Biological movements look uniform: Evidence of motor-perceptual interactions.

Six experiments demonstrate a visual dynamic illusion.Previous work has shown that in 2dimensional (2D) drawing movements, tangential velocity and radius of curvature covary in a constrained manner.The velocity of point stimuli is perceived as uniform if and only if this biological constraint is satisfied.The illusion is conspicuous: The variations of velocity in the stimuli exceed 200%.Yet movements are perceived as uniform.Conversely, 2D stimuli moving at constant velocity are perceived as strongly nonuniform.The illusion is robust: Exposure to true constant velocity fails to suppress it.Results cannot be explained entirely by the kinetic depth effect.The illusion is evidence of a coupling between motor and perceptual processes: Even in the absence of any intention to perform a movement, certain properties of the motor system implicitly influence perceptual interpretation of the visual stimulus.