Academic journal article Perception and Psychophysics

Person Identification from Biological Motion: Effects of Structural and Kinematic Cues

Academic journal article Perception and Psychophysics

Person Identification from Biological Motion: Effects of Structural and Kinematic Cues

Article excerpt

Human observers are able to identify a person based on his or her gait. However, little is known about the underlying mechanisms and the kind of information used to accomplish such a task. In this study, participants learned to discriminate seven male walkers shown as point-light displays from frontal, half-profile, or profile view. The displays were gradually normalized with respect to size, shape, and walking frequency, and identification performance was measured. All observers quickly learned to discriminate the walkers, but there was an overall advantage in favor of the frontal view. No effect of size normalization was found, but performance deteriorated when shape or walking frequency was normalized. Presenting the walkers from novel viewpoints resulted in a further decrease in performance. However, even after applying all normalization steps and rotating the walker by 90°, recognition performance was still nearly three times higher than chance level.

Humans, being a highly social species, rely on efficient mechanisms to derive information about individual conspecifics in their immediate environment. We observe what another person is doing, we can attribute biological traits, emotional states, and intentional behavior to an individual, and we can identify other people individually if we have met them before. Socially relevant information is transmitted through many different sensory modalities and perceptual mechanisms, including the way a person moves. In normal situations, we almost never have to rely on animate motion alone, because other sources of information are usually present at the same time. However, if isolated under laboratory conditions the significance of animate motion as an important source of socially relevant information becomes obvious.

The most popular method to achieve this is based on biological motion point-light displays, which were introduced into experimental psychology by Gunnar Johansson more than 30 years ago (Johansson, 1973). Only a small number of dots representing the major joints of a walking person results in a vivid percept of a human body (Johansson, 1976). The use of such point-light walkers showed that besides identifying particular actions (Dittrich, 1993) we can also attribute sex to a walker (Barclay, Cutting, & Kozlowski, 1978; Cutting, 1978; Kozlowski & Cutting, 1977; Mather & Murdoch, 1994; Troje, 2002), and identify individual persons (Cutting & Kozlowski, 1977; Stevenage, Nixon, & Vince, 1999) solely based on their motion patterns. In some cases, only parts of the body seem to be sufficient for us to recognize specific properties of a performed action and the actor (Pollick, Lestou, Ryu, & Sung-Bae, 2002; Pollick, Paterson, Bruderlin, & Sanford, 2001).

Although the impressive performance of the human visual system to effortlessly organize a small number of moving dots into the coherent percept of a person is well documented, the principles underlying information encoding in biological motion and the mechanisms for its retrieval are poorly understood. A number of studies aimed at understanding the perceptually significant differences between male and female walking patterns have been conducted (Cutting, 1978; Mather & Murdoch, 1994; Troje, 2002), but virtually nothing is known about encoding of person identity. Here, we present the results of an experiment that we designed to investigate the role of a number of different stimulus attributes for person identification from biological motion displays.

The first experiment on person identification from point-light displays was conducted by Cutting and Kozlowski ( 1977), who showed that point-light displays are sufficient for the recognition of friends. Stimuli were point-light displays of six walkers moving across a computer screen in the sagittal plane. The observers were familiar with the individuals displayed but not with their point-light depictions. The observers performed rather poorly at the beginning, but even though no feedback was given, they improved to a level of correct responses that was well above chance performance. …

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