The measure of pupil size is a noninvasive indicator of reactions that occur spontaneously during stimulus presentation, do not require overt responses (Laeng, Sirois, & Gredebäck, 2012 Tamietto et al., 2009), and can be observed in infants (Jackson & Sirois, 2009 Wass, de Barbaro, & Clackson, 2015 Wetzel, Buttelmann, Schieler, & Widmann, 2016), patients with psychiatric or neurological disorders (Anderson et al., 2013 Bakes et al., 1990 Martineau et al., 2011 Steinhauer & Hakerem, 1992 Wang et al., 2016), and even nonhuman primates (Iriki, Tanaka, & Iwamura, 1996 Kret, Tomonaga, & Matsuzawa, 2014 Machado, Bliss-Moreau, Platt, & Amaral, 2011 Wang, Boehnke, Itti, & Munoz, 2014 Weiskrantz, Cowey, & Le Mare, 1998). These studies have highlighted the potential of using low-cost pupil size measurement for diagnosis or to examine executive function deficits in early stages of the disorders. Alterations in pupil size and/or pupil response have also been observed in psychiatric disorders and have been proposed as indicators of autonomic dysfunction in autism spectrum disorder (Anderson, Colombo, & Unruh, 2013 Martineau et al., 2011), anxiety or depressive disorders (Bakes, Bradshaw, & Szabadi, 1990 Wehebrink, Koelkebeck, Piest, de Dreu, & Kret, 2018), and schizophrenia (Steinhauer & Hakerem, 1992). Another study demonstrated disruptions in pupil responses during voluntary movement preparation in these patients (Wang, McInnis, Brien, Pari, & Munoz, 2016). Patients with Parkinson’s disease, for instance, have been shown to exhibit a larger pupil diameter after light adaptation, as well as a reduced amplitude of contraction and a prolonged contraction time during the light reflex (Micieli et al., 1991). But as many different disorders are characterized by an imbalance in the sympathetic and the parasympathetic system, the number of studies incorporating measures of pupil size into clinical investigation is growing. The pupil has since long been studied extensively as an index of the level of consciousness in coma patients (Teasdale & Jennett, 1974). Although other neurotransmitters, such as serotonin, are known to influence dilation, these effects are similarly known to be mediated via the locus coeruleus–norepinephrine complex (Yu, Ramage, & Koss, 2004). Therefore, under constant low light levels, pupil size is a reliable and accessible measure of norepinephrine levels (Aston-Jones & Cohen, 2005 Koss, 1986 Nieuwenhuis, Aston-Jones, & Cohen, 2005). This dilation response is distinct from the strong contractions exhibited during the pupillary light reflex, which is mediated by acetylcholine (via the iris sphincter muscle). Pupil dilation is regulated by the sympathetic nervous system and mediated almost exclusively via norepinephrine from the locus coeruleus (through stimulation of α-adrenoceptors on the iris dilator muscle and postsynaptic α 2-adrenoceptors within the relatively closely located Edinger–Westphal nucleus, which projects to the ciliary ganglion controlling the dilation of the iris Yoshitomi, Ito, & Inomata, 1985). Given that pupil diameter is easily measured by standard eyetracking technologies and can provide fundamental insights into cognitive and emotional processes, it is hoped that this article will further motivate scholars from different disciplines to study pupil size. These guidelines are accompanied by an open source MATLAB script (available at ). Finally, we provide step-by-step guidelines that will help those interested in pupil size to preprocess their data correctly. In this article we first provide a short review of the literature on pupil size measurements, then we highlight the most important sources of noise and show how these can be detected. Before analyzing pupil size, it is therefore of crucial importance first to detect this noise and deal with it appropriately, even prior to (if need be) resampling and baseline-correcting the data. The different systems used for recording pupil size are almost as variable as its applications, and all yield, as with many measurement techniques, a substantial amount of noise in addition to the real pupillometry data. As a result, research involving pupil size measurements has been reported in practically all psychology, psychiatry, and psychophysiological research journals, and now it has found its way into the primatology literature as well as into more practical applications, such as using pupil size as a measure of fatigue or a safety index during driving. Changes in pupil size can reflect diverse cognitive and emotional states, ranging from arousal, interest and effort to social decisions, but they are also widely used in clinical practice to assess patients’ brain functioning. Pupillometry has been one of the most widely used response systems in psychophysiology.
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