The critical area for spatial summation increases with retinal eccentricity for which photoreceptor types?

• A. Only rods
• B. Only cones
• C. Both rods and cones
• D. Neither

Answer: (C)

Spatial summation is about how many photoreceptors contribute to driving a single downstream neuron. Near the fovea, cones feed into certain ganglion cells with little pooling, giving small receptive fields and high spatial precision. As you move toward the periphery, more photoreceptors converge onto the same downstream cells, so the receptive fields enlarge. This increase in pooling with eccentricity happens for both cone and rod pathways: cones still undergo more convergence away from the center, and rods—abundant in the peripheral retina—have strong pooling into their downstream circuits as you go outward. So the smallest retinal area needed to elicit a response grows with eccentricity for both types, making the best answer that both photoreceptor types show this pattern.