What photoreceptive state should the subject be in before recording an ERG?

• A. Fully dark adapted
• B. Light adapted
• C. Moderate light adaptation
• D. Unadapted baseline

Answer: (A)

The test hinges on how photoreceptors respond to different lighting states. For ERG recordings that aim to measure rod function (scotopic response), you want the retina in a state of full darkness so rods are highly sensitive and dominate the signal. In darkness, rods recover their sensitivity and respond to very dim stimuli, giving a clean readout of rod activity with minimal cone influence. If the eye were light-adapted, cones would drive the response, masking the rod signals and making the results difícil to interpret as a rod-specific ERG. So the subject should be fully dark adapted before recording, typically after a period of complete darkness (around 20–30 minutes) in a light-tight environment.