Which photoreceptors are primarily responsible for detecting low light levels?

• A. Cones
• B. Bipolar cells
• C. Ganglion cells
• D. Rods

Answer: (D)

Rods are the photoreceptors that handle vision in low-light conditions. They are extremely sensitive and use a photopigment called rhodopsin that can respond to very dim light, even single photons. Many rods connect to a single bipolar cell, which amplifies the signal and makes the whole system highly sensitive, though at the expense of fine spatial detail and color information. This design lets you detect shapes and movement in the dark, especially in the peripheral vision where rods are more abundant. Cones, by contrast, require brighter light and provide color vision and sharp detail, which is why color and high-acuity vision rely on them. Bipolar and ganglion cells aren’t photoreceptors themselves; they’re the downstream neurons that pass along the rod or cone signals to the brain.