As the time between test flashes increases, the second signal can arrive up to how many msec later to still be summated?

• A. 50 ms
• B. 100 ms
• C. 150 ms
• D. 200 ms

Answer: (B)

Temporal summation in vision depends on how long the photoreceptor and its downstream neurons keep integrating light. If a second flash arrives while the first response is still elevated, their effects add together, producing a stronger overall signal. The practical integration window for cones, which handle brighter, color vision, is about 100 milliseconds. Within this window, a second flash can arrive up to roughly 100 ms after the first and still be incorporated into the same response. If the interval is longer than about 100 ms, the first response has largely returned toward baseline, so the two flashes no longer sum effectively. So, the second signal can arrive up to about 100 ms later and still be summated. Shorter intervals like 50 ms would also sum, but they’re not the maximum; longer intervals like 150 or 200 ms would generally not sum.