Pupil size correlates with near-threshold detection performance irrespective of stimulus colour, eccentricity, or retinal adaptation-state

In visual near-threshold detection tasks, larger pre-stimulus pupil size is associated with improved accuracy. However, previous studies have used black and white peripherally presented stimuli, leaving open the question of whether the relationship persists when targets differ in their colour or eccentricity. Here we addressed this question with three experiments that systematically varied the lighting conditions and target properties in a visual near-threshold detection task (data collected in 2022-2024). Light conditions ranged from dark to dim to bright, with the dark condition including a period of dark-adaptation. Possible target colours were blue and red on a black background (dark condition), blue and red on a grey background (dim condition), or yellow and cyan on a white background (bright condition). Possible target eccentricities ranged from parafoveal to peripheral, in a continuous manner (experiment 3) or as two predefined near and far eccentricities (experiments 1 & 2). Across all experiments we show that larger pre-stimulus pupil size is associated with improved performance. This large-pupil advantage is not systematically modulated by the colour or eccentricity of the targets, the illumination of the testing room, or retinal adaptation-state. We conclude that the phenomenon is robust, indicating that pupil size affects vision in a behaviourally relevant manner, regardless of the exact conditions. Significance statementBy showing that pupil size is robustly associated with performance in visual near-threshold detection tasks, we provide evidence for the functional role of endogenous pupil size fluctuations in visual perception.