PERCEIVED VELOCITY OF LUMINANCE, CHROMATIC AND NON-FOURIER STIMULI - INFLUENCE OF CONTRAST AND TEMPORAL FREQUENCY

We measured perceived velocity as a function of contrast for luminance and isoluminant sinusoidal gratings, luminance and isoluminant plaids , and second-order, amplitude-modulated, drift-balanced stimuli, For a il types of stimuli perceived velocity was contrast-invariant for fast moving patterns at or above 4 deg/sec. For slowly moving stimuli the log of perceived velocity was a linear function of the log of the cont rast, The slope of this perceived velocity-vs-contrast line (velocity gain) was relatively shallow for luminance gratings and luminance plai ds, but was steep for isoluminant gratings and isoluminant plaids, as well as for drift-balanced stimuli, Independent variation of spatial a nd temporal frequency showed that these variables, and not velocity al one, determine the velocity gain, Overall, the results indicate that s low moving stimuli defined by chromaticity or by second-order statisti cs are processed in a different manner from luminance defined stimuli, We propose that there are a number of independent mechanisms processi ng motion targets and it is the interplay of these mechanisms that is responsible for the final percept.