During measurement of temporal summation, how long is the time window within which quanta must be summed together?

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

Answer: (C)

Temporal sum depends on how long the photoreceptor’s response to a photon lasts. In vision, a photon’s effect on a photoreceptor persists long enough that subsequent photons arriving within about 100 milliseconds can add to the same response. This ~100 ms window is set by the photoreceptor’s membrane time constant and the phototransduction cascade, so quanta arriving within this interval combine to push the cell toward signaling. If photons arrive sooner than this window, they sum; if they come later, the earlier response has largely decayed and they don’t contribute as effectively, reducing summation. That’s why roughly 100 milliseconds is the typical integration window for temporal summation.