behaviors, such as speech and gestures, and automatic responses regulated by dopaminergic and noradrenergic neural networks, including pupil dilation and blinking. Dysfunctions in dopamine and noradrenaline activity, common in ADHD, impair automatic social processes such as physiological synchrony and anticipation, leading to broader difficulties in social cognition and interpersonal relationships. Shifting the lens from overt behaviors to automatic processes that lie at the core of social interaction, the present study examined whether adults with ADHD indication (ADHD-Ind) respond differently to a simulated social cue from those without ADHD-Ind. Sixty young adults (16 ADHD-indicated, 44 controls; 20-35 years) watched nine 6-s clips where on-screen eyes blinked twice or four times while eye-tracking recorded blink timing and pupil size. While global measures did not differ between groups, time-locked analyses of blink synchrony to simulated social cues emerged in the control group-particularly around the onset of simulated blinks. In contrast, synchrony effects are attenuated or absent in individuals with ADHD indications. For pupil dynamics, group differences emerged in the sparse simulated blinking condition, where ADHD-Ind showed reduced dilations from - 80 to 560 ms compared to controls and between - 40 and 400 ms compared to the frequent condition. No group differences appeared in the frequent condition. Further, Higher ADHD symptomatology predicted lower blink synchrony and alterations in the pupil time course. The study underscores the added value of automatic, non-verbal synchrony as potential communicative biomarkers for Neuro-Developmental Disorders, and specially in ADHD. Together, the findings suggest that, in alignment with social cue dynamics, timing- and context-specific alterations-rather than global differences-impede social functioning in individuals with ADHD.