The delayed rod afterimage

A flashed background, presented to a dark-adapted eye, can saturate the rod system, making an incremental test patch invisible. But as the afterimage decays, the test can be distinguished. Increment thresholds measured within the decaying afterimage exhibit Weber's law over a wide range. The Penn and Hagins model of rod kinetics correctly predicts Weber's law, but makes incorrect predictions of the latency for the detection to occur. A new model, involving two exponential decays, is able to accommodate the latency data, as well as Weber's law. The model also makes good predictions of the results when the stimulus duration is increased from 100 msec to 1 sec.