The effect of contrast on perceived speed and flicker

Slowly moving low contrast patterns appear to drift more slowly than higher contrast patterns. It has been reported that this effect of contrast is reversed for flickering patterns such that they appear to flicker faster than high contrast patterns. This apparent difference in the effect of contrast on perceived speed and flicker may place important constraints upon models of speed encoding in the human visual system. We have measured perceived speed and flicker over a range of spatial and temporal frequencies. The results indicate that contrast has qualitatively (but not quantitatively) similar effects upon perceived speed and flicker. The results also indicate that the effect of contrast upon perceived speed is likely to be inherited from the effect of contrast upon perceived flicker. These findings allow a relaxation of previous constraints upon models of speed encoding.