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Chapter 6

Chapter 6 - PSYB51

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Mathias Niemier

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Chapter 6 – Space Perception and Binocular Vision  Realism: philosophical position arguing that there is real world to sense  Positivists: philosophical position arguing that all we really have to go on is evidence of senses, so world might be nothing more than elaborate hallucination rd  Euclidean: referring to geometry of world, so named in honor of Euclid, the ancient Greek geometer of the 3 century BCE. In Euclidean geometry, parallel lines remain parallel as they are extended in space, objects maintain same size and shape as they move around in space, the internal angles of triangle always add to 180 degrees and so forth  Geometry of retinal images of that world is decidedly non-Euclidean o Geometry becomes non-Euclidean when 3 dimensional world is projected onto curved, 2 dimensional surface of retina  Retinal area occupied by an object gets smaller as the object moves farther away from the eyeball  We reconstruct the world from 2 non-Euclidean inputs – 2 distinct retinal images  Advantages of having 2 eyes o Confers same evolutionary advantage as having 2 lungs, kidneys, brain hemispheres – can lose one and still be able to see o Enable you to see more of world  Visual field limited to about 190 degrees from left to right, 110 degrees of which is covered by both eyes o Field more restricted vertically – about 60 degrees up from center of gaze and 80 degrees down  Overlapping, binocular visual fields give predator animals better chance to spot small, fast moving objects in front of them  Binocular summation: combination of signals from each eye in ways that make performance on many tasks better with both eyes than with either eye alone o Under large circumstances, we don’t get large benefit from binocular summation o 50% to 75% benefit assumes 2 completely independent observers, but 2 eyes are still embedded in one person  Binocular disparity: differences between 2 retinal images of same scene. Disparity is basis for stereopsis, a vivid perception of 3D of world that isn’t available with purely monocular vision  Monocular: with one eye  Stereopsis: ability to use binocular disparity as cue to depth  Depth cues: info about third dimension (depth) or visual space. May be monocular or binocular o Monocular depth cue: depth cue that is available even when world is viewed with one eye alone o Binocular depth cue: depth cue that relies on info from both eyes Monocular Cues to 3D Space  Escher was master of rules that govern our perception of space  On basis of retinal images and an implicit understanding of physics and geometry, each cue provides hint about likely structure of space in front of us and disposition of objects in that space  Occlusion o Some of the cues to layout of 3D world were introduced earlier in book, because hints to layout of space can also be hints about structure of objects in that space o Occlusion: cue to relative depth order in which, for example, one object obstructs the view of part of another object o Occlusion is present in almost every visual scene and many argue that it is most reliable of all depth cues o Nonmetrical depth cue: depth cue that provides info about depth order but not depth magnitude o Metrical depth cue: depth cue that provides quantitative info about distance in 3 dimension  Size and Position Cues o Image of retina formed by an object out in world gets smaller as object gets farther away o Projective geometry: for purposes of studying perception of 3D world, geometry that describes transformations that occur when 3D word is projected onto a 2D surface o Relative size: comparison of size between items without knowing absolute size of either one o Texture gradient: depth cue based on geometric fact that items off same size form smaller images when they’re farther away. An array of items that change in size across image will appear to form surface in depth  Larger objects in one area and smaller objects in another – because smaller is interpreted as farther away, arrangement creates perception of ground plane receding into distance o Relative height: as depth cue, observation that objects at different distances from viewer on ground plane will form images at different heights in retinal image. Objects farther away will be seen as higher in image  The smaller image of more distant rabbit will be projected higher in visual field o Texture fields that provide an impression of 3D are really combinations of relative size and relative height cues o If we know what size something out to be, that knowledge can be a depth cue in its own right o Familiar size: depth cue based on knowledge of typical size of objects like humans or pennies o Occlusion is nometrical cue, only providing only depth order o Relative size and height don’t tell us exact distance to an object or between objects o Relative metrical depth cues: depth cue that could specify, for example, that object A was twice as far as object B without providing info about absolute distance from to either A or B o Familiar size could be absolute metrical depth cue o Absolute metrical depth cue: depth cue that provides absolute info about distance in 3 dimension o If visual system knew actual size of object and visual angle of object’s projection on retina, could calculate exact distance from object to eye  Aerial Perspective o Haze/aerial perspective: depth cue based on implicit understanding that light is scattered by atmosphere. More light is scattered when we look through more atmosphere. Thus, more distant objects are subject to more scatter and appear fainter, bluer, and less distinct  Linear Perspective o Linear perspective: depth cue based on fact that lines that are parallel in 3D world will appear to converge in 2D image o Core piece of geometry is that lines that are parallel in 3D world will appear to converge in 2D image, except when parallel lines lie in a plane that is parallel to plane of 2D image o Vanishing point: apparent point at which parallel lines receding in depth converge  Pictorial Depth Cues and Pictures o Pictorial depth cues: cue to distance or depth used by artists to depict 3D depth in 2D pictures o Realistic picture or photograph is result of projecting 2D world onto 2D surface of film or canvas o To correctly interpret the shape of 3D objects from 2D dimensional pictures, take orientation of flat surface of image into account so that they can understand both that picture is a picture and not the real thing and, at the same time, calculate an accurate impression of the thing that is portrayed o Marty Banks showed that our visual system compensates for perceptual distortion in first