If Eyes Are a Window to the Soul, Our Pupils May Reveal PTSD
Pupillometry shows how PTSD disrupts healthy autonomic nervous system responses.
New pupillometry research reveals how post-traumatic stress disorder (PTSD) may affect both branches of the autonomic nervous system. By measuring how someone's pupils dilate and constrict in response to changes in ambient light and also while viewing emotive images, the researchers were able to index both parasympathetic nervous system (PNS) and sympathetic nervous system (SNS) responses. The findings (McKinnon, Gray, & Snowden, 2020) appear in the July 2020 issue of Biological Psychology.
Previous research (Steinhauer et al., 2004) established that stimulation of the sympathetic "fight or flight" pathway (SNS) causes pupils to dilate and get bigger. Conversely, stimulation of the calming "rest and digest" parasympathetic pathway (PNS) causes pupils to constrict and get smaller; inhibition of the parasympathetic system causes pupils to stay dilated.
Based on the link between parasympathetic function and pupil diameter, if someone doesn't have high vagal tone and lacks a robust vagus nerve-mediated PNS response, his or her pupils tend to stay dilated and constrict more slowly in the aftermath of experiencing a "fight or flight" inducing stressor.
Notably, another study (Wang et al., 2016) also found that parasympathetic nervous system dysfunction is associated with disruptions of the pupil light reflex (PLR), which causes someone's pupils to constrict in response to increased amounts of light hitting the retina.
Because of this well-established link between pupil dilation/constriction and autonomic nervous system functions, pupillometry (which measures changes in pupil size) offers a window into how both branches of the autonomic nervous system respond to environmental stimuli and various stressors.
The latest UK-based pupillometry study (2020) was led by Aimee McKinnon (who is now at Oxford University) during her tenure at Cardiff University in conjunction with Cardiff's Robert Snowden and Nicola Gray of Swansea University in Wales. This study (N = 65) included a cohort of 20 participants who met the diagnostic criteria for PTSD, 28 participants who had been exposed to trauma but didn't have PTSD, and a control group of 17 people without trauma exposure or PTSD.
Unlike someone without trauma exposure or PTSD, the researchers found that the pupils of participants with PTSD did not constrict quickly in response to increases in light; this suggests a link between PTSD and PNS-mediated pupil constriction. As the authors explain: "Individuals with PTSD showed reduced pupil constriction to stimulus onset suggesting reduced parasympathetic arousal."
The researchers also used pupillometry to measure changes in pupil size while viewing positive or negative emotive images as an index autonomic nervous system responses. Unexpectedly, the researchers found that the pupils of those with PTSD showed hyperarousal and an exaggerated dilation in response to both negative images (e.g., graphic violence) and positive images (e.g., exciting sports action).
"The greater response to the threatening pictures in patients with PTSD was expected and is in line with the fact that people with PTSD are hypersensitive to their surroundings due to an overactive sympathetic nervous system response," Snowden said in a Cardiff news release. "However, the lack of constriction is quite novel and suggests that they also have problems with their parasympathetic system. The research suggests that these people are in a constant state of vigilance and react strongly to arousing images."
In their paper's abstract, the authors sum up about how these pupillometry findings relate to autonomic nervous system functions and PTSD:
“Initial pupil constriction (a marker of parasympathetic function) was reduced for the PTSD group, while dilation due to the emotional content of the image (a marker of sympathetic activity) was greater in the PTSD group. Individuals with PTSD demonstrated enhanced physiological arousal to both threat-related and positive images."
Taken together, these pupillometry results suggest that patients with PTSD may be more likely to experience reduced parasympathetic arousal and increased sympathetic arousal of their autonomic nervous systems. For those living with PTSD, both positive and negative emotional stimulation may trigger an immediate "fight or flight" response that may be difficult to counterbalance due to reduced parasympathetic functionality.
"[Our research] shows that the hyper-response of the pupil is in response to any arousing stimulus, and not just threatening ones," Gray said in a Swansea news release. "This may allow us to use these positive pictures in therapy, rather than relying upon negative images, that can be quite upsetting to the patient, and therefore make therapy more acceptable and bearable. This idea now needs testing empirically before it is put into clinical practice."
"These findings allow us to understand that people with PTSD are automatically primed for threat and fear responses in any uncertain emotional context, and to consider what a burden this must be to them in everyday life," McKinnon concluded. "It also suggests that it is important for us to recognize that, in therapy, it is not just the fear-based stimuli that need deliberately re-appraising."
Christopher Bergland is a world-class endurance athlete, coach, author, and political activist.