The critical area for spatial summation is larger for which photoreceptor type?

• A. Cones
• B. Rods
• C. Both
• D. None

Answer: (B)

Spatial summation depends on how many photoreceptors feed into the same downstream neuron, which is tied to the size of the receptive field created by convergence. Rods in the peripheral retina converge many-to-one, forming large receptive fields, so light from a relatively large patch can sum together to elicit a response. This means the area over which light can be integrated—the critical area for spatial summation—is large for rods. In contrast, cones, especially in the fovea, have little convergence and very small receptive fields, so they support high spatial resolution with a much smaller integration area. Overall, the ability for light to sum over a broad area is greater with rods, making the critical area for spatial summation larger for rods.