Abstract
Pupil size is influenced by cognitive and non-cognitive factors. One of the strongest modulators of pupil size is scene luminance, which complicates studies of cognitive pupillometry in environments with complex patterns of visual stimulation. To help understand how dynamic visual scene statistics influence pupil size during an active visual search task in a visually rich 3D virtual environment (VE), we analyzed the correlation between pupil size and intensity changes of image pixels in the red, green, and blue (RGB) channels within a large window (~14 degrees) surrounding the gaze position over time. Overall, blue and green channels had a stronger influence on pupil size than the red channel. The correlation maps were not consistent with the hypothesis of a foveal bias for luminance, instead revealing a significant contextual effect, whereby pixels above the gaze point in the green/blue channels had a disproportionate impact on pupil size. We hypothesized this differential sensitivity of pupil responsiveness to blue light from above as a “blue sky effect,” and confirmed this finding with a follow-on experiment with a controlled laboratory task. Pupillary constrictions were significantly stronger when blue was presented above fixation (paired with luminance-matched gray on bottom) compared to below fixation. This effect was specific for the blue color channel and this stimulus orientation. These results highlight the differential sensitivity of pupillary responses to scene statistics in studies or applications that involve complex visual environments and suggest blue light as a predominant factor influencing pupil size.
Highlights
In classic cognitive pupillometry studies, it has been critical to equate luminance across stimuli and/or experimental conditions to isolate cognitive influences on pupil size and ensure that results are not driven by confounds due to variable luminance
The analyses presented in this paper were focused on characterizing the relationship between pupil size and RGB scene statistics over the course of a visual search task in a virtual environment
We tested whether three findings from previous literature on the relationship between pupil size and luminance would generalize to a naturalistic task: (i) pupil size is more strongly influenced by luminance inputs near the fovea, (ii) pupillary responses to light are modulated by color spectrum effects, with a specific sensitivity to blue light, and in Experiment 2, and (iii) that pupil size is more strongly influenced by blue light, when it is located above fixation
Summary
In classic cognitive pupillometry studies, it has been critical to equate luminance across stimuli and/or experimental conditions to isolate cognitive influences on pupil size and ensure that results are not driven by confounds due to variable luminance. While this careful control of luminance has led to a great deal of knowledge about the relationship between pupil size and cognition (Aston-Jones and Cohen, 2005; Sirois and Brisson, 2014; Mathôt, 2018; Hoffing et al, 2020; Joshi and Gold, 2020), it limits the ability to generalize research findings to contexts in which luminance cannot be controlled. Evidence from individuals with nonfunctioning rod and cone photoreceptors (Czeisler et al, 1995; Lockley et al, 1997) and transgenic mice without photosensitive RGCs (Lucas et al, 2001, 2003) suggest that both types of retinal cells contribute to the PLR; much less is understood about their relative contributions to the PLR, especially in uncontrolled settings with naturalistic and dynamic visual scenes
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