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Final Notes

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University of Guelph
PSYC 2390
Lana Trick

Sensation and Perception Final NotesDepth perception Chapter 8Depth perception consists of finding out how far away things areVisual angle tells you how far away and how big things areD Binocular depth cuesRely on the fact that there are 2 eyes and in particular they rely on comparing the left eye image and the right eye image left eye and right eye dont see the same thingVisual system uses the difference between the left eye and the right eye to measure distancedifferences between images in the left and right eyesocorresponding retinal points the places on each retina that would overlap if one retina could be slid on top of anotherohoropter an imaginary surface that passes through the point of fixation and indicates the location of objects that fall on corresponding points on the two retinasononcorresponding retinal points create angles of disparity which give us cues about the location of objectsostereopsis the impression of depth resulting from binocular disparitystereogram present two versions of an image which vary very slightly and when it is seen through a viewer it appears to be 3Drandom dot stereogram one part of the image is shifted over slightly which gives the impression of depthIV Close up on Binocular depth perceptionAHow is depth information derived from stereopsisStereopsis one eye looks at one picture and another eye looks at a different picture creating a 3D imageIf there are 2 squares one square will seem closer than the other Julesz1Solving the stereocorrespondence problemInvolves finding the spot in the left image that corresponds to the part in the right imagecorrespondence problem how does the visual system match the images from the left and right eyesowhen it is solved incorrectly you get the illusion of depthaAutostereograms Solving the stereocorrespondence problem incorrectlyMade so there is repetition in the imageAlso called Magic eye diagramsVisual system is being invited to make the illusory problemSome parts seem incorrectly farther away than otherscarefully made with repetition in the image that causes you to solve the stereo correspondence problem incorrectly2Calculating the retinal disparity Calculating the difference in position of the object from the left eye to the right eyehow different the left and right eye images area3 possible outcomes Diagrams on courselinkItems on the horopter which is the fixation plane the area you are looking at1Zero disparity the object is exactly at the same point on the left eye than on the right eye when an object is on the horopter it has 0 disparity if the image is on the horopter the object is the same distance from reference point middle of the eye and there is no disparity it is in the same position on the left and the right eye2Crossed disparity when a point is in front of the horopter position of object falls on opposite sides of reference point If the object is in front of the horopter this creates crossed disparities and the object is on opposite sides of the reference point in the two eyes3Uncrossed disparity object falls beyond the fixation plane object falls same side of the reference point but is not in the same place for each eye If the object is behind the horopter this is uncrossed disparities and the object is on the same side of the reference point in both eyes but in different positionsB There are only a limited range of distances that you can derive stereodepthIf something is too close or too far away you get double images Outside Panums fusion area Panums fusion area area in which you can fuse the left eye image and the right eye image to get binocular depthDepth Perception Across Speciesanimals with frontal eyes use binocular disparity toperceive depthinsects use mainly movement parallax bats use echolocation C How is stereodepth information derived in the brain1 Observation An interesting observation about the tilt aftereffectstare at left tilted lines then look at vertical lines and you will see right tilted lines a What does this mean can adapt with your left eye and can still get the tilt aftereffect with your right so this must mean that there are cells that take info from both eyes binocular cells which are effected during this process 2 Binocular cells in striate cortex80 of cells are binocular in the striate cortexthese cells do not develop if you do not get the correct visual stimulation during the critical periodDisparity sensitive cells in the visual cortexcell responds to movement in either direction responds the most to 30 minutes of arc degree doesnt respond well to monocularaEffects of early deprivationCritical period kittens are born blind and their visual system develops in the first four monthsThe test included the kittens wearing an eye patch one day on the right eye the next day on the left and so onKitten never has information from both eyes therefore kitten never has binocular perception if you are deprived from information in both eyes then binocular perception will never developStriabismus imbalance in eye musclesex wall eyed cross eyed one eye image is suppressed from day to day if one passes the critical period then binocular depth will be effected and people will be unable to have binocular depth in the futureIf a person never developed binocular cells then the tilt aftereffect will not cross over to the other eye if the left eye is used to stare at left tilted lines and right eye then looks at horizontal lines it will see horizontal lines rather than right tilted linessome neurons respond to pictorial depthsome neurons respond to binocular disparity eg cats reared with their eyes alternately covered were not able to use binocular disparity and did not have these neurons
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