Academic journal article Attention, Perception and Psychophysics

Curved Apparent Motion Induced by Amodal Completion

Academic journal article Attention, Perception and Psychophysics

Curved Apparent Motion Induced by Amodal Completion

Article excerpt

Published online: 9 November 2011

© Psychonomic Society, Inc. 2011

Abstract We investigated whether amodal completion can bias apparent motion (AM) to deviate from its default straight path toward a longer curved path, which would violate the well-established principle that AM follows the shortest possible path. Observers viewed motion sequences of two alternating rectangular tokens positioned at the ends of a semicircular occluder, with varying interstimulus intervals (ISIs; 100-500 ms). At short ISIs, observers tended to report simple straight-path motion-that is, outside the occluder. But at long ISIs, they became increasingly likely to report a curved-path motion behind the occluder. This tendency toward reporting curved-path motion was influenced by the shape of tokens, display orientation, the gap between tokens and the occluder, and binocular depth cues. Our results suggest that the visual system tends to minimize unexplained absence of a moving object, as well as its path length, such that AM deviates from the shortest path when amodal integration of motion trajectory behind the curved occluder can account for the objective invisibility of the object during the ISI.

Keywords Apparent motion . Occlusion . Amodal completion . Tunnel effect

In a cluttered environment of opaque objects and surfaces, occlusion of an object behind another is ubiquitous (Gibson, 1979). Occlusion poses a difficult challenge to the visual system because retinal projection of physically continuous objects does not provide local sensory information corresponding to parts hidden behind occluding surfaces. However, we do not have the impression as we view the world that it is filled with object fragments; rather, the objects that we see appear to be complete ones, a phenomenon called amodal completion or amodal continuation (Kanizsa, 1979; Michotte, Thinès, & Crabbé, 1964/ 1991; Shimojo & Nakayama, 1990).

Amodal completion in static scenes have been extensively studied (e.g., Anderson, Singh, & Fleming, 2002; Bruno, Bertamini, & Domini, 1997; Gerbino & Salmaso, 1987; He & Nakayama, 1994; Kanizsa & Gerbino, 1982; Kellman & Shipley, 1991; Rauschenberger, Peterson, Mosca, & Bruno, 2004; Sekuler, Palmer, & Flynn, 1994; Singh, 2004; Watanabe, 1995). But the need for amodal completion is even more pervasive in a dynamic setting (Gibson, 1979). A moving object disappears and reappears as it passes behind other surfaces, or covers and uncovers others when it moves in front. Moreover, stationary objects are frequently occluded by an intervening surface due to the change in an observer's viewpoint, since mobile observers are able to move their eyes, heads, and bodies.1 However, as compared with the static case, dynamic amodal completion has been investigated relatively little.

One of the most famous demonstrations of amodal continuation in dynamic scenes is the tunnel effect (Burke, 1952; Michotte et al., 1964/1991; Wertheimer, 1912/1961). Suppose that a moving object gradually disappears next to a large stationary screen (tunnel), and then another similar object appears on the other side of the screen. In this situation, object motion is temporarily invisible between the end of the first (pretunnel) and the beginning of the second (posttunnel) object motion. Nonetheless, observers unanimously report (under the appropriate spatiotemporal display conditions) that a single object moves behind the tunnel, suggesting that the representation of the moving object is amodally maintained during its disappearance, which links the two spatiotemporally discontinuous visible motion phases. Furthermore, Burke and Michotte et al. found that people could perceive not only the presence of an amodally moving object, but also its location and motion-related properties behind the tunnel, such as apparent speed and trajectory of the invisible motion. Strikingly, even when the posttunnel object path was not a linear continuation of the pretunnel object path, such that the two paths formed an angle less than 180° but were co-circular at the entering and exiting points, observers could still perceive the continuity of movement behind the tunnel, via a relatively long, curved trajectory connecting two visible object motion phases (Burke, 1952; Michotte et al. …

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