# Pain-induced effects on the pupillary light response under high and low illumination conditions

## TL;DR

This study explores how pain affects pupil reactions to light under different brightness levels and finds that pain influences these reactions through the nervous system.

## Contribution

The study demonstrates how pain modulates pupillary responses and clarifies the role of the autonomic nervous system in this process.

## Key findings

- Pain alters pupillary light response parameters in line with parasympathetic and sympathetic nervous system activity.
- Light intensity influences pupillary responses but does not interact with pain effects.
- Pain ratings and heart rate correlate with changes in pupillary parameters.

## Abstract

The present study investigated the impact of two different light intensities on the pain-modulated pupillary light response (PLR). Additionally, it aimed to demonstrate parasympathetic and sympathetic influences on PLR parameters in response to pain, as predicted by functional models.

A total of 24 participants were included in a randomized, repeated-measures design. The PLR was measured in response to both dark and bright light stimuli within two test cycles. Pain was induced using the cold pressor test (CPT), which involved immersing participants' feet in ice water. PLR measurements were taken during baseline and ice-water immersion within each test cycle. The assessed PLR parameters included initial diameter (INIT), latency (LAT), amplitude (AMP), and re-dilation time (ReDIL25). Along with these parameters, heart rate (HR) and pain ratings were also computed and analyzed.

The CPT caused moderate pain in participants, and the resulting PLR parameters were found to be congruent with the expected parasympathetic and sympathetic nervous system activities. Although the luminance of the stimulus did influence PLR parameters, no interaction with pain exposure was found.

The results showed that different aspects of pain experienced by an individual, as modulated through the sympathetic and parasympathetic nervous systems, are visible in their pupillary reactions to light. Notably, within the range used in the current study, light intensity did not significantly affect the pain-related PLR effects.