If an ERG shows activity from both rods and cones, which stimulus condition is likely?

• A. Strong flash to dark-adapted eye
• B. Weak flash to light-adapted eye
• C. Flicker in light adaptation
• D. No adaptation

Answer: (A)

In an ERG, rod and cone systems are driven differently depending on how the eye is prepared (adapted) and how strong the light stimulus is. When the eye is dark-adapted, rods are highly sensitive and ready to respond to light. If you deliver a strong flash, the light is bright enough to also activate the cone pathways, which normally require more light to respond. So a strong flash to a dark-adapted eye engages both photoreceptor types, producing an ERG that shows activity from rods and cones together.

This is less likely with a weak flash in a light-adapted state, where you’d mainly get cone activity. Flicker in light adaptation emphasizes cone function, and no adaptation wouldn’t reliably yield a robust mixed rod–cone response.