The Effects of Short-Term Light Adaptation on the Human Post-Illumination Pupil Response

We determine the effect of short-term light adaptation on the pupil light reflex and the melanopsin mediated post-illumination pupil response (PIPR). Inner and outer retinal photoreceptor contributions to the dark-adapted pupil response were estimated. In Experiment A, light adaptation was studied using short wavelength lights ranging from subthreshold to suprathreshold irradiances for melanopsin signaling that were presented before (5-60 seconds) and after (30 seconds) a melanopsin-exciting stimulus pulse. We quantified the pupil constriction and the poststimulus response amplitudes during dark (PIPR) and light (poststimulus pupil response, PSPR) adaptation. In Experiment B, colored prestimulus adapting lights were univariant for melanopsin or rod excitation. Increasing the prestimulus duration and irradiance of adapting lights increased the pupil constriction amplitude when normalized to the dark-adapted baseline but reduced its amplitude when normalized to the light-adapted baseline. Light adaptation at irradiances suprathreshold for melanopsin activation increased the PIPR amplitude, with larger changes at longer adaptation durations, whereas the PSPR amplitude became more attenuated with increasing irradiances, independent of duration. Rod versus melanospin univariant adaptation did not alter the constriction amplitude but increased the PIPR amplitude in the rod condition. Correlations between millimeter pupil constriction and PIPR amplitudes were eliminated when normalized to the baseline diameter. The findings have implications for standardizing light adaptation paradigms and the choice of pupil metrics in both laboratory and clinical settings. Light and dark adaptation have opposite effects on the pupil metrics, which should be normalized to baseline to minimize significant correlations between constriction and PIPR amplitudes.