Michaela Klimova¹, MiYoung Kwon¹; ¹Northeastern University

Despite the prevalence of mesopic (dim light) environments, their effects on human visual processing remain poorly understood, as most vision research and assessments are conducted under idealized photopic (bright) conditions. Animal studies suggest that mesopic conditions enlarge retinal ganglion cell receptive fields and reduce surround antagonism, thereby enhancing light sensitivity but compromising spatial resolution. However, how mesopic conditions affect human cortical spatial integration remains unclear. To address this, we investigated cortical population receptive fields (pRF) and surround suppression, alongside behavioral measures of surround suppression, under mesopic and photopic conditions. We began by acquiring fMRI BOLD responses from visual areas V1-V3 in 11 normally sighted observers. In Exp. 1, we estimated pRF properties with standard stimuli and procedures, while Exp. 2 measured cortical surround suppression using high-contrast center and surround grating stimuli. We also obtained full contrast sensitivity functions (CSFs) with and without surround suppression from a subset of observers. CSFs were fit using a difference of Gaussians model, enabling a psychophysical comparison to the fMRI-derived pRF size and shape estimates. To further link cortical measures with perceptual spatial resolution, we assessed mesopic and photopic visual acuity in all observers. Our results reveal that, mesopic conditions, contrary to findings in animal retinal studies, were associated with smaller pRF sizes across early visual cortical areas (p < 0.01). Surround suppression remained robust, with no significant differences in suppression strength between mesopic and photopic conditions at the group level. Interestingly, observers with greater reductions in mesopic visual acuity exhibited larger decreases in V1 surround suppression, highlighting the critical role of surround suppression in contrast coding and spatial resolution. Diverging from retinal electrophysiology studies, our findings suggest that the human early visual cortex may employ compensatory mechanisms to maintain spatial resolution in response to degraded retinal input under mesopic conditions.