Which photoreceptors have the higher threshold for detecting light?

• A. Rods
• B. Rods and cones
• C. Ganglion cells
• D. Cones

Answer: (D)

Cones have the higher threshold for detecting light. The idea is that different photoreceptors respond to different light levels: rods are extremely sensitive and can respond to very dim light, even a single photon, which means they have a very low detection threshold and drive vision in low-light conditions. Cones require more light to reach their activation threshold, so they operate best in bright, daytime conditions and are responsible for color vision and fine detail. Ganglion cells aren’t photoreceptors themselves; they’re downstream neurons that respond to the input from photoreceptors after transduction. So the photoreceptors that need more light to respond are cones.