A chimeric point-light walker

Ambiguity has long been used as a probe into visual processing. Here, we describe a new dynamic ambiguous figureöthe chimeric point-light walkeröwhich we hope will prove to be a useful tool for exploring biological motion. We begin by describing the construction of the stimulus and discussing the compelling finding that, when presented in a mask, observers consistently fail to notice anything odd about the walker, reporting instead that they are watching an unambiguous figure moving either to the left or right. Some observers report that the initial percept fluctuates, moving first to the left, then to the right, or vice versa; others always perceive a constant direction. All observers, when briefly shown the unmasked ambiguous figure, have no difficulty in perceiving the novel motion pattern once the mask is returned. These two findingsöthe initial report of unambiguous motion and the subsequent `primed' perception of the ambiguityö are both consistent with an important role for top ^ down processing in biological motion. We conclude by suggesting several domains within the realm of biological-motion processing where this simple stimulus may prove to be useful. DOI:10.1068/p5010 Figure 1. Point-light Ringo. The Beatles'Abbey Road album cover has been altered to demonstrate how a human form can easily be recovered from a few points of light when real or implied motion is present. (Original photograph by Iain MacMillan 1969, copyright Apple Corps Ltd; figure adapted from Thornton and Shiffrar 1996.) form in action can still be obtained. Over the past 30 years, a number of researchers have employed point-light stimuli to explore the sensitivity of the visual system to objects and actions defined primarily by biological motion (eg Ahlstro« m et al 1997; Bertenthal and Pinto 1994; Dittrich 1993; Kozlowski and Cutting 1977, 1978; Mather et al 1992; Pavlov and Sokolov 2000; Sumi 1984; Thornton et al 1998; Troje 2002; Verfaillie 1993). Recently, Jan Vanrie and colleagues demonstrated that symmetric forms of these point-light figures are perceptually bistable. For instance, a figure walking towards an observer at 458, travelling from 11 o'clock to 5 o'clock, can also be seen as a figure moving away from the observer at 458, from 7 o'clock to 1 o'clock (Vanrie et al 2003). In other areas of vision research, relatively simple static ambiguous figures, such as the Necker cube (Gregory 1970; Necker 1832) and the hawk^ duck illusion (Bernstein and Cooper 1997; Tinbergen 1939), as well as dynamic ambiguous displays, such as motion-defined rotating cylinders (eg Hiris and Blake 1996) and Ternus displays (Burt and Sperling 1981; Ternus 1926/1955), have long been used to explore the mechanisms of perception. Ambiguous point-light walkers are appealing as they may provide novel insights into the way we process biological motion. The purpose of this brief report is to introduce a new form of dynamic ambiguous figureöthe chimeric point-light walker. We begin by describing the new stimulus and detailing the simple technique used to create it. Next, we discuss how such figures, when combined with concurrent noise masks, provide a powerful demonstration of top ^ down processing of dynamic stimuli. In the remainder of the paper, we outline a number of potential research areas where we feel this stimulus could be applied, and briefly present some initial empirical findings.