Dynamic properties of vision-V: perception lag and reaction time in relation to flicker and flash thresholds

In order to relate perception lag to flicker thresholds, both were measured by the same subjects under the same conditions. In a preliminary series of experiments, perception lag obtained by the three methods, double flash, eye and ear, and reaction time, were compared for foveal flashes at a dark background. The course of perceptive delay was measured as a function of the intensity of long flashes over a large intensity range. At medium intensities the results of the three methods agree. At low intensities, decreasing the intensity towards the threshold increases reaction time progressively faster than double-flash settings in the first decade above the threshold. The eye-and-ear method was not found feasible in this range. At high intensities the methods cannot be proved to give different results but the intensities are limited by disturbance of the adaptive state caused by the test flashes themselves. In a second series of experiments, reaction times and double-flash settings at a low and a high background level were compared. The flicker fusion characteristics of the same stimuli were determined by modulating them harmonically. The change of perception lag at the low background level was considerably greater than that at the high level on varying the flash intensity over a comparable range. The difference between the results of the two methods near the threshold is even more obvious with a background. Visual latency and flicker thresholds have been related on the basis of three hypothetical general system properties. The course of perception lag obtained by double-flash settings is in agreement with that calculated from the flicker fusion curve. Latency curves at different background levels can be found from a generalized function and two parameters which are characteristic for sensitivity and inertia and which can be found either from flicker or flash thresholds.