The second principle is that when presented with ambiguous dynamic stimuli, our perception generally corresponds to the slowest motion compatible with the stimulus. Displacements below this limit (whose magnitude may depend on the texture) yield veridical motion percepts, whereas displacements larger than this limit lead to a loss of coherent motion perception. First, when a random texture is displaced, there is an upper limit, denoted d max, to the step size that leads to perception of coherent, rigid motion ( 1– 4). Several decades of research on the perception of motion have revealed two fundamental principles-the loss of coherent motion perception above an upper limit and the preference for minimal motion-both of which we challenge here. In summary, across a large variety of stimuli, we find that when incoherent motion noise is preceded by a small bias, instead of perceiving little or no motion-as suggested by the minimal-motion principle-observers perceive jumps whose amplitude closely follows their own d max limits. In addition, when a texture is actually displaced by a large step beyond the upper step size limit of d max, a breakdown of coherent motion perception is expected however, in the presence of an inducer, observers again perceive coherent displacements at or just above d max. Jump magnitude is robust across jump directions and different types of transient. Our experiments with transients, such as texture randomization or contrast reversal, show that the magnitude of the jump depends on spatial frequency and transient duration-but not on the speed of the inducing motion signals-and the direction of the jump depends on the duration of the inducer. This visually striking effect, which we call “high phi,” challenges well-entrenched assumptions about the perception of motion, namely the minimal-motion principle and the breakdown of coherent motion perception with steps above an upper limit called d max. When human observers are exposed to even slight motion signals followed by brief visual transients-stimuli containing no detectable coherent motion signals-they perceive large and salient illusory jumps.
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