Abstract
Pupillometry, thanks to its strong relationship with cognitive factors and recent advancements in measuring techniques, has become popular among cognitive or neural scientists as a tool for studying the physiological processes involved in mental or neural processes. Despite this growing popularity of pupillometry, the methodological understanding of pupillometry is limited, especially regarding potential factors that may threaten pupillary measurements' validity. Eye blinking can be a factor because it frequently occurs in a manner dependent on many cognitive components and induces a pulse-like pupillary change consisting of constriction and dilation with substantive magnitude and length. We set out to characterize the basic properties of this "blink-locked pupillary response (BPR)," including the shape and magnitude of BPR and their variability across subjects and blinks, as the first step of studying the confounding nature of eye blinking. Then, we demonstrated how the dependency of eye blinking on cognitive factors could confound, via BPR, the pupillary responses that are supposed to reflect the cognitive states of interest. By building a statistical model of how the confounding effects of eye blinking occur, we proposed a probabilistic-inference algorithm of de-confounding raw pupillary measurements and showed that the proposed algorithm selectively removed BPR and enhanced the statistical power of pupillometry experiments. Our findings call for attention to the presence and confounding nature of BPR in pupillometry. The algorithm we developed here can be used as an effective remedy for the confounding effects of BPR on pupillometry.
Highlights
Even when ambient light is controlled, our pupil keeps dilating and constricting as mental operations or internal states of various sorts transpire in the course of carrying out diverse cognitive tasks [1,2,3,4,5,6,7,8]
By inspecting the pupillary response profiles that were associated with isolated single events of eye blinking, we learned that pupil size reacts to a single blink similar to how it responds to an abrupt change in retinal illuminance, which we dubbed blink-locked pupillary response (BPR)
By inspecting how the blink rate changes as a function of task epoch or as a function of cognitive states, we learned that BPR is not a mere nuisance but should be treated as a serious confounder that threatens the internal validity of experiments
Summary
Even when ambient light is controlled, our pupil keeps dilating and constricting as mental operations or internal states of various sorts transpire in the course of carrying out diverse cognitive tasks [1,2,3,4,5,6,7,8] This pupil-size dynamics is known to be tightly coupled with the activation of norepinephrine-containing neurons in the locus coeruleus (LC-NE system) and in other brain regions associated with the LC-NE system such as the colliculi and cingulate cortex [9,10,11,12]. Thanks to recent technical advancements, pupil size can be measured with high temporal resolution in non-invasive manners even when animals or humans are allowed to move their eyes and head relatively freely. Pupil size became a popular physiological measure among cognitive scientists and neuroscientists, being considered a “peripheral window” into internal cognitive and neural processes [13,14].
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