You can see pain in the eye: pupillometry as an index of pain intensity under different luminance conditions.

Abstract Pupil dilation is regulated autonomically and it may be a valid measure of pain, but pupillometry for pain intensity recordings has not been evaluated under different luminance conditions. We hypothesized that the pupil response may serve as an objective indicator of pain intensity even if luminance conditions differ which is often the case in experiments with pictures. In 20 healthy females we applied a tonic pressure pain to the fingers (20 s). During pain induction, participants looked at pictures of three different levels of luminance. Pupil dilation was recorded continuously. Immediately after pain onset, there was a significant pupil dilation which reached its maximum about 2 s after pain onset. While this maximum pupil dilation did not differ with pressure intensity, the pupil dilation was larger for the higher pressure intensity in the period from 10 s after pressure onset to pressure offset. Even under different luminance conditions, pupillometry can serve as an objective indicator of pressure pain intensity. Thus, it seems promising to use pupillometry with complex experimental designs combining pain and pictorial stimuli.