Impact of Rod-Dominant Mesopic Conditions on Spatial Summation and Surround Suppression in Early Visual Cortex

Mesopic (dim light) conditions are prevalent in everyday environments, yet most human vision research is conducted under idealized, photopic (bright) conditions. Electrophysiological studies suggest that under mesopic conditions, contrast-encoding retinal ganglion cell receptive fields expand their center width while diminishing surround inhibition. These retinal modifications enhance light capture by increasing the summation area but they limit spatial resolution. However, the impact of mesopic conditions on human cortical spatial integration mechanisms remains unclear. To address this, we investigate how mesopic conditions affect early visuocortical processing, specifically spatial summation and surround suppression. Across two experiments, we acquired fMRI BOLD responses from 11 normally sighted participants of both sexes under photopic and mesopic conditions in visual areas V1-V3. The first experiment estimated population receptive field (pRF) properties while the second experiment assessed cortical surround suppression. Photopic and mesopic psychophysical surround suppression, contrast sensitivity function (CSF), and visual acuity were also measured. At the cortical level, mesopic conditions were associated with smaller pRF sizes, while surround suppression remained robust. At the perceptual level, mesopic conditions led to reduced acuity, lower CSF, and weaker suppression, diverging from the observed cortical effects. Importantly, individual differences linked these findings: participants who exhibited greater mesopic reductions in visual acuity also showed larger decreases in early visuocortical surround suppression, underscoring its role in contrast coding and spatial resolution. Altogether, our fMRI findings contrast with retinal electrophysiology and suggest that early visual cortex may employ distinct, perhaps compensatory, mechanisms in response to reduced retinal input under mesopic conditions.