Improving a Pupillometry Signal Through Video Luminance Modulation

Leon Schreiber, Wafaa Wardah, Vera Schmitt, Sebastian Möller, Robert P. Spang

Abstract: Objectively measuring affective states remains a recurring challenge in psychology and user experience research. A promising proxy for perceived arousal is the momentary assessment of the pupil diameter. However, the pupillometric signal is highly susceptible to variations in stimulus luminance, which can substantially confound the results. We introduce a method to improve the accuracy of pupillometric signals as an arousal predictor by eliminating frame-by-frame luminance variations in visual stimuli (i.e. scaling the brightness value for all pixels such that each frame has the same average luminance). This process was termed "equalization". We tested our approach in a study with 31 participants between the ages of 21 and 64. Our study focused on two metrics: (1) the perceived quality of the stimuli should not be affected by the process, and (2) the predictive power of the pupillometric signal for the perceived arousal should be greater for the equalized videos than for the non-equalized ones. We used a within-subjects design, where each participant rated their affective response and five distinct quality dimensions of four videos (two in the equalized and two in the non-equalized condition). Our results indicate that the equalization process does improve the predictive power of the pupillometric signal by a substantial amount. Statistical analyses also show that the participants rated all quality dimensions equivalently for an equalized and non-equalized variant of the same stimulus, with only small differences that are limited to a subset of the perceptual dimensions. While potentially limiting the efficacy of the process in some scenarios, the strengthened explanatory power of the pupillometric signal leaves room for a wide range of possible applications.