Reply to Mathôt and Naber: Neuroimaging shows that pupil mimicry is a social phenomenon

Electrophysiological concomitants of pupillary synchrony

Pupillary synchrony or contagion is the automatic unconscious mimicry of pupil dilation in dyadic interactions. This experiment explored electrophysiological event-related potential (ERP) concomitants of pupillary synchrony. Artificial pupils (black dots) were superimposed on either partial faces (eyes, nose, brow) or random textures. Observers were asked to judge dot size (large, medium, or small). There was clear evidence of pupillary synchrony with observer pupil dilation greater to large dots than to small or medium dots. The pupillary synchrony increased in magnitude throughout the trial and was found both with faces and with textures. When the stimuli were partial faces with artificial pupils (dots), there was ERP activity related to target dot size in the period at P250 and P3. A face specific N170 was also found. When the stimuli were random textures with dots, there was ERP activity at P1 and in the interval from 140 to 200 ms post-stimulus onset. The use of ERP with pupillometry revealed results for faces that were consistent with a social explanation of pupillary synchrony whereas results for textures were consistent with a local luminance explanation.

Individual differences in pupil dilation to others' emotional and neutral eyes with varying pupil sizes

Sensitivity to others' emotional signals is an important factor for social interaction. While many studies of emotional reactivity focus on facial emotional expressions, signals such as pupil dilation which can indicate arousal, may also affect observers. For example, observers' pupils dilate when viewing someone with dilated pupils, so-called pupillary contagion. Yet it is unclear how pupil size and emotional expression interact as signals. Further, examining individual differences in emotional reactivity to others can shed light on its mechanisms and potential outcomes. In the current study, adults' (N = 453) pupil size was assessed while they viewed images of the eye region of individuals varying in emotional expression (neutral, happy, sad, fearful, angry) and pupil size (large, medium, small). Participants showed pupillary contagion regardless of the emotional expression. Individual differences in demographics (gender, age, socioeconomic status) and psychosocial factors (anxiety, depression, sleep problems) were also examined, yet the only factor related to pupillary contagion was socioeconomic status, with higher socioeconomic status predicting less pupillary contagion for emotionally-neutral stimuli. The results suggest that while pupillary contagion is a robust phenomenon, it can vary meaningfully across individuals.

Infants' brain responses to pupillary changes in others are affected by race

Sensitive responding to eye cues plays a key role during human social interactions. Observed changes in pupillary size provide a range of socially-relevant information including cues regarding a person’s emotional and arousal states. Recently, infants have been found to mimic observed pupillary changes in others, instantiating a foundational mechanism for eye-based social communication. Among adults, perception of pupillary changes is affected by race. Here, we examined whether and how race impacts the neural processing of others’ pupillary changes in early ontogeny. We measured 9-month-old infants’ brain responses to dilating and constricting pupils in the context of viewing own-race and other-race eyes using functional near-infrared spectroscopy (fNIRS). Our results show that only when responding to own-race eyes, infants’ brains distinguished between changes in pupillary size. Specifically, infants showed enhanced responses in the right superior temporal cortex when observing own-race pupil dilation. Moreover, when processing other-race pupillary changes, infants recruited the dorsolateral prefrontal cortex, a brain region linked to cognitive control functions. These findings suggest that, early in development, the fundamental process of responding to pupillary changes is impacted by race and interracial interactions may afford greater cognitive control or effort. This critically informs our understanding of the early origins of responding to pupillary signals in others and further highlights the impact of race on the processing of social signals.