Which photoreceptors mediate scotopic vision and which mediate photopic vision?

• A. Cones mediate scotopic vision; rods mediate photopic vision.
• B. Rods mediate photopic vision; cones mediate scotopic vision.
• C. Rods mediate both scotopic and photopic vision.
• D. Rods mediate scotopic vision; cones mediate photopic vision.

Answer: (C)

Under low light, vision relies on rods because they are highly sensitive to light, can respond to single photons, and provide motion detection but not color or fine detail. In bright light, cones take over, granting color vision and high acuity, while rods become less active or saturate. So, rods mediate scotopic vision, and cones mediate photopic vision. Note that in intermediate lighting (mesopic) both contribute, but photopic vision is defined by cone activity and scotopic by rod activity.