Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMotion perceptionPSY 310Greg FrancisLecture 23Where would you grab this branch?Purdue UniversityMotion and survival Almost all animals with vision are very sensitive to motion Motion contributes to object perception and figure-groundidentification Camouflaged objects become visible with motion (Hidden item) Small details in motion dramatically change figure-groundrelationships (Moving rings) Object properties vary with motion (Teapot) Like everything else in sensation and perception Our percepts of motion generally track some importantcharacteristics of things in the real world However, the percepts are based on certain computations ofvisual information and do not always correspond to true motionProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTypes of motion Our textbook describes four ways of producing amotion percept There are actually many more Real movement Apparent movement Induced movement Motion aftereffectPurdue UniversityReal motion For a physical object to get from one place toanother, it must cover some path in between We do not detect all movements Some things move too fast Light Blurs Some things move too slow Hour hand of a clockProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityApparent Motion When objects move, there is a continuous path ofmotion But, continuous paths are not necessary for motion to beseen Motion in movies, TV, computers is all apparentmotionTime1Time2Purdue UniversityKorte’s laws Apparent motion was highly studied at the turn ofthe 20th century Korte (1915) noted that to get good motion, you neededto increase the ISI between the stimuli as the distancebetween them increased ISI just rightProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityKorte’s laws Apparent motion was highly studied at the turn of the 20thcentury Korte (1915) noted that to get good motion, you needed toincrease the ISI between the stimuli as the distance betweenthem increased ISI too brief (simultaneity)Purdue UniversityKorte’s laws Apparent motion was highly studied at the turn of the 20thcentury Korte (1915) noted that to get good motion, you needed toincrease the ISI between the stimuli as the distance betweenthem increased ISI too long (separate dots)Prof. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityKorte’s laws Compare to CogLab data (68 subjects)Purdue UniversityInduced movement A non-moving object can appear to be moving ifobjects around it are moving Sometimes with the flow of motion Example from textbook Sometimes the opposite direction Nearby car or train moves, but it feels like you havemoved Moon-in-the-clouds illusion Hard to demo because they work best in isolatedconditions Motion dominates the field of view, while the induced object isisolated within the fieldProf. Greg Francis6PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMovement aftereffect competition betweenopposite directions ofmotion Left-right Up-down habituating gate offset of one direction leads torebound in other Motion aftereffectPurdue UniversityDetecting motion Just like depth, brightness, color, and many other aspects ofperception We do not have direct awareness of motion Instead, the visual system computes motion based onpatterns on the retina Both space and time And using information about movement of our bodies / eyes Your textbook gives a description of a simple circuit todetection motion It is not a good model of motion detection Although it could be elaborated to work properlyProf. Greg Francis7PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTextbook model It’s instructive to discuss the problem with the textbookmodel because it highlights some aspects of motionperception Here’s a little diagram of the modelinhibitionexcitationexcitationmotion signalPurdue UniversityTextbook model Suppose a dot moves from left to rightinhibitionexcitationexcitationmotion signalProf. Greg Francis8PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTextbook model When the dot is here it activates these cellsinhibitionexcitationexcitationmotion signalPurdue UniversityTextbook model Which inhibits the other cell Meanwhile the dot is movinginhibitionexcitationexcitationmotion signalProf. Greg Francis9PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTextbook model When the dot hits here, the cell may still be inhibited, even thoughthe receptor is excited No motion detectedinhibitionexcitationexcitationmotion signalPurdue UniversityTextbook model Suppose a dot moves from right to leftinhibitionexcitationexcitationmotion signalProf. Greg Francis10PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTextbook model When the dot is here it activates these cellsinhibitionexcitationexcitationmotion signalPurdue UniversityTextbook model The signal continues even while the dot movesinhibitionexcitationexcitationmotion signalProf. Greg Francis11PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTextbook model When the dot reaches the other receptor, that receptor respondsinhibitionexcitationexcitationmotion signalPurdue UniversityTextbook model When the dot reaches the other receptor, that receptor responds And inhibits the other cell, but it is too late to stop the motion signalinhibitionexcitationexcitationmotion signalProf. Greg Francis12PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTextbook model What’s the problem? A motion detector should not respond to a stationary dot But this circuit doesinhibitionexcitationexcitationmotion signalPurdue UniversityReichardt detector A better model looks similar, but has different calculations Here the signals from the two receptors multiply The signal on the left is delayed relative to the one on the rightdelayexcitationexcitationmotion signalProf. Greg Francis13PSY 310: Sensory and Perceptual ProcessesPurdue UniversityReichardt detector Suppose a dot moves from right to left Here it activates the receptor There is no motion signal because the x-cell needs input from bothsourcesdelayexcitationexcitationmotion signalPurdue UniversityReichardt detector The dot
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