Perception of biological motion without local image motion

Abstract

A vivid perception of the moving form of a human figure can be obtained from a few moving light points on the joints of the body. This is known as biological motion perception. It is commonly believed that the perception of biological motion rests on image motion signals. Curiously, however, some patients with lesions to motion processing areas of the dorsal stream are severely impaired in image motion perception but can easily perceive biological motion. Here we describe a biological motion stimulus based on a limited lifetime technique that tests the perception of a moving human figure in the absence of local image motion. We find that subjects can spontaneously recognize a moving human figure in displays without local image motion. Their performance is very similar to that for classic point-light displays. We also find that tasks involving the discrimination of walking direction or the coherence of a walking figure can be performed in the absence of image motion. Thus, although image motion may generally aid processes such as segmenting figure from background, we propose that it is not the basis for the precept of biological motion. Rather, we suggest biological motion is derived from dynamic form information on body posture evolving over time.

Figure 1

Standard biological motion stimuli (a) consist of a frame animation of the motion of light points attached to the joints of a moving human figure (7). In our sequential position stimulus (b) light points were positioned anywhere on the limbs and jumped to another randomly selected position for each frame.

Figure 2

Percentage of subjects that recognized a walking human figure from the classic point-light walker, the sequential position (SP) walker (8p), the sequential position walker with four points per frame (4p), and an upside-down display of the sequential position walker (UD). n indicates the number of subjects for that experiment.

Figure 3

Rate of correctly discriminated walking direction as function of lifetime (in 52-ms long increments) for different number of stimulus points. Data are collapsed over walking direction and represent the average over three subjects. Error bars are ± 1 SE.

Figure 4

Noise tolerance as function of lifetime for direction discrimination of the sequential position walker. Noise tolerance is expressed as the number of noise dots at which 70.7% correct is reached. Data are collapsed over walking direction and represent the average over 3 subjects. Error bars are ± 1 SE.