Point-light stimuli (an example is shown on the left) were first created by Johansson in 1973 by filming in the dark people with lights attached to the major joints. When subjects are shown a single snapshot of the resulting movie they cannot recognize a human figure, but as soon as the movie is being played then all the moving dots are instantaneously and robustly organized into a coherent and vivid percept. The main question is how can we have such a vivid perception despite the impoverishment of the stimulus.


Two complementary theories have been proposed.

(1) Perception relies on form information, accumulated over time, possibly by fitting a form template to the stimulus (as an example a sketched human body is fit to the point-light stimulus shown above).
(2) Perception is mainly elicited by local motion information. In particular perception is vivid because optic flow associated with point-light stimuli is similar to the optic flow of natural stimuli experienced in everyday life.


Evidence supporting form-based theories.

Evidence supporting form-based theories was recently presented by Beintema and Lappe (PNAS, 2002). In their paper they have proposed a novel point-light stimulus (Sequential Position Walker, SPS, shown on the left) in which they impoverished motion information by randomly displacing the dots along the limbs. Since subjects could reliably recognize the direction of walking of such impoverished stimuli it has been concluded that subjects could only rely on for information (possibly by fitting a form template as shown on the right).


... is really form information driving the perception in the case of the SPS?

We propose that actually what elicits the perception in the SPS is motion and not form information. To prove this we created a novel point-light stimulus (Critical Features Stimulus, CFS, shown on the left). The stimlus is composed of 4 regions. In two of them (roughly corresponding to the "head" and "body" in a point-light stimulus) dots move in a completely random way, while in the regions corresponding to the "hands" and "feet", dots have a sinusoidal motion along the x axis, but a complete random motion along the y axis. Surprisingly you can still perceive a person walking, although matching a form template is in this case strongly impaired (example shown on the right)


Psychophysical Results

Methodology
Recognition percentages
Response Time
  • Subjects were presented either the SPS or the CFS and they had to judge the walking direction.
  • Subjects were uniformed about which of the two stimuli was being presented
  • No time constraints to answer

Conclusions

  • Perception can be elicited by stimuli that violate strongly the kinematics of the human body
  • Motion information is most important
  • Only coarse form information needed


    • Modeling Results

    Methodology
    Recognition percentages
    Response Time
    • Physiologically plausible model of the ventral (form) and dorsal (motion) pathway.
    • Different degrees of expertise were modeled by training the model with either full body (naive subjects) and point-light (expert subjects) stimuli

    Conclusions

  • Recognition of the direction of walking of the SPS mainly obtained using the motion pathway.
  • Only coarse form information is necessary.
  • A possible cue might be opponent motion in correspondence of the "hands" and "feet" regions.


    • Bibliography

    Casile, A., Giese, M.A. (2005) Critical Features for the recognition of biological motion. Journal of Vision 5(4), 348-360, [PDF]

    Casile A, Giese M (2003): Roles of motion and form in biological motion recognition. In: O. Kaynak, E. Alpaydin, E. Oja, L. Xu (eds.) Artificial Networks and Neural Information Processing.Lecture Notes in Computer Science 2714, pp. 854-862.[PDF].

    Giese M A, Poggio T (2003): Neural mechanisms for the recognition of biological movements and action. Nature Reviews Neuroscience 4, 179-192..