CONTRAST AFFECTS FLICKER AND SPEED PERCEPTION DIFFERENTLY

We have previously shown that. contrast affects speed perception, with lower-contrast, drifting gratings perceived as moving slower. In a re cent study, we examined the implications of this result on models of s peed perception that use the amplitude of the response of linear spati o-temporal filters to determine speed. In this study, we investigate w hether the contrast dependence of speed can be understood within the c ontext of models in which speed estimation is made using the temporal frequency of the response of linear spatio-temporal filters. We measur ed the effect of contrast on flicker perception and found that contras t manipulations produce opposite effects on perceived drift rate and p erceived flicker rate, i.e., reducing contrast increases the apparent temporal frequency of counterphase modulated gratings. This finding ar gues that, if a temporal frequency-based algorithm underlies speed per ception, either flicker and speed perception must not be based on the output of the same mechanism or contrast effects on perceived spatial frequency reconcile the disparate effects observed for perceived tempo ral frequency and speed. (C) 1997 Elsevier Science Ltd.