The effect of intermittent illumination on the visual acuity threshold

Senders (1949) reported that with intermittent illumination short flashes needed less energy than larger ones to make a visual acuity target visible. This apparent breakdown of the reciprocal relationship between time and intensity, which has been found almost universally by earlier workers under a very wide range of experimental conditions, raised a number of interesting theoretical issues such as the relationship between visual acuity and intensity discrimination thresholds, and the plausible importance of eye movements in each. Unfortunately we have been unable to repeat Senders' result in spite of the most determined efforts to reproduce her experimental conditions. We are unable to suggest why our results differ from hers, but feel that our experiments at least show that her data are very difficult to reproduce and hence no general theoretical argument can be based on them.

In view of this we felt at liberty to predict results on the basis of a model, we had tentatively constructed, which we regarded as integrating Bloch's and Talbot's laws. This enabled us to make predictions about the effect of frequency of repetitive stimulation on thresholds. Experiments were carried out to ascertain how far these predictions were substantiated. Agreement was remarkably good.