Catching Mind Wandering With Pupillometry: Conceptual and Methodological Challenges

Abstract

Mind-wandering (MW) refers to the shift of attention away from an ongoing task and/or external environment towards mental contents (e.g., memories, prospective thoughts) unrelated to the task. Physiological measures (e.g., pupil size, EEG, and fMRI) have often been acquired as objective markers for MW states, which has greatly helped their study as well as triangulation with other measures. Pupillometry in particular has been used as a covert biomarker of MW because it is reliably modulated by several distinct processes spanning arousal, emotion, and attention, and it signals attentional lapses. Yet, coupling MW and the measurement of pupil size has led to seemingly contrasting results. We argue that, common to the studies reviewed here, one reason is resolving to the measurement of tonic pupil size, which reflects low-frequency, slow changes in one's physiological state, and thus implicitly assumes that MW is a static, long-lasting process. We then additionally focus on three major axes of variability in the reviewed studies: (i) the definition and measurement of MW; (ii) the impact of contextual aspects, such as task demands and individual arousal levels; (iii) the identification and tracking of MW in combination with pupillary measures. We provide an overview of these differences and put forward recommendations for using physiological measures-including, but not limited to, pupil size-in MW research effectively. In conclusion, pupillometry can be a very informative tool for MW research, provided that it is used with the due methodological caution.

Keywords: mind‐wandering; pupil dynamics; pupillometry; task‐unrelated thoughts; tonic pupil size.

FIGURE 1

This image is meant to illustrate the difference between tonic and phasic pupil size. The top panel depicts the time course of pupil size in a participant who was passively staring at the screen, without any particular request (i.e., free‐viewing and, perhaps, mind‐wandering). The tonic pupil size in this case is one summary value, for example, the mean, of pupil size in the entire window (or another arbitrary time window); it reflects the relative balance between the sympathetic and parasympathetic autonomic nervous system in this specific moment in time and light conditions, thus in a rather crude way (bottom‐left panel). On the other hand, phasic changes in pupil size return a much more nuanced picture. Here (bottom‐right panel), constriction and dilation events are highlighted, in a bottom‐up fashion, by using the method of Joshi et al. (2016), that is, by assessing when differences between consecutive timepoints crossed zero. Phasic changes in pupil size can be spontaneous, that is, naturally occurring at variable frequencies, or event‐related, when triggered by specific events and cognitive processes (e.g., cognitive load, light reflex). Here we argue that phasic pupil size is a much more appropriate variable to consider when researching mind wandering (MW) and its multifaceted nature because more directly tied to the deployment of these qualitatively different cognitive processes and characterized by higher temporal resolution. Ideally, phasic changes in pupil size should be reconstructed starting from an event identified as triggering MW. This would be the ideal scenario because it shields from arbitrary assumptions about the length and duration of each MW event. That said, assessing spontaneous phasic changes, even if using a fixed‐length time window, is likely to provide more informative and multifaceted summary values than the simple tonic pupil measurement, in that more reflective of the heterogeneous chains of thoughts under consideration.