At lower speeds, an object moving through a background with less contrast will appear slower. At around 8 Hz, percieved speed is not affected by contrast. And at higher speeds, reducing contrast will result in overestimation of speed. That is, the perceieved speed of the object will increase.
That’s the thrust of the research by Thompson et al., who also present a model to explain their findings. In opposition to the two Bayesian models suggested by earlier researchers, theirs is a “very simple” ratio model, which includes only two parameters. We are assured that these parameters are physiologically plausible.
The actual vision effect is interesting and deserves further study. Why does this contrast not effect perceieved speed at 8 Hz? What’s so special about that frequency?
P. Thompson, K. Brooks, S. T. Hammett, Speed can go up as well as down at low contrast: Implications for models of motion perception, Vision Research 46 (2006), pp. 782-786.