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Depth Cues Part 2.docx

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PSYC 2390
Lana Trick

Depth Cues Part 2 10/22/2012 1:45:00 PM Depth Cues Part 2  2D retina with 3D world  must use visual angle to proximate depth o Smaller visual angle when farther away o Dimensional ambiguous  Actual size  How far away  Cues from the eye: convergence, lens  Pictorial cues (monocular cues  only need 1 eye to see them)  Don‟t always work depending on situation o Atmospheric only works for distance, not for close up Motion-based Cues: Relative motion / Motion Parallax  Often viewer that is moving  Object that is close to you seem to be moving rapidly in opposite direction as you are  Object that is far away from you seem to be moving slower in the same direction o Farther away than object that you are fixating on o Farther it is, the slower it seems to be moving Accretion and Deletion  Accretion – growing larger  Deletion – growing smaller o The thing behind is covered by object in front (gets smaller) then gets bigger again (accretes) Motion in Depth  A.k.a. looming (something is growing larger in visual field) o Baseball Binocular Depth Cues: (a.k.a. Stereopsis)  Can do things up to 30% faster if binocular depth cues are being perceived correctly  Requires 2 eyes (input from both) o Both eyes have different points of view (jumping of image when switching eyes), eyes are 6 cm apart  Jumps farther when fixating on object closer to you  Difference of location of image between left and right eyes  Stereoscopes  have 2 slightly different images that are seen by left and right eye and when you view them together, see picture in depth  3D - superimposed images o When look through blue lens, only see red image o When looking through red lens, only see blue image o 2 images (blue and red) are slightly different when put together, appears to be „poking out‟ o can do the same thing with images made of polarized light (vibrate in only one direction, i.e. horizontal and vertical)  Random dot diagrams or random dot stereogram (Julesz) o Looks like black and white dots o Proved that observers can perceive depth in displays that contain no depth information other than disparity o Random dot diagram  Part of one image is duplicated on the other image but slightly shifted  When presented simultaneously, appears as though that part of the image is „poking out‟ Binocular Depth Perception 10/22/2012 1:45:00 PM Binocular Depth Perception How is depth information derived from stereopsis? 1. Solving the stereo-correspondence problem (First stage)  Solving the stereo-correspondence problem correctly o Stereopsis: the impression of depth that results from information provided by binocular disparity o Must determine which part of left image corresponds to part on right image o Stereo  2 eyes o Correspond  same o Can do this in images and in random dot diagrams o Corresponding retinal points: the places on each retina that would overlap if one retina could be slid on top of the other  Autostereograms: an illusion produced when you solve the stereo- correspondence problem incorrectly o Correctly realized that 2 images on retina came from same fixation in image o Illusory position: misread 2 images on retina came from same object but they actually came from different objects in image  Makes objects look either farther or closer than they actually are  Need image with repetition in it 2. Calculating retinal disparity (Second stage)  3 possible outcomes (see diagrams)  Retinal disparity: retinal difference a.k.a. binocular disparity o Compare difference of object on left eye on object on right eye  Angle of disparity: difference between where image falls and corresponding point (absolute disparity) o Provides information about distance of objects o Bigger the angle, the closer the object is o Angle of disparity is zero for the object you are fixating on (falls on fovea) o Changes every time a person changes what they are looking at (3 fixations per second  Relative disparity: stays relatively the same, the difference between two objects‟ absolute disparities o Disparities may have changed but the differences between the two stays the same as long as the object themselves haven‟t moved Neurons that Respond to Pictorial Depth  Monkey matching experiments  Found that neurons fire for specific depth displays indicated by pictorial depth cues to textural gradients and disparity Neurons that Respond to Binocular Disparity  Neurons that respond to specific amounts of disparity  Called binocular depth cells or disparity selective cells o Responds best when stimuli presented to left and right eyes create a specific amount of absolute disparity Connecting Binocular Depth Cells and Depth Perception  Proof through selective rearing experiments (see effects of early deprivation below)  Microstimulation o Set small electrode into the cortex and pass electrical charge through to activate neurons near electrode o Neurons sensitive to some disparities are clustered together  Stimulate one area, stimulate cluster of neurons all sensitive to specific disparity o Monkeys trained to indicate depth created  When stimulating neurons different than disparity shown to monkey, monkey would indicate judgment towards disparity signaled by neurons  Brain imagining techniques show that many areas of brain are involved in creating binocular disparity o Absolute disparity  primary visual cortex o Relative disparity  higher visual system, temporal lobe There are only a limited range of distances that you can derive stereo-depth:  Panum‟s fusion area o Double images produced when object is too far away or too close (outside of Panum‟s fusion area, too far from horopter) o Region around horopter in which you can match differences in position in object in left and right eye and get stereo-depth from it o Only limited range in which you can get binocular depth How is stereo-depth information derived in the brain?  Striate cortex o Ocular dominance columns  cells respond better to one eye more than the other  80% of cells respond to both  Information between eyes is being compared o Binocular cells: cells powered by either eye Observation: An interesting finding about the tilt after-effect  Seeing lines of opposite direction after being adapted to diagonal lines and seeing vertical lines  Can look and adapt with 1 eye but look at vertical lines with other eye o See the illusion anyways  What does this mean? o Adapting left eye (for example) but getting illusion in right eye o Illusion is passing over between eyes o Shows you there is some cell in brain that gets info from either eye  Doesn‟t matter what eye it comes from, it responds the same  Looking at something with left eye, are activating same cell that you would if you were looking at image with right eye Binocular Cells in the Striate Cortex  Effects of early deprivation o Require stimulation during certain points in life or they do not develop properly o Critical periods  if not developed during this time, never developed properly  Development of binocular cells have critical periods  “kitty pirate” studies  One day, had patch on one eye, next day had patch on the other eye and keep alternating  Cats are naturally born blind  Develop ability to see over time  If within 4 weeks to 4 months do not get proper stimulation, do not get proper vision
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