Abstract not found

Recent psychophysical experiments suggest that humans can recover only relief structure from motion (SFM); i.e., an object’s 3D shape can only be determined up to a stretching transformation along the line of sight. Here we propose a physiologically plausible model for the computation of relief SFM, which is also applicable to the related problem of motion parallax. We assume that the perception of depth from motion is related to the firing of a subset of MT neurons tuned to both velocity and disparity. The model MT neurons are connected to each other laterally to form modulatory interactions. The overall connectivity is such that when a zero-disparity velocity pattern is fed into the system, the most responsive neurons are not those tuned to zero disparity, but instead are those having preferred disparities consistent with the relief structure of the velocity pattern. The model computes the correct relief structure under a wide range of parameters and can also reproduce the SFM illusions involving coaxial cylinders. It is consistent with the psychophysical observation that subjects with stereo impairment are also deficient in perceiving motion parallax, and with the physiological data that the responses of direction- and disparity-tuned MT cells covary with the perceived surface order

Two coaxial, ambiguously rotating objects tend to be perceived as corotating. Such grouping could be the consequence of bottom-up, cooperative interactions between the stimuli, or the top-down selection of object properties consistent with a model of the objects or scene. However, we find that the coupling between an ambiguous and unambiguous object is sharply reduced, presenting a challenge for both explanations of grouping. We describe experiments that support the idea that top-down feedback is necessary to select and stabilize a perceptual interpretation for ambiguous figures. Reduced coupling between objects of differing ambiguity can be explained if the feedback is global and proportional to ambiguity.

Five experiments were designed to determine whether a rotating, transparent 3-D cloud of dots (simulated sphere) could influence the perceived direction of rotation of a subsequent sphere. Experiment 1 established conditions under which the direction of rotation of a virtual sphere was perceived unambiguously. When a near–far luminance difference and perspective depth cues were present, observers consistently saw the sphere rotate in the intended direction. In Experiment 2, a near–far luminance difference was used to create an unambiguous rotation sequence that was followed by a directionally ambiguous rotation sequence that lacked both the near–far luminance cue and the perspective cue. Observers consistently saw the second sequence as rotating in the same direction as the first, indicating the presence of 3-D visual inertia. Experiment 3 showed that 3-D visual inertia was sufficiently powerful to bias the perceived direction of a rotation sequence made unambiguous by a near–far luminance cue. Experiment 5 showed that 3-D visual inertia could be obtained using an occlusion depth cue to create an unambiguous inertia-inducing sequence. Finally, Experiments 2, 4, and 5 all revealed a fast-decay phase of inertia that lasted for approximately 800 msec, followed by an asymptotic phase that lasted for periods as long as 1,600 msec. The implications of these findings are examined with respect to motion mechanisms of 3-D visual inertia.

ABSTRACT—Ambiguous visual information often produces unstable visual perception. In four psychophysical experiments, we found that unambiguous tactile information about the direction of rotation of a globe whose three-dimensional structure is ambiguous significantly influences visual perception of the globe. This disambiguation of vision by touch occurs only when the two modalities are stimulated concurrently, however. Using functional magnetic resonance imaging, we discovered that touching the rotating globe, even when not looking at it, reliably activates the middle temporal visual area (MT1), a brain region commonly thought to be crucially involved in registering structure from motion. Considered together, our results show that the brain draws on somatosensory information to resolve visual conflict. People’s daily activities are guided by an amalgam of sensory inputs from different modalities. These sensory modalities, although typically segregated in textbooks, function together to specify behaviorally important objects and events. To give just a few examples, sound and vision interact to influence speech perception (McGurk & MacDonald, 1976) and to specify the nature of dynamic events such as collision (Sekuler, Sekuler, & Lau, 1997). Similarly, sound can influence the perceived roughness of a touched surface (Guest, Catmur, Lloyd, & Spence, 2002), and touch can influence visual perception of surface texture (Heller, 1982) and surface slant (Ernst, Banks, & Bulthoff, 2000). In the work reported here, we sought to extend the analysis of multimodal perception to an aspect of visual perception—structure from motion—about which there is some evidence concerning possible underlying neural mechanisms. In particular, we exploited the kinetic depth effect: the perception of a three-dimensional (3D) object on the basis of differential optic flow (Doner, Lappin, & Perfetto, 1984; Wallach & O’Connell, 1953). When viewing the 2D parallel projection of a rotating 3D object, one may experience spontaneous reversals in the perceived direction of rotation (Fisichelli, 1947; Howard, 1961;

Although motion parallax is closely associated with observer head movement, the underlying neural mechanism appears to rely on a pursuit-like eye movement signal to disambiguate perceived depth sign from the ambiguous retinal motion information [Naji, J. J., & Freeman, T. C. A. (2004). Perceiving depth order during pursuit eye movement. Vision Research, 44, 3025–3034; Nawrot, M. (2003). Eye movements provide the extra-retinal signal required for the perception of depth from motion parallax. Vision Research, 43, 1553–1562]. Here, we outline the evidence for a pursuit signal in motion parallax and propose a simple neural network model for how the pursuit theory of motion parallax might function within the visual system. The first experiment demonstrates the crucial role that an extra-retinal pursuit signal plays in the unambiguous perception of depth from motion parallax. The second experiment demonstrates that identical head movements can generate opposite depth percepts, and even ambiguous percepts, when the pursuit signal is altered. The pursuit theory of motion parallax provides a parsimonious explanation for all of these observations.