The visibility of transient changes of luminance.

1. The threshold intensities of various transient changes from a base line luminance in the mid-photopic range have been measured.2. Rectangular changes in luminance show no difference between increments and decrements, a threshold intensity decreasing inversely with durations up to 16 msec, a lesser reduction in threshold up to 64 msec and for longer durations a slight increase in threshold.3. Two brief flashes separated by an interval show incomplete summation at 10 msec, almost no summation at 65 msec and a second reduction in threshold at 100 msec.4. Two brief flashes in opposite directions negate each other at very small intervals but complement each other at longer intervals, maximally at 65 msec.5. A model of the detector system is proposed that passes the stimulus wave form through a band-pass filter and takes a running average of the variance of the filter output. Threshold is reached if this average attains a certain level.6. The model satisfactorily accounts for the observations that any two wave forms occurring together and not exceeding a total of 100 msec in duration will interact so that the relationship between them at the combined threshold will be an ellipse; and that this ellipse will have no obliquity if one wave form is the differential of the other.7. Measurements of the thresholds of two brief flashes separated by various intervals define the attenuation characteristics of the band-pass filter invoked to explain the thresholds of repetitive stimuli.8. The model reconciles the apparent disparity between the thresholds of single flashes of varying duration and pairs of flashes of varying interval.9. The statistical efficiency of such a system is discussed.