Possible Mechanisms for Elevated Pupil Size in Autistic Adults

Background: Increases in pupil size dilation has been associated with emotional salience and greater cognitive load. A series of studies by Tsukahara, et al. (2016) have reported a positive correlation between tonic pupil size and higher fluid intelligence and working memory performance, suggesting that persons with high tonic pupil size also have a higher capacity or cognitive load. Studies of autistic samples have found higher tonic pupil sizes in autism (AUT) compared to neurotypical (NT) samples, which could be due to cognitive factors but could also be due to elevated emotional arousal. To date, there has not been a study examining the relationship between tonic pupil size, cognitive functioning, and emotional distress in an autism sample.

Objectives: To explore the relationship between tonic pupil size, cognitive ability, and emotional distress in both neurotypical and autistic adults.

Methods: Participants were autistic (n=26) and neurotypical (n=24) adults who were part of a larger study on emotion learning in autism. We collected tonic pupil size using using Eyelink 1000 Plus eye-tracking system while participants stared at a fixation cross in the center of the screen for 60 seconds. We administered the WAIS-IV to all participants to identify cognitive ability using standard subtests and domains. We measured emotional distress using the Depression, Anxiety, and Stress Scale Short Form and the Penn State Worry Questionnaire. We calculated differences between the AUT and NT groups using t-tests in all measures, and also examined possible correlations between variables using a Pearson’s correlation.

Results: Analyses confirmed previous findings of larger tonic pupil size in AUT compared to NT group participants (p= .011). The AUT group also had a higher emotional distress scores on the DASS (p<.001) and PSWQ (p<.001). Groups were evenly matched for Verbal Comprehension and Perceptual Reasoning domains of the WAIS-IV but the AUT group scored lower on the Working Memory (p=.011), and Processing Speed (p<.001) domains. However, there were no significant correlations between pupil size and cognitive ability or emotional distress, for either group separately or for the combined samples.

Conclusions: At least in this small sample, the higher pupil size in AUT does not seem to be related to increased cognitive abilities as extrapolated from Tsukahara, et al. (2016). However, we also found no correlations with emotional distress. Cognitive and emotional reactivity--how one responds to a challenging task--may be interesting to study in relation to pupil size. At present, however, we do not have an explanation for the consistent physiological difference of larger tonic pupil size in autism and future studies of both physical and psychological factors are needed to shed light on this phenomenon.