Which procedure measures temporal summation psychophysically?

• A. Bright adaptation the subject, then present a single flash of fixed size
• B. Present circular test flashes of various sizes for a fixed duration and measure the number of quanta needed for detection
• C. Use a single flash of fixed size and duration
• D. Dark adapt the subject, present circular test flashes of various sizes for a fixed duration, measure number of quanta needed for detection

Answer: (D)

Temporal summation reflects how the visual system pools light energy over a brief time window to reach detection. Psychophysically, you probe this by examining how the amount of energy (quanta) needed for a detection response changes when the stimulus is delivered within a fixed, controlled time window, ideally after placing the eye in a dark-adapted state so the measurements reflect the intrinsic temporal integration of the retina.

The chosen approach does exactly that: you dark-adapt the subject to boost sensitivity, present circular flashes of different sizes but with a fixed duration, and measure how many quanta are required for detection. Keeping the duration constant means any changes in the threshold energy as you vary the stimulus size reveal how photons are integrated over that fixed time period across the retina. This gives a psychophysical readout of temporal summation because you’re quantifying how energy within a single temporal window is pooled to produce a percept.

The other options don’t isolate this temporal aspect as cleanly. One flash with fixed size doesn’t map out how energy accumulates over time; varying size with a fixed duration probes spatial (not temporal) summation, and the lack of duration variation in most choices makes it hard to characterize how detection depends on time.