Which combination defines a photopic ERG?

Which combination defines a photopic ERG?

• A. Rods with dark adaptation and a weak flash
• B. Cones with light adaptation and a strong flash
• C. Rods with light adaptation and a strong flash
• D. Cones with dark adaptation and a weak flash

Answer: (B)

Photopic ERG measures the cone-driven electrical response under bright conditions. To isolate cones, you first light-adapt the eye to saturate the rod system so rods don’t contribute, and then deliver a strong flash that reliably activates cones. This setup—cones with light adaptation and a strong flash—yields a waveform dominated by cone activity, which is exactly what a photopic ERG is designed to assess. If you were dark-adapted, you’d be testing rod function (scotopic ERG). A weak flash wouldn’t reliably evoke a cone-dominated response and could still involve some rod activity, muddying the cone-specific signal.

Photopic ERG measures the cone-driven electrical response under bright conditions. To isolate cones, you first light-adapt the eye to saturate the rod system so rods don’t contribute, and then deliver a strong flash that reliably activates cones. This setup—cones with light adaptation and a strong flash—yields a waveform dominated by cone activity, which is exactly what a photopic ERG is designed to assess.

If you were dark-adapted, you’d be testing rod function (scotopic ERG). A weak flash wouldn’t reliably evoke a cone-dominated response and could still involve some rod activity, muddying the cone-specific signal.