Why do larger retinal receptive fields have lower thresholds for weak light?

Larger retinal receptive fields have lower thresholds for weak light because of the convergence of multiple photoreceptors onto single bipolar and ganglion cells. This anatomical arrangement allows for spatial summation, where the electrical effects of light stimuli falling on many individual rods accumulate at the ganglion cell.

Because these effects are cumulative, a weak light stimulus spread across a large number of rods can be as effective at triggering a response as a stronger stimulus concentrated on a smaller area. Consequently, the threshold luminance required to evoke a visual sensation is lower for larger receptive fields. This process is particularly significant in the retina outside the fovea, where high densities of rods converge to enhance sensitivity to dim light.12