picture, where context enables us to take orientation of the surface into account o Anamorphosis/anamorphic projection: use of rules to linear perspective to create a 2D image so distorted that it looks correct only when viewed from a special angle or with a mirror that counters the distortion o Rules are known as “anamorphic art”  Motion Cues o Number of additional sources of info are available to our visual system when we view real world scenes that can’t be reproduced in static 2D picture o Motion parallax: an important depth cue that is based on head movement. Geometric info obtained from an eye in 2 different positions at 2 different times is similar to info from 2 eyes in different positions in head at same time o How does motion provide cue for depth? – parallax refers to geometric relationship where eyes move, objects closer to you shift position more than objects farther away when you change your viewpoint o Motion parallax provides relative metrical info about how far away objects are o Downside to motion parallax is that it works only if head moves  Accommodation and Convergence o Eyes need to be focused to see objects at different distances clearly o Accommodation: process by which eye changes its focus (in which lens gets fatter as gaze is directed toward nearer objects) o Convergence: ability of 2 eyes to turn inward, often used in order to place 2 images of a feature in world on corresponding locations in 2 retinal images (typically on fovea of each eye). Convergence reduces disparity of feature to 0 o Divergence: ability of 2 eyes to turn outward, often used in order to place 2 images of a feature in world on corresponding locations in 2 retinal images (typically on fovea of each eye). Reduces disparity of that feature to 0 o The more we have to converge and the more the lens has to bulge in order to focus on object, the closer it is o Studies show that visual system takes advantage o both cues for objects closer than limit  Convergence is used more than accommodation  Cues are only ones besides familiar size that can tell us exact distance to an object Binocular Vision and Stereopsis  Binocular disparity: differences between images falling on our 2 retinas  Stereopsis: impression of 3D of objects “popping out in depth”  Story of the route from binocular disparity to stereoposis is a story of the visual system exploiting the regularities of projective geometry to recover the 3D world from its projections, onto 2D surfaces  Corresponding retinal points: geometric concept stating that points on retina of each eye where monocular retinal images of a single object are formed are at the same distance from fovea in each eye. Two foveas are also corresponding points  Any object lying on the imaginary circle that runs through the 2 eyeballs and object on which Bob is fixated should project to corresponding retinal points  Vieth-Muller circle: location of objects whose images fall on geometrically corresponding points in the 2 retinas. If life were simple.  If 2 eyes looking at one spot, surface of 0 disparity running through that spot  Horopter: location of objects whose images lie on corresponding points. The surface of 0 disparity  Any object placed on that imaginary surface in the world will form images on corresponding retinal locations – horopter and Vieth-Muller circle aren’t the same  Important – there is a surface of 0 disparity whose position in the world depends on current state of convergence of the eyes  Objects that lie on horopter are seen as single objects when viewed with both eyes.  Objects significantly closer to or farther away from surface of 0 disparity form images on decidedly noncorresponding points in 2 eyes, and we see 2 of each of those objects  Diplopia: double vision. If visible in both eyes, stimuli falling outside of Panum’s fusional area will appear diplopic  Objects that are close to horopter but not quite on it can still be seen as single objects  Panum’s fusional area: region of space, in front of and behind the horopter, within which binocular single vision is possible  The larger the disparity, the greater the distance in depth of object from the horopter  Direction in depth is given by sign of disparity  Crossed disparity: sign of disparity created by objects in front of the plane of fixation (horopter). Crossed is used because images of objects located in front of horopter appear to be displaced to left in right eye, and to right in left eye. Always means “in front of the horopter  Uncrossed disparity: sign of disparity created by objects behind plane of fixation (horopter). Uncrossed used because images of objects located behind horopter will appear to be displaced to right in right eye, and to the left in the left eye. Always means “behind the horopter” Stereoscopes and Stereograms th  Although scientists had studied geometry of binocular vision for millennia, not until 19 century was binocular disparity properly recognized as a depth cue  Stereoscope: device for presenting one image to one eye and another image to other eye. Stereoscopes can be used to present dichoptic stimuli for stereopsis and binocular rivalry. Provided visual system treats binocular disparity as a depth cue, regardless of whether disparity is produced by actual or simulated images of a scene  Wheatstone stereoscope had 2 different images in 2 different places o Brewster and Holmes invented viewers that held a card with a double image – captured by cameras with 2 lenses separated by about 2 inches, distance between average human’s eyes – allows to mimic images produced by projective geometry of human binocular vision  Free fusion: technique of converging (crossing) or diverging the eyes in order to view a stereogram without stereoscope  When using free fusion, opposite misalignments become binocular disparity and visual system converts that disparity into perception of depth  Depth that you see depends on whether you converged or diverged your eyes  Possible to free-fuse images by diverging eyes  Because images falling on 2 retinas in divergence method are reversed compared to convergence situations, disparities are reversed and perceived depth will be reversed  Approximately 3-5% of population lacks stereoscopic depth perception  Stereoblindness: inability to make use of binocular disparity as a depth cue. Used to describe individuals with vision in both eyes. Someone who has lost one or both eyes is not typically described as “stereoblind” o Usually a secondary effect of childhood visual disorders such as strabismus, - 2 eyes are misaligned Random Dot Stereograms  For 100 years, after invention of stereoscope, supposed that stereopsis occurred relatively late in processing of visual stimuli  Idea was that first step in free-fusing images wou
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