Under photopic conditions, with the same setup of two simultaneous flashes delivering 5 quanta each, how many lights are perceived?

• A. One light
• B. Four lights
• C. Two lights
• D. No light

Answer: (C)

Under bright, cone-dominated vision you have good spatial resolution, so the visual system can distinguish separate light sources. Each flash delivers about 5 quanta, which is enough for the cones to detect and produce a percept. Since there are two simultaneous flashes from distinct locations, your brain can localize and count them as two separate lights even though they arrive at the same moment. If the two flashes were from the exact same spot, they might fuse into a single percept, but with two distinct sources you perceive two lights.