Academic journal article Perception and Psychophysics

Temporal Properties in Masking Biological Motion

Academic journal article Perception and Psychophysics

Temporal Properties in Masking Biological Motion

Article excerpt

The perception of biological motion using point light animation techniques was investigated in several experiments. Animations simulating walking were presented with additional masking dots. The temporal properties of the walking motion or the temporal relationship between the walking and masking motions were systematically manipulated. Results showed that (1) perception of biological motion was sensitive to even small temporal perturbation within the walker, (2) the effectiveness of a mask depended upon the temporal phase difference between the mask and point light walker, (3) relatively small temporal differences between the mask and point light walker decreased the effectiveness of the mask, and (4) these effects were not due simply to observers detecting the phase offsets in the display. Temporal properties of the motion are important in perceiving the human form in action, just as in other types of figure-ground segregation. This information may be processed by both motion and form pathways for processing biological motion.

The motions of 10 to 12 points of light placed on the major joints of the body are sufficient to give the strong impression of the human form and the nature of the activity being performed (Johansson, 1973). Such biological motion displays offer a rich source of information about the action and the actor portrayed. Potentially, they can carry information about the gender of the actor (Barclay, Cutting, & Kozlowski, 1978; Kozlowski & Cutting, 1977, 1978; Mather & Murdoch, 1994; Pollick, Lestou, Ryu, & Cho, 2002; Runeson & Frykholm, 1983; Troje, 2002), the identity of the actor (Beardsworth & Buckner, 1981; Cutting & Kozlowski, 1977; Hill & Pollick, 2000; Stevenage, Nixon, & Vince, 1999), the effort of the action (Runeson & Frykholm, 1981), the emotions of the actor (Brownlow, Dixon, Egbert, & Radcliffe, 1997; Dittrich, Troscianko, Lea, & Morgan, 1996; Pollick et al., 2002; Pollick, Paterson, Bruderlin, & Sanford, 2001; Walk & Homan, 1984), the expectations and intentions of the actor (Runeson & Frykholm, 1983), or the size of the actor (Jokisch & Troje, 2003); they can even carry information for the perception of sign language (Poizner, Bellugi, & Lutes-Driscol, 1981). Inverting biological motion displays, however, even though it preserves the relationships between the moving dots, is known to disrupt the perception of biological motion (Grossman & Blake, 2001 ; Mitkin & Pavlova, 1990; Pavlova & Sokolov, 2000; Shipley, 2003; Sumi, 1984; Troje, 2003). The effect of inversion on biological motion detection may be due to the unfamiliar form that the inverted point lights now create (Bertenthal & Pinto, 1994) or to unfamiliar dynamics in relation to gravity (Shipley, 2003).

One common way to investigate biological motion is to mask the displays by simultaneously presenting other moving points of light in the display area. Cutting, Moore, and Morrison (1988) showed that a scrambled walker mask disrupted perception of an embedded point light walker more effectively than did translating masks, circular masks, or random motion masks. Furthermore, a scrambled walker mask with the same step-size as that of the walker maximizes disruption, and with a sufficient number of mask elements, performance does not improve with presentations longer than 800 msec.

In detecting masked biological motion, the central task is to segregate the figure (biological motion) from the ground (the masking dots). Figure-ground segregation can be supported by many cues (e.g., orientation, color, depth, motion). However, in masked biological motion, the walker and mask dots have the same orientation, color, depth, and so on. As Cutting et al. (1988) showed, in general the more similar the motion of the mask dots to the motion of the walker dots, the more effective the mask. This is not surprising, given that as the similarity of the mask and walker dot motions increase, there are fewer cues to support figure-ground segregation. …

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