Separating Event-related Pupil Responses To A Structure-from-motion Stimulus - Info and Reading Options

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During states of perceptual bistability, the observer’s perception typically switches stochastically between two alternative perceptual interpretations despite the physical properties of the stimuli staying the same. Perception is said to be bistable in these situations because the stimulus is inherently consistent with two stable percepts, and each percept tends to remain for a few seconds at a time before a perceptual switch. Perceptual bistability has been extensively studied because perceptual switches are indicative of underlying neural mechanisms that are important for our understanding of visual perception in general. The noradrenergic arousal system associated with the locus coeruleus has been suggested to be one possible modulating mechanism because pupil dilations, typically considered an reaction to noradrenaline release, have been suggested to signify occurrences of perceptual switches (Einhäuser, Stout, Koch & Carter, 2008; Hupé, Lamirel & Lorenceau, 2009). In particular, one study has found, using a variety of bistable stimuli, that pupil dilations were closely related to timing of the switches such that pupil diameter increased immediately preceding reported perceptual switches (Einhäuser et al., 2008). However, observers reported perceptual switches either overtly by pressing keys on a keyboard, or covertly tracked and counted the number of switches they experienced. Thus due to possible influences of moth motor responses and effortful cognitive processing on pupil responses (Joshi & Gold, 2020; Mathôt, 2018), pupil responses in those situations may not be strictly tied to perceptual switches themselves. Because these and other similar task-related actions always occur at the same time as, or immediately following a genuine perceptual switch, this temporal correspondence complicates interpretation of dilations as they could be due to the actions of pressing keys or the mental arithmetic of counting switches. Therefore, pupil responses may have been caused by the task-relevant actions and their corresponding neural events, immediately following the perceptual switches, rather than the switch events themselves (Hupé et al., 2009; Brascamp, Hollander, Wertheimer, DePew & Knapen, 2021). In support of this notion, more recent work, in which task relevance was manipulated, found drastically different pupil responses depending on whether observers needed to overtly report switches. Using a dichoptic setup inducing binocular rivalry between separate planes of dots moving in opposite directions, observers’ perception alternates between the representation that originated from the left eye’s retinal image and one that arises from the right eye’s retinal image. In other words, they sometimes perceived left-moving dots and sometimes perceived right-moving dots. They found that, compared to the “report” conditions where observers were instructed to report switches by pressing a key on a keyboard, those in the “ignore” conditions where they were told to ignore switches showed significant constrictions following perceptual switches, in sharp contrast to the dilations in earlier findings (Brascamp et al., 2021). Importantly, in the ‘report’ conditions, dilation was also present and overlapped with the later portion of the perception-tied pupil constriction, suggesting that dilations found in earlier work could be accounted for by the task-relevant motor responses. Though pupils are found to react differently between these two studies, different stimuli were used in the process. Notably, among the stimuli used by Einhaüser et al. (2008) was a structure-from-motion rotating cylinder that, though also bistable for most observers, likely involves different mechanisms compared to binocular rivalry. Typically, percepts during observation of those stimuli are more susceptible to higher-level influences such as attention (Meng & Tong, 2004) or intention (Brouwer & van Ee, 2005); and perception of these stimuli based also tends to recruit higher-level brain areas such as the MT (Bradley, Chang & Anderson, 1998). On the other hand, perceptual alternation during binocular rivalry is generally considered more stimulus-driven (Blake & Logothetis, 2002), as it is less susceptible to higher-level influences such as attention (Meng & Tong, 2004). Further, neurophysiological evidence shows low-level suppression at as early as LGN during binocular rivalry (Wunderlich et al., 2005). Therefore it is possible that discrepancy in pupil responses between binocular rivalry and structure-from-motion could be attributed to different underlying mechanisms. In the current project, we aim to examine pupil responses during observation of a structure-from-motion stimulus again with the included design of an “ignore” condition allowing extraction of perceptual switches from observers’ eye movement without manual reporting. Similar to the paradigm used in Brascamp et al. (2021), we apply an algorithm based on optokinetic nystagmus (OKN) to infer moments of perceptual switches during observations of a transparent rotating sphere and observe pupillary responses corresponding to perceptual switches that are independent of manual reports.

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