Filling-in of the foveal rod scotoma and confidence preference for central vision under mesopic viewing conditions

Different photoreceptors support vision depending on ambient light levels: cones mediate photopic vision at high light levels, while rods mediate scotopic vision at low light levels. In mesopic vision at intermediate light levels, both rods and cones are active. Due to the absence of rods in the center of the visual field, the fovea, a foveal scotoma occurs in scotopic vision. Previous research showed that this scotoma can be perceptually filled in using surrounding visual information, and that humans place more trust in such inferred central information than in veridical peripheral input. However, it is unknown whether this phenomenon also occurs in mesopic vision, when cones are still active in the fovea. To this end, we investigated perceptual filling-in of the foveal rod scotoma and confidence judgments under photopic, scotopic, and mesopic conditions. Under mesopic conditions, rods and cones were stimulated independently using a tetrachromatic projector. Participants were asked to discriminate the continuity of two sequentially presented stimuli and to indicate their confidence using a forced-choice task. In foveal rod-isolating conditions, participants could not distinguish between continuous and discontinuous stimuli and showed a bias toward perceiving them as continuous, under both scotopic and mesopic conditions. This indicates that rod-mediated foveal gaps were perceptually filled in by surrounding information, even when cone input was available under the mesopic condition. Furthermore, participants consistently preferred the centrally filled-in information over veridical peripheral information, indicating that the confidence bias toward inferred information within the foveal rod scotoma persists even in mesopic vision.