Assessing the roles of change discrimination and luminance integration: evidence for a hybrid race model of perceptual decision making in luminance discrimination.

Many models of perceptual processing assume that participants integrate stimulus evidence over time, for example, random walk models. This class of models is tested in a luminance discrimination paradigm in which the onsets of the stimuli are either instantaneous (stepped) or slowly ramped. The ramped portion of ramped stimuli occurs prior to the stepped stimuli onsets. Consequently, there is more luminance energy in ramped stimuli. Therefore, if participants integrate luminance energy, they should perform better to ramped stimuli. This did not occur in 4 experiments. Participants performed better to stepped stimuli than ramped stimuli in earlier foreperiods and the reverse in later foreperiods. A new model is proposed in which participants monitor both integrated luminance energy and quick temporal changes in luminance, but they do so in a serial fashion. First, participants monitor temporal changes in luminance; later, they monitor integrated luminance energy.