| Motion Perception |
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The observer's visual input is generally insufficient to uniquely determine the 'true' velocity in a visual scene. In monocular vision for example, the visual input will be a 2D projection of a 3D scene. The motion cues present in the 2D projection will by default be insufficient to reconstruct the motion present in the 3D scene. Put differently, many 3D scenes will be compatible with a single 2D projection. The problem of motion estimation generalizes to Binocular Vision when we consider occlusion or motion perception at relatively large distances, where binocular disparity is a poor cue to depth. These issues become more apparent when we look at Visual Illusions involving motion. A well known example is the barberpole illusion. When a diagonally-striped pole is rotated around its longer axis, so that the stripes are moving in the direction of the pole's shorter axis, it nonetheless appears the stripes are moving in the direction of its longer axis. In addition to the problems of motion perception mentioned above, a number of issues arise due to the physiology of the brain. Each Neuron in the visual system is sensitive to visual input in a small part of our Visual Field , as if each neuron is looking at the visual input through a small apperture. At the resolution of this apperture visual cues can often be approximated by straight lines. The motion direction of a straight line is fundamentally ambiguous, because the motion component parallel to the line cannot be inferred based on the visual input. In cases where motion cannot be determined based on visual input alone, the Visual System is thought to rely on prior assumptions. In the second figure the visual input and prior assumptions together make it appear the stripes are moving to the bottom-right. Individual neurons initially estimate motion locally within their receptive field. Because each neuron will suffer from the aperture problem the estimates from many neurons are then integrated into a global motion estimate. This appears to occur in Area MT/V5 in human Visual Cortex . Motion estimation has connections to both Psychology (i.e. Visual Perception ) and Computer Science . SECOND-ORDER MOTION PERCEPTION Motion stimuli are classified into ''first-order'' stimuli, in which the moving contour is defined by Luminance , and ''second-order'' stimuli in which the moving contour is defined by Contrast , Texture , flicker or some other quality that does not result in an increase in motion energy in the Fourier Spectrum of the stimulus (Chubb & Sperling, 1988; Cavanagh & Mather, 1989). There is much evidence to suggest that early processing of first- and second-order motion is carried out by separate pathways (Nishida, et al, 1997). Second-order mechanisms have poorer temporal resolution and are Low-pass in terms of the range of Spatial Frequencies that they respond to. Second-order motion produces a weaker Motion Aftereffect unless tested with dynamically flickering stimuli (Ledgeway & Smith, 1994). EMPIRICAL EXPLANATIONS: USE OF STATISTICAL INFORMATION Just as the visual qualities of Static images are determined probabilistically, image sequences that elicit Sensations of motion are evidently determined by statistical Information . Objective and Perceived Motion ''A solution to the Retinal Stimulus ambiguity is to accumulate experience interacting with moving objects, such that motion perception gradually come to accord with the Statistics of the possible displacements underlying the stimulus.'' In objective terms, generates quite definite Percepts that usually (but not always) allow the observers to deal successfully with the real-world cause of the retinal stimulus. A solution to this problem is evidently to accumulate in any two sequential images; 2) the Possible Appearance of some, but not necessarily all, the elements in the second image not present in the first; 3) the possible disappearance of some, but not necessarily all, the elements in the first image compared to the second; and 4) the possible Deformation of the stimulus during the Interval between the two images. Since the relative Contribution of these correspondences and differences to the physical displacements underlying the stimulus cannot be determined by inspection of stimulus per se, the Problem posed by this inevitable uncertainty can only be solved empirically by generating motion percepts based on past experience of what such stimuli have typically turned out to be. To assess this are generated in case of a straight line moving in fronto- Parallel plane; 2) using this information to derive a set of probability distributions of the possible real-world displacements underlying the stimulus; 3) deriving a Principle for combining these probability distributions based on the statistical structure of the underlying events in the stimulus; and 4) devising a procedure for predicting motion perception based on this Joint probability (see Yang, et al., 2001a). The way subjects perceived the direction and speed of a line moving in any of a variety of apertures in the fronto-parallel plane can be accounted for quite well on this basis. For instance, the perceived direction of motion of a line oriented at 45 degrees and translating horizontally from left to right, should change when constrained by a circular aperture, now being seen moving downward and to the right (a shift in direction of 45 degrees) (see DEMONSTRATION #15 Moreover, the perceived speed of the line in this circumstance should be about 30% less than the perceived speed without the aperture in place. These predicted effects for this and a variety of other apertures correspond well with what subjects in fact see (Figures 2,3,4)[http://www.purveslab.net/research/explanation/motion/motion.html . This wholly empirical framework can apparently account for the full range of Phenomena first described by Hans Wallach 65 years ago. Objects Translating and Rotating ''The unusual Qualities of these Novel percepts are all predicted by the probability Distribution of the Possible stimulus sources, implying that motion perception is always generated on a wholly empirical basis''. The motion of objects that are both translating and rotating can also be framework, we investigated the Special challenge presented by the motion of such objects behind an aperture. When a stimulus of this Sort presents a particular sequence of Linked appearance and disappearance at the apeture Boundary , the line appears to be rotating only; furthermore, the perceived centers of rotation fall on a Cycloid , even when one aperture Shape is replaced by another. The unusual Qualities of these novel percepts are all predicted by the probability distribution of the possible stimulus sources, implying that motion perception is always generated on a wholly empirical basis (see Figures 2B,C) {Link without Title} . OTHER USES OF THE TERM This article deals with Visual motion perception. Other detection mechanisms (for Locomotion in particular) are e.g. based on the Accelerometer principle. REFERENCES
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