Here, we tested whether pupil size reacts to the violation of statistical regularities when these are not consciously perceived as such by participants and, most importantly, the violations are not task-relevant. Evidence for pupillary response to violations of expectations in statistical learning would also make pupil size a compelling physiological marker to track learning progression. This process has been reported for the first time in the auditory domain, showing that 8-months babies were able to learn differences in statistical relationship between non-meaningful syllables ( Saffran et al., 1996) since then, statistical learning has been demonstrated in many other studies, for example in the context of language acquisition ( Saffran et al., 1999 Arciuli and Torkildsen, 2012), but also in visual processing ( Turk-browne et al., 2005, 2008 Alamia and Zénon, 2016). Such unconscious predictions occur, for instance, in the context of statistical learning, the process through which we learn the statistical regularities of the environment ( Kim et al., 2009 Turk-Browne et al., 2009). Although a few studies have reported pupil dilation to surprising stimuli in the absence of behavioral responses ( Gomes et al., 2015 Liao et al., 2016 Zekveld et al., 2018), these findings were obtained in contexts in which surprising events were conscious.Ĭonsequently, it remains unknown whether the eye pupil responds to unconscious violations of predictions, or whether it relies on conscious processes. Nonetheless, these results were obtained in the context of behavioral tasks, in which the surprising observation indicated the need for behavioral adaptation to new contingencies, and possibly emotional surprise response ( Meyer et al., 1997). In agreement with this view, pupil size has been shown to respond to self-information, or surprisal, which can be defined as the negative log probability of an event, i.e., how unexpected an observation is ( Friedman et al., 1973 Raisig et al., 2010 Preuschoff et al., 2011 Nassar et al., 2012 O'Reilly et al., 2013 Damsma and van Rijn, 2017). The fundamental, causal link between cognition and arousal remains unknown but authors have hypothesized that uncertainty, and its reduction through brain processing, may be the common denominator of arousal responses to cognition ( Yu and Dayan, 2005 Preuschoff et al., 2011 Zénon et al., 2019). However, pupil size is also affected by global brain arousal state ( Bradley et al., 2008 Reimer et al., 2016), which is modulated by a wide range of cognitive factors ( Hess and Polt, 1964 Beatty and Lucero-Wagoner, 2000 Bradley et al., 2008 Mathot et al., 2014). The main function of the eye pupil is to adjust the quantity of light reaching the retina by dilating or constricting in response to environmental luminance changes ( De Groot and Gebhard, 1952). ![]() ![]() All in all, our results establish novel insights about the mechanisms driving global arousal levels, and it provides new possibilities for reliably measuring unconscious processes. Further, we show that the concurrent electrophysiological response shares similarities with mismatch negativity, suggesting the involvement of anterior cingulate cortex. Here, we demonstrate that unexpected changes in the environment, even when processed unconsciously and without being relevant to the task, lead to an increase in arousal levels as reflected by the pupillary response. However, whether this response arises as a conscious response or reflects a more fundamental mechanism outside the scrutiny of awareness is still unknown. SIGNIFICANCE STATEMENT Pupil size dilates following increase in mental effort, surprise, or more generally global arousal. ![]() ![]() Arousal response to surprisal outside the scope of conscious perception points to the fundamental relationship between arousal and information processing and indicates that pupil size can be used to track the progression of implicit learning. These pupil responses correlated with central potentials, evoking an anterior cingulate origin. We found that pupil size dilated following these surprising events, in the absence of awareness of transition statistics, and only when attention was allocated to the stimulus. We measured pupil and brain-evoked potentials to stimuli that violated transition statistics but were not relevant to the task. In the present study, male and female adult humans performed an implicit learning task across a series of three experiments. Whether this response can be observed regardless of conscious perception is still unknown. Pupil size under constant illumination reflects brain arousal state, and dilates in response to novel information, or surprisal.
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