Necessary but not sufficient: motion perception is required for perceiving biological motion.

Researchers have argued that biological motion perception from point-light animations is resolved from stationary form information. To determine whether motion is required for biological motion perception, we measured discrimination thresholds at isoluminance. Whereas simple direction discriminations falter at isoluminance, biological motion perception fails entirely. However, when performance is measured as a function of contrast, it is apparent that biological motion is contrast-dependent, while direction discriminations are contrast invariant. Our results are evidence that biological motion perception requires intact motion perception, but is also mediated by a secondary mechanism that may be the integration of form and motion, or the computation of higher-order motion cues.