Johansson (1973) attached small points of light at the joints of human actors, and filmed them moving about in the dark. Observers viewing the film reported vivid impressions of human figures, even though the images contained just a few isolated bright points. The animation on the right was created using a computer rather than real actors, but the result is similar. Research has shown that the figure can be perceived even when it is masked by irrelevant noise dots. Observers can judge attributes such as the gender and mood of the walker reliably from displays based on real people.
Evidence indicates that the movements of individual dots in the display are important for biological motion perception. But research also indicates that subtle clues about body shape are also extracted from the spatial arrangement of the moving dots, so the effect relies on combining information about movement and form.
See:
Beintema, J. A., & Lappe, M. (2002). Perception of biological motion without local image motion. Proceedings of the National Academy of Sciences USA, 99, 5661–5663.
Johansson, G. (1973). Visual perception of biological motion and a model for its analysis. Perception and Psychophysics, 14, 201–211.
Mather, G., Radford, K., & West, S. (1992). Low-level visual processing of biological motion. Proceedings of the Royal Society of London, B249, 149–155. PDF
Mather G, Murdoch L (1994) Gender discrimination in biological motion displays based on dynamic cues. Proceedings of the Royal Society of London, Series B, 258, 273-279. PDF
Thornton, I. M., Rensink, R. A., & Shiffrar, M. (2002). Active versus passive processing of biological motion. Perception, 31(7), 837-854.
Troje, N. F., & Westhoff, C. (2006). The inversion effect in biological motion perception: Evidence for a “life detector”? Current Biology, 16(8), 821-824.