Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMotion perceptionPSY 310Greg FrancisLecture 24How do you see motion here?Purdue UniversityAperture problem A detector that only sees part of a scene cannotprecisely identify the motion direction or speed of anedgePurdue UniversityAperture problem The same thing is true for whole gratings It’s a property of physics, not of the brain Everything is the same inside the aperturePurdue UniversityAperture problem There is no real “solution” to the aperture problem Only different assumptions about how the world works and howto interpret motion signals Breathing square illusion Combing motion signals together as parts of differentobjects leads to different percepts Two squares (right side) Cross or lines? It’s perceptual organization all over again Gestalt laws Figure-groundPurdue UniversityMotion organization We often group together motion signals from different positions toproduce a common interpretation Walker Motion pattern of dots gives rise to more elaborate motion percepts Given that many different motion signals are present in a scene,how do we group them together? 360 Turning kickPurdue UniversityMovies The motion in movies is all apparent motion Individual still pictures are shown one after the otherProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMovies The motion inmovies is allapparent motion Individual stillpictures are shownone after the otherPurdue UniversityMovies The amount of movement from one frame to the next is oftenquite largePurdue UniversityMovies The amount of movement from one frame to the next is often quitelarge And there are many different movements Head turns, Board moves, Shoulder moves back Foot moves down, Body moves forwardPurdue UniversitySimplify things To identify the heuristics used by the visual system, use many fewerstimuli Apparent motion Occlusion heuristic An object can move behind another object and disappear Triangle-squaresFrame 1 Frame 1Purdue UniversitySimplify things To identify the heuristics used by the visual system, use many fewerstimuli Apparent motion Occlusion heuristic Objects can converge together Dots convergingFrame 1 Frame 1Purdue UniversitySimplify things To identify the heuristics used by the visual system, use many fewerstimuli Occlusion heuristic Objects can converge together,but the presence of an occluder leads to adifferent interpretation Dots occludingFrame 1 Frame 1Prof. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityEye movements Reichardt detectors respond to the motion of light patterns on theretina But these patterns do not always produce a motion percept There are several kinds of eye movements Vestibulo-ocular reflex: as your body or head moves, the eyesadjust to keep an image focused on the fovea (no motion onretina, no motion percept) Pursuit: as an object moves, the eyes move to keep its imagefocused on the fovea (no motion on retina, but a motion percept!) Saccades: rapid movement of the eyes that focus on a new objectat a new position (motion on retina, but no motion percept!) Vergence: rotation of the eyes together to look at an object thatvaries in depth (motion on retina, but no motion percept!)Purdue UniversityEye movements Your brain combinescommands to move the eye Corollary discharge ofmuscle commands With the motion pattern onthe retina To discount motion fromeye movement To combine motion of animage that is pursued It’s not the actualmovement from themuscles, it’s the commandto move the musclesExpected motion signals on the retinaCommandto moveeyesActual motion signals on the retinacombineequalsMotion signalsfrom objects in the worldPurdue UniversityAfterimage movement An afterimage produces nomotion signals on the retina But when you move theeyes, there is expected tobe motion So you perceive motion asif the object were movingthe same amount as theexpected motion As if you were tracking theobjectExpected motion signals on the retinaCommandto moveeyesActual motion signals on the retinacombineequalsMotion signalsfrom objects in the worldPurdue UniversityFlash lag It’s a complicated system and it doesn’t always work perfectly Perception of a flashed object seems to lag a moving object Flash Lab demoPurdue UniversityMotion It’s a complicatedsystem and it doesn’talways work perfectly Small eye movementscan introduce motionsignals even when thephysical objects donot movePurdue UniversityMotion Small eye movements can introduce motion signals evenwhen the physical objects do not moveProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityConclusions Aperture problem Movies Motion organization Eye movementsPurdue UniversityNext time Optic flow Moving through the environment Ecological approach to
View Full Document
Unlocking...