Mesopic Luminance Inflates Crowding in a Pattern Consistent With Rod-to-Cone Ratio Across the Visual Field

Purpose: Visual crowding, the inability to recognize objects amid clutter, is a fundamental bottleneck for peripheral vision. Its spatial extent, measured as critical spacing, increases with eccentricity and varies across the visual field, mirroring the topography of ganglion cell density. Yet, it remains unclear whether the spatial extent of crowding changes under mesopic luminance and whether any such change covaries with local rod prevalence, given that rod pathways are more highly convergent than cone pathways. Here, we examined the effect of mesopic luminance on the spatial extent of crowding. Methods: Twelve young adults performed a letter-identification task with and without flankers at three eccentricities (4°, 8°, and 12°) along four cardinal meridians under photopic and mesopic luminance. Critical spacing, defined as the center-to-center distance between target and flankers at 79.4% identification accuracy, was measured using an adaptive staircase. Results: Mesopic luminance increased crowding in a manner dependent on both eccentricity and visual field location. Critical spacing was larger under mesopic than photopic conditions, with the effect growing with eccentricity and being most pronounced along vertical meridians. This pattern was consistent with higher rod-to-cone ratios with larger eccentricities and along vertical meridians, based on human histology. Single-letter recognition at the same locations remained above 90% and did not differ across luminance conditions. Across the 12 locations, mesopic inflation covaried with the distribution of rod-to-cone ratios (R = 0.95, P < 0.001). Conclusions: These findings suggest that the spatial pattern of crowding under mesopic luminance is consistent with early sensory constraints, although the observed relationship remains correlational.