The Changing of Perceived Speed as a Function of Stimulus Contrast: An Attempted Replication with a Variety of Stimuli

It has been reported that the perceived speed of a target stimulus varies with its physical contrast (Thompson, 1982 Vision Research 22 377 – 380). Yet the effect is somewhat ephemeral with intersubject variations, effects of presentation mode, and test speed. So far this effect has only been reported for grating stimuli and it is unclear whether the effect generalises to other stimuli.

Subjects reported which of two patterns appeared to move more quickly. Using a forced-choice method we compared performance with three different stimuli: simple gratings in linear motion, radial gratings in rotary motion, and Julesz random-dot patterns in linear motion. We further compared simultaneous and successive presentations.

For grating stimuli at slow speeds we obtained an effect of contrast which was greater for simultaneous presentations, thus replicating Stone and Thompson's results (1992 Vision Research 32 1535 – 1549). However, at similar temporal frequencies there is no reliable effect of contrast for the radial gratings. With the random-dot patterns we generally found little effect of contrast; however, we did obtain a small effect at the slowest speeds, but only when presenting the two patterns simultaneously. Our results suggest that the contrast effect on perceived speed does not readily generalise to all stimuli. We discuss the implications of these results in terms of attentional tracking mechanisms and models of motion perception.