In photopic vision, less spatial summation leads to which effect on sensitivity?

• A. Increases sensitivity
• B. Reduces sensitivity
• C. No change in sensitivity
• D. Varies with lighting

Answer: (C)

In photopic vision, the visual system relies on cones with very small receptive fields to keep high spatial resolution. Spatial summation means pooling signals from multiple receptors to boost detectability, which increases sensitivity but reduces detail. In bright light, cones are already operating with minimal pooling, so the detectability of a given stimulus isn’t driven by how many receptors contribute to the signal. Reducing pooling further doesn’t significantly change the signal-to-noise balance for typical photopic stimuli, so sensitivity stays essentially the same. This contrasts with scotopic (rod) vision, where larger pooling markedly boosts sensitivity at the cost of acuity.