In the described psychophysical experiment, what is the primary measurement used to determine the threshold for temporal summation?

• A. Number of quanta
• B. Duration of stimulus
• C. Size of stimulus
• D. Color of stimulus

Answer: (A)

Temporal summation in vision relies on how the visual system pools light over a brief moment. The key measurement for the threshold in this kind of psychophysical experiment is the total number of photons, or quanta, arriving during the integration window. Detection depends on this photon count, not on how long the flash lasts by itself, so the threshold is expressed in quanta. In practice, researchers vary duration and intensity to see how many quanta are needed for detection, illustrating that fewer photons can suffice if spread over a longer time to reach the same total, and more photons are needed if the stimulus is shorter. Color or size can affect sensitivity in other contexts, but they don’t define the threshold for temporal summation as directly as photon count does.