Under scotopic conditions, two simultaneous flashes occur at two locations, and each flash results in 5 quanta absorbed by photoreceptors. How many lights are perceived?

Under scotopic conditions, two simultaneous flashes occur at two locations, and each flash results in 5 quanta absorbed by photoreceptors. How many lights are perceived?

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

Answer: (A)

Under scotopic conditions the visual system relies on rods, which are extremely sensitive but have low spatial resolution because many rods connect to the same downstream neurons. This causes signals from a broad area to be pooled as a single input to the brain. When two simultaneous flashes occur at different locations, each delivering 5 quanta to the photoreceptors, the rod signals from those regions are summed within the same neural pathways. The brain cannot separate these two inputs into two distinct percepts, so the flashes fuse into a single perceived light. In low light, sensitivity is prioritized over resolving detail, so two light sources are not usually seen as two separate lights.

Under scotopic conditions the visual system relies on rods, which are extremely sensitive but have low spatial resolution because many rods connect to the same downstream neurons. This causes signals from a broad area to be pooled as a single input to the brain. When two simultaneous flashes occur at different locations, each delivering 5 quanta to the photoreceptors, the rod signals from those regions are summed within the same neural pathways. The brain cannot separate these two inputs into two distinct percepts, so the flashes fuse into a single perceived light. In low light, sensitivity is prioritized over resolving detail, so two light sources are not usually seen as two separate lights.