Two test flashes of 10 minutes of arc are shown 20 ms apart, and two test flashes of again 10 minutes of arc are shown next, this time 120 ms apart. Each emits 100 quanta. Which test flashes are seen as one: the 20 ms interval only, 120 ms interval only, or both?

Two test flashes of 10 minutes of arc are shown 20 ms apart, and two test flashes of again 10 minutes of arc are shown next, this time 120 ms apart. Each emits 100 quanta. Which test flashes are seen as one: the 20 ms interval only, 120 ms interval only, or both?

• A. 20 ms interval only
• B. 120 ms interval only
• C. Both
• D. Neither

Answer: (A)

Temporal integration in the visual system lets the retina sum light energy over a brief time window. If two flashes occur within that window, their signals overlap and you perceive them as a single event; if the gap is longer than the window, the first response decays before the second arrives and you see two separate flashes. In this scenario, the shorter interval lies inside that integration window, so the two flashes effectively combine and are seen as one. The longer interval exceeds the window, so the two flashes are perceived separately. Since both pairs emit the same amount of light, only the short-interval pair merges into one percept while the long-interval pair does not.

Temporal integration in the visual system lets the retina sum light energy over a brief time window. If two flashes occur within that window, their signals overlap and you perceive them as a single event; if the gap is longer than the window, the first response decays before the second arrives and you see two separate flashes.

In this scenario, the shorter interval lies inside that integration window, so the two flashes effectively combine and are seen as one. The longer interval exceeds the window, so the two flashes are perceived separately. Since both pairs emit the same amount of light, only the short-interval pair merges into one percept while the long-interval pair does not.