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Abstract

We measured perceived speed as a function of contrast for luminance and isoluminant sinusoidal gratings, luminance and isoluminant plaids, and second order amplitude- modulated drift-balanced stimuli. For all types of stimuli perceived speed was contrast- invariant for fast moving patterns at or above 4 Hz. For slowly moving stimuli the log of perceived speed was a linear function of the log of the contrast. The slope of this speed- versus-contrast line (velocity gain) was relatively shallow for luminance gratings and luminance plaids, but was steep for isoluminant gratings and isoluminant plaids, as well as for the drift-balanced stimuli. Independent variation of spatial and temporal frequency showed that these variables, and not velocity, determine the velocity gain. Over all the results indicate that slow moving stimuli defined by chromaticity or by second-order statistics are processed in different manner from luminance defined stimuli. We propose that there are a number of independent mechanisms processing motion targets and it is the interplay of these mechanisms that is responsible for the final percept.