![]() MT is also involved in the perceptual decision process for ambiguous structure-from-motion stimuli (Bradley et al., 1998 Brouwer & van Ee, 2007 Dodd, Krug, Cumming, & Parker, 2001). More recent evidence shows that MT is also involved in processing of depth, because it contains disparity-selective neurons (Bradley, Chang, & Andersen, 1998 Bradley, Qian, & Andersen, 1995) and neurons that are selective for depth from motion parallax (Nadler et al., 2008, 2009). Area MT contains direction-selective neurons and is a key structure for processing of motion for perception (Britten, Shadlen, Newsome, & Movshon, 1992 Newsome, Wurtz, Dursteler, & Mikami, 1985 Salzman, Murasugi, Britten, & Newsome, 1992) as well as smooth pursuit eye movements (Dursteler, Wurtz, & Newsome, 1987 Lisberger & Movshon, 1999). There is considerable overlap between the neural networks for motion perception, smooth pursuit eye movements, and depth perception. Motion parallax typically involves self-motion of an observer, and in the kinetic depth effect, the different layers of motion belong to one object. Although both motion parallax and kinetic depth emphasize the close relationship between motion and depth, there are some important differences to motion transparency. In contrast to motion parallax, the observer is typically stationary and the object is rotating. Another example for the close relationship between motion and depth is the kinetic depth effect or structure from motion (Wallach & O'Connell, 1953), where depth or three-dimensional form can be extracted from two-dimensional projections of three-dimensional objects, if the objects are rotating. There is behavioral (Naji & Freeman, 2004 Nawrot & Joyce, 2006) and physiological (Nadler, Angelaki, & DeAngelis, 2008 Nadler, Nawrot, Angelaki, & DeAngelis, 2009) evidence that the extraretinal signal of smooth pursuit eye movements is used to disambiguate the retinal input. Objects in front or behind the fixation move in opposite directions on the retina, but the assignment of direction to depth sign is ambiguous. In this case, the retinal speed alone is only informative about the absolute distance in depth relative to fixation, i.e., objects close to fixation move slower on the retina than objects far away from fixation. However, usually observers fixate a stationary object under these conditions, so that the eyes move in a direction opposite to the translation. As a consequence of the translational motion, close objects move faster on the retina than far objects. Motion parallax is an informative depth cue if an observer is translating in a scene. Smooth pursuit eye movements play a crucial role in motion parallax (Rogers & Graham, 1979). The common effect of dot number and motion adaptation suggests that global motion strength can induce a bias to perceive the stronger motion in the back. ![]() The differences between perceived depth order and initial pursuit preferences and the slow adjustment of pursuit indicate that perceived depth order is not determined solely by the eye movements. After 300 to 500 ms, smooth pursuit eye movements adjusted to perception and followed the surface whose direction had to be indicated. Smooth pursuit eye movements showed an initial preference for surfaces containing more dots, moving in a non-adapted direction, moving at a faster speed, and being composed of larger dots. Surfaces containing more dots, moving opposite to an adapted direction, moving at a slower speed, or moving in the same direction as the eyes were more likely to be seen in the back. Here, we investigated the influence of different surface features on the perceived depth order and the direction of smooth pursuit eye movements. Little is known about the surface features that are used to resolve this ambiguity. When two overlapping, transparent surfaces move in different directions, there is ambiguity with respect to the depth ordering of the surfaces.
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