Balance and imbalance in dark and bright (OFF and ON) visual channels

The visual system uses separate neuron channels to distinguish figures that are darker or brighter than their background. We displayed letters as dark and/or bright pulses, with pulse energy scaled as a function of background energy, and assessed the probability of letter recognition as a function of pulse energy and polarity. Similarity of effect for dark and bright pulses was tested with single pulses and with flicker displays. The role of frequency and polarity within flicker cycles was evaluated with reference to provisions of the Talbot-Plateau law. Some energy ranges of dark and bright energy produced comparable changes in the probability of letter recognition, while others did not. The results also provide insight into what stimulus conditions will produce a previously reported “anomalous contrast” effect.