Biological movement

Most people are very good at interpreting the movements of other people, and can decide very rapidly whether someone is walking, running, limping, or whatever. How successful would we be at interpreting biological movement if the visual information available to us were greatly reduced? Johansson (1975) addressed this issue by attaching lights to actors' joints (e.g., wrists, knees, ankles). The actors were dressed entirely in black so that only the lights were visible, and they were then filmed as they moved around in the dark (Figure 3.7). Reasonably accurate perception of a moving person could be achieved even with only six lights and a short segment of film. Most observers could describe accurately the posture and movements of the actors, and it almost seemed as if their arms and legs could be seen.

Subsequent research has indicated that observers can make very precise discriminations when viewing point-light displays. Cutting and Kozlowski (1977) found that observers were reasonably good at identifying themselves and others known to them from point-light displays. Kozlowski and Cutting (1978) discovered that observers were correct about 65% of the time when guessing the sex of someone walking. Judgements were better when joints in both the upper and lower body or only the lower body were illuminated, presumably because good judgements depend on some overall bodily feature or features.

Some of the most interesting findings with point-light displays were reported by Runeson and Frykholm (1983). In one experiment, they asked the actors to lift a box weighing four kilograms and to carry it to a table, while trying to give the impression that the box weighed 6.5, 11.5, or 19 kilograms. Observers detected the actors' intentions from the pattern of lights, and so their perception of the weight of the box did not vary across conditions.

In another experiment, Runeson and Frykholm (1983) showed films of actors throwing sandbags to targets at different distances. The observers were good at judging how far the actors had intended to throw the bags, even though there were no lights on the bags. Finally, Runeson and Frykholm (1983) asked the actors to carry out a sequence of actions naturally or as if they were a member of the opposite sex. Observers guessed the gender of the actor correctly 85.5% of the time when he or she acted naturally, and there was only a modest reduction to 75.5% correct in the deception condition.

Theoretical accounts

Does our ability to perceive biological motion accurately involve complex cognitive processes? Much of the evidence suggests that it does not. For example, Fox and McDaniel (1982) presented two different motion displays side by side to infants. One display consisted of dots representing someone running on the spot, and the other showed the same activity but presented upside down. Infants four months of age spent most of their time looking at the display that was the right way up, suggesting that they were able to detect biological motion.

More evidence suggesting that the detection of biological motion occurs straightforwardly was reported by Johansson, von Hofsten, and Jansson (1980). Observers who saw the moving lights for only one-fifth of a second perceived biological movement with no apparent difficulty.

These findings are consistent with Johansson's (1975) view that the ability to perceive biological motion is innate. However, it is clearly possible that four-month-old infants have learned from experience how to perceive biological motion. Runeson and Frykholm (1983) argued for a Gibsonian position, according to which aspects of biological motion provide invariant information. These invariants can be perceived with the impoverished information available from point-light displays, and can be identified even when there are deliberate attempts to deceive observers.

There have been various attempts to identify the invariant or invariants that might be used by observers to make accurate sex judgements. Cutting, Proffitt, and Kozlowski (1978) pointed out that men tend to show relatively greater side to side motion (or swing) of the shoulders than of the hips, whereas women show the opposite. The reason for this is that men typically have broad shoulders and narrow hips in comparison to women. The shoulders and hips move in opposition to each other, that is, when the right shoulder is forward, the left hip is forward. One can identify the centre of moment in the upper body, which is the neutral reference point around which the shoulders and hips swing. The position of the centre of moment is determined by the relative sizes of the shoulders and hips, and is typically lower in men than in women. Cutting et al. (1978) found that the centre of moment correlated well with the sex judgements made by observers.

Cutting (1978) extended these findings. He used artificial moving dot displays (i.e., the lights were not attached to people) in which only the centre of moment was varied. Judgements of the sex of "male" and "female" walkers were correct over 80% of the time, suggesting the importance of centre of moment. However, Cutting used a greater range of variation in the centre of moment than would be found in real human beings, and the general artificiality of his situation suggests some caution in generalising his findings to real-life situations.

Mather and Murdoch (1994) also used artifical point-light displays. Most previous studies had involved movement across the line of sight, but the "walkers" in their displays appeared to be walking either towards or away from the camera. There are two correlated cues that may be used by observers to decide whether they are looking at a man or a woman in point-light displays:

1. Structural cues based on the tendency of men to have broad shoulders and narrow hips, whereas women have the opposite tendency; these structural cues form the basis of the centre of moment.

2. Dynamic cues based on the tendency for men to show relatively greater body sway with the upper body than with the hips when walking, whereas woman show the opposite.

Sex judgements were based much more on dynamic cues than on structural ones when the two cues were in conflict. Thus, the centre of moment may be less important than was assumed by Cutting (e.g., 1978).

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