Inter- and intraindividual variability of the pupillary unrest index

Abstract

The recording of slow pupillary oscillations in complete darkness is a promising approach for objective evaluation of daytime sleepiness at the physiological level. The aim of the present study was to analyze the magnitude of between- and within-subject variation of the pupillary unrest index (PUI) in a sample of healthy individuals. The present data were collected within the framework of a mobile phone study on possible effects of radiofrequency electromagnetic fields (RF-EMF) on the central nervous system. Pupillary behavior was monitored in 30 young healthy male volunteers (age 18–30 years) on ten non-consecutive experimental days at 11:00 a.m. and 04:00 p.m., using infrared video pupillography (compact integrated pupillograph, CIP). Since RF-EMF had no impact on the PUI, data were pooled for the present analysis. Mixed-model analyses of variance (ANOVAs) showed that the PUI was subject to higher interindividual variation as compared to intraindividual variation at both times of measurement. This resulted in intraclass correlation coefficients pointing to a substantial stability of these interindividual differences. A comparison of the PUI results with currently used cutoff values revealed that more than 50% of recordings from young, healthy, non-sleep-disturbed males showed values beyond the “normal” range. The significant interindividual variability implies that the PUI is not only a state marker. Evaluations of such measurements should therefore consider the PUI as a possible trait marker to ensure comparability and correct interpretation.