Higher-level cognitive processes like attention, memory, or mental imagery can produce pupil responses, without any actual difference in luminance of the visual stimulus. Interestingly, the inverse scenario remained untested to date: when a physical luminance difference is perceptually eliminated from awareness, does pupil size still track attention to the stimulus? In this Registered Report, Vilotijević and Mathôt (2024) sought to test this experimentally using a perceptual fading phenomenon where two Gaussian patches with different luminances fade from consciousness and are thus perceived as mid-level uniform grey (or at least the subjective difference is much reduced). This fading manipulation, as well as a control condition without perceptual fading, were presented in separate blocks. Participants were instructed to covertly attend one of the patches.

The authors hypothesised that if pupil size reflects attentional selection, these pupil responses in the fading condition​ should be eliminated or at least reduced, and this should evolve with time as the stimuli are perceptually fading. Their results show that pupil responses during covert attention are indeed reduced during perceptual fading - but they are not eliminated. Interestingly, this reduction did not depend on time or self-reports of the strength of perceptual fading. The findings therefore suggest that pupil dilation tracks subjective brightness differences.

 

One inherent issue with experiments like these is that the experimental and control conditions necessarily involve a physical difference in the stimulus. Here, the fading condition had the same spatial configuration of light and dark stimuli throughout a block while in the control (non-fading) condition the light and dark stimuli alternated sides between trials. It is therefore impossible to completely rule out that the physical difference affects the results. However, the only alternative to this would be an experimental design in which the stimuli never change, but only the subjective perceptual state varies. Such a design is completely at the mercy of the participant's subjective state and therefore loses experimental control and statistical sensitivity. The present results confirmed the authors' prediction that there are indeed differences in overall pupil responses during the fading and control conditions, irrespective of covert attention.

 

Critically, the fact that the attention effect did not vary with time or subjective self-reports of the illusion supports the authors' interpretation that this reflects higher-level cognition: the mere act of attending to the dark side - even if the actual appearance has faded - could cause a sustained pupil dilation. This would be consistent with the type of pupil effects for memory and mental imagery that motivated the present study. However, a simpler alternative is that the experience of perceptual fading was incomplete (as possibly suggested by Figures 2D and 2F) but that self-reports fail to capture this subjective experience accurately. Perhaps a future study could compare the magnitude of the attentional pupil effects when the initial stimulus is completely removed. If similar differences in pupil response persist this would suggest that the present results are due to high-level modulation or the residual low-level luminance difference.

 

The Stage 2 manuscript was evaluated by two reviewers and the recommender over two rounds of review. One reviewer again advised additional robustness checks to rule out eye movement confounds, an issue they had already raised during Stage 1 review. The researchers provide clear evidence that this is unlikely to have confounded their findings. This has been added to the supplementary data repository. Following this review and revision, the recommender judged that the Stage 2 criteria were met and awarded a positive recommendation.

 

URL to the preregistered Stage 1 protocol: https://osf.io/bmtp6
 
Level of bias control achieved: Level 6. No part of the data or evidence that was used to answer the research question was generated until after IPA.
 
List of eligible PCI RR-friendly journals:

 

 

 

1. Vilotijević, A. & Mathôt, S. (2024). The effect of covert visual attention on pupil size during perceptual fading [Stage 2]. Acceptance of Version 2 by Peer Community in Registered Reports. https://osf.io/ku8qc?view_only=f331df53b50f431386fabba9e386b387

 

Previous research has shown that higher-level cognitive processes like attention, memory or mental imagery can produce pupil responses, without any actual difference in luminance of the visual stimulus. Interestingly, the inverse scenario remains untested: when a difference in luminance is perceptually eliminated from awareness, does pupil size still track cognitive factors? In this study, Vilotijević and Mathôt (2024) seek to test this experimentally using a perceptual fading phenomenon where two Gaussian patches with different luminances fade from consciousness and are thus perceived as mid-level uniform grey. The researchers will present this fading manipulation and a control condition where no fading occurs in separate blocks. Participants are instructed to covertly attend one of the patches. If pupil size reflects attentional selection, pupil responses should be reduced in the fading condition, and this should evolve with time as the stimuli are perceptually fading.

 

The Stage 1 manuscript was evaluated by three reviewers and the Recommender over three rounds of review. One reviewer advises conducting robustness checks to rule out eye movements confounds. This constitutes exploratory analyses at Stage 2 that probably require some flexibility. Despite this, the Recommender decided to grant in-principle acceptance of this study. While these robustness tests can put the eventual results in context, the general hypotheses and predictions specified in this version are clear and straightforward.

 

URL to the preregistered Stage 1 protocol: https://osf.io/bmtp6
 
Level of bias control achieved: Level 6. No part of the data or evidence that will be used to answer the research question yet exists and no part will be generated until after IPA.
 
List of eligible PCI RR-friendly journals:

 

 

 

1. Vilotijević, A. & Mathôt, S. (2024). The effect of covert visual attention on pupil size during perceptual fading. In principle acceptance of Version 3 by Peer Community in Registered Reports. https://osf.io/bmtp6