Figure 2.

Perceptual contrast effects. A, Psychometric curves from a single human subject comparing a reference speed of 20°/s with a range of test speeds (horizontal axis) at four levels of contrast (color of the data point/curve; see legend). The point at which the psychometric curves cross the horizontal dashed line (Test Faster is 50%) defines the PSE for that contrast. The solid curves are cumulative Gaussians that were fitted to the data points (see Materials and Methods). B, Average data from human subjects (n = 6) illustrating the change in PSE with contrast for various speeds. All PSE changes are expressed as a ratio of perceived to reference speed. The thick gray line represents a linear regression to the data averaged over all reference speeds. Error bars represent the SE in the population mean. C, Psychometric curve from a single macaque subject (monkey M) comparing a reference speed of 16°/s to a range of test speeds. The squares represent comparisons between equal (70%) contrast stimuli. The open circle shows the percentage of trials (recorded over multiple days; n = 1100) in which a 20% contrast reference stimulus was reported to be faster than the 70% test stimulus moving at the physically identical speed. Error bars show 95% confidence intervals.