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Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,*,2004 Prentice Hall,Chapter 2:Visual Perception,PowerPoint by Glenn E.Meyer,Trinity University,2004 Prentice Hall,Cognition 2/e,Dr.Daniel B.Willingham,1,What Makes Visual Perception Hard?,Vision is hard as the pattern of light that falls on your retina is consistent with with many different scenes.,For example,is the figure to the left a square or a cube face on?Maybe it is the base of a four sided pyramid?,2,Visual Perception Hard-Continued?,We will have to deal with ambiguities.,This is done by the visual system making assumptions.,So what is the visual system for?,1)It identifies objects,2)It helps us navigate in the world,3,Visual Perception Hard-Continued?,Chief problem of the visual system is the,inverse projection problem,:,The problem of recovering three-dimensional shape from a two-dimensional projection,like the projection on the retina.A two dimensional project may represent different three dimensional objects.,Can you see the Necker Cube on the Left as either of the cubes below?,OR,?,Thus,visual system must deal with indeterminacy in,shape,and,orientation,.,4,Visual Perception Hard-Continued?,The Second Important Problem-,Surface Features,The Visual System must deal with an objects surface features:color,how dark or light it is,etc.,How does the visual system deal with luminance,the amount of light the eye receives?,There are indeterminacies in,Light Source,Reflectance,Shadow,The Mach Card as an Example of Indeterminacy?,Is the gray part a gray surface of a two part object or is a shadow of the white part?,5,Visual Perception Hard-Continued?,The Third Important Problem-,Object size and distance are indeterminate in a two dimensional representation.,For example,the images of the sun and moon seem the same size but they are at different distances and are different sizes.Could you tell which is what by visual inspection alone?,Is Face Perception Special?,Prosopagnosia-syndrome that affects face recognition,A sheep farmer lost his ability to recognize faces but still could recognize his sheep (MacNeil and Warrington,1983),However,imaging of the fusiform gyrus suggests that the loss is really one of visual expertise(Gauthier,et al,1999),The issue is still debated.,6,How Are Visual Ambiguities Resolved?,Shape,Brightness,Distance and Size,Top-Down Influences in Vision,An Alternative:,The Ecological Approach,7,Shape,The visual system uses assumptions in processing objects,Object orientations are assumed,not,to be unusual,Likelihood principle(Helmhotz):,Suggestion that among the many ways of interpreting an ambiguous visual stimulus,the visual system will interpret it as the stimulus that is most likely to occur in the worldExample lines parallel in a 2-dimensional representation are likely to be parallel in the 3-dimensional world,Frame of Reference:,The position or orientation of an object is defined relative to something else.For example,which object is a square and which is a diamond?,Palmer,et al 1988,8,Brightness,Assumptions:,Surfaces are uniformly colored,Gradual changes could be caused by shadow,Three Factors Contribute to Luminance,Light Source,Shading,Shadow see Photo 2.1 for a fine example,Visual system needs to analyze complex scenes to find simple meaningful components.This is depicted in:,Adelsons(1998)illusion in Fig.2.5,Gilchrists(1997)demonstration of the effects of local contrast tested,binocularly,&,monocularly,see Fig.2.6,9,Distance,and,Size,How can we determine the true size and distance of objects?Through the use of cues.,Three Cues in the Visual System,Accommodation lens shape changes as you focus on objects,Convergence angle of the eyes as you focus on objects,Stereopsis based on,retinal disparity,the difference in position of an objects image on each retina,For example see Fig 2.8 and the discussion of the,correspondence problem,.,Random stereograms,are a prime example.,Correspondence problemTo use disparity as a cue to depth,one must match up the left and right retinal images.The correspondence problem refers to the difficulty that retinal images may contain many possible matches,Random dot stereograms Special stimuli with no cues to depth except retinal disparity.Based on shifting patterns of random dot elements to the left or right in two versions of identical dot matrices.Developed by Julesz.,10,Distance and Size-Continued,Cues in the Environment,Familiar Size,Using ones knowledge of the typical size of an object as a cue to the likely size and distance of an object.,For example,if a child appears larger than an adult,it is likely that the child is closer to the observer.,Pictorial Cues Cues to distance that can be used in 2-dimensional pictures,:,Occlusion:An object that occludes another is closer,Texture Gradient:A field is assumed to have a uniform texture gradient,so if more detail is visible in part of the field,it is assumed to be closer,Linear Perspect
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