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Read Pinker article for Thurs. Seeing SeeingininStereo Stereo Seeing in Stereo It’s It’svery veryhard hardtotoread readwords wordsififthere there are aremultiple multipleimages imageson onyour yourretina retina Seeing in Stereo It’s It’svery veryhard hardtotoread readwords wordsififthere there are aremultiple multipleimages imageson onyour yourretina retina But how many images are there on your retinae? Binocular Disparity • Your eyes have a different image on each retina – hold pen at arms length and fixate the spot – how many pens do you see? – which pen matches which eye? Binocular Disparity • Your eyes have a different image on each retina – now fixate the pen – how many spots do you see? – which spot matches which eye? Binocular Disparity • Binocular disparity is the difference between the two images Binocular Disparity • Binocular disparity is the difference between the two images • Disparity depends on where the object is relative to the fixation point: – objects closer than fixation project images that “cross” – objects farther than fixation project images that do not “cross” Binocular Disparity • Corresponding retinal points Binocular Disparity • Corresponding retinal points Binocular Disparity • Corresponding retinal points Binocular Disparity • Corresponding retinal points Binocular Disparity • Points in space that have corresponding retinal points define a plane called the horopter The Horopter Binocular Disparity • Points not on the horopter will be disparate on the retina (they project images onto non-corresponding points) Binocular Disparity • Points not on the horopter will be disparate on the retina (they project images onto non-corresponding points) • The nature of the disparity depends on where they are relative to the horopter Binocular Disparity • points nearer than horopter have crossed disparity • points farther than horopter have uncrossed disparity Binocular Disparity • Why don’t we see double vision? Binocular Disparity • Why don’t we see double vision? • Images with a small enough disparity are fused into a single image Binocular Disparity • Why don’t we see double vision? • Images with a small enough disparity are fused into a single image • The region of space that contains images with close enough disparity to be fused is called Panum’s Area Binocular Disparity • Panum’s Area extends just in front of and just behind the horopter Stereopsis • Our brains interpret crossed and uncrossed disparity as depth • That process is called stereoscopic depth perception or simply stereopsis Stereopsis • Stereopsis requires that the brain can encode the two retinal images independently Stereopsis • Primary visual cortex (V1) has bands of neurons that keep input from the two eyes separate Stereopsis • If the brain only gets normal signals from one eye early in life, that eye’s neurons crowd out the other eye’s neurons Amblyopia • Amblyopia is a visual deficit in which one eye has poor vision because the brain never developed the ability to use signals from that retina Amblyopia • Amblyopia is a visual deficit in which one eye has poor vision because the brain never developed the ability to use signals from that retina • Usually caused by – strabismus - when eyes don’t lock onto the same point – anisometropia - when one eye has very bad optics and the other is normal Amblyopia • People with Amblyopia can’t see stereograms
