Frequencies and latencies in detecting two-flash stimuli

Dark-adapted Ss responded as soon as they detected near-threshold stimuli, which usually comprised two flashes that were equal in luminance and duration. The percentage of trials on which the S responded and the median latency of responses were determined as functions of the interval between flash onsets. At short intervals: the percentage was independent of the interval and attributable to the stimulus energy alone (Bloch’s law); the latency increased gradually to the value for the first flash alone and was attributable to not only the energy, but also to its temporal distribution. At long intervals: the percentage decreased to a steady level that was either equal to or higher than the value for the first flash; the latency increased to a peak and then decreased to the value for the first flash; both response measures suggested inhibition and probability summation between the flashes.