Which phenomenon describes how the sensitivity of the retina changes with the entry location of light at the pupil, due to directional sensitivity of cones?

Which phenomenon describes how the sensitivity of the retina changes with the entry location of light at the pupil, due to directional sensitivity of cones?

• A. Stiles Crawford effect
• B. Purkinje shift
• C. Photochromatic interval
• D. Rod-cone break

Answer: (A)

The idea being tested is that cones have directional sensitivity, so where light enters the pupil changes how strongly the retina is stimulated. Cone outer segments are oriented toward the fovea and act a bit like waveguides. Light that comes in near the center of the pupil travels along favorable angles and is absorbed more efficiently by the cones, making the stimulus appear brighter. Light entering toward the edge hits the cones at less optimal angles, producing a weaker response for the same physical brightness. This angular dependence of cone stimulation is known as the Stiles-Crawford effect. It’s most noticeable under bright, cone-dominated (photopic) conditions and helps explain why a small light spot can look brighter or dimmer depending on its entry location at the pupil. The other options describe different phenomena—spectral shifts with lighting adaptation, changes in sensitivity across the rod–cone transition, or other chromatic effects—not the directional, pupil-entry dependence produced by cone geometry.

The idea being tested is that cones have directional sensitivity, so where light enters the pupil changes how strongly the retina is stimulated. Cone outer segments are oriented toward the fovea and act a bit like waveguides. Light that comes in near the center of the pupil travels along favorable angles and is absorbed more efficiently by the cones, making the stimulus appear brighter. Light entering toward the edge hits the cones at less optimal angles, producing a weaker response for the same physical brightness. This angular dependence of cone stimulation is known as the Stiles-Crawford effect. It’s most noticeable under bright, cone-dominated (photopic) conditions and helps explain why a small light spot can look brighter or dimmer depending on its entry location at the pupil. The other options describe different phenomena—spectral shifts with lighting adaptation, changes in sensitivity across the rod–cone transition, or other chromatic effects—not the directional, pupil-entry dependence produced by cone geometry.