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 sti mulus evidence over time, for example, random walk models. This class of mo dels is tested in a luminance discrimination paradigm in which the onsets o f the stimuli are either instantaneous (stepped) or slowly ramped. The ramp ed portion of ramped stimuli occurs prior to the stepped stimuli onsets. Co nsequently, there is more luminance energy in ramped stimuli. Therefore, if participants integrate luminance energy, they should perform better to ram ped stimuli. This did not occur in 4 experiments. Participants performed be tter 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 lum inance, but they do so in a serial fashion. First, participants monitor tem poral changes in luminance; later, they monitor integrated luminance energy .