Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySize perceptionPSY 310Greg FrancisLecture 22Why the cars look like toys.Purdue UniversitySize constancy Our visual system is useful for identifying the properties ofobjects in the world Surface (color, texture) Location (depth) Size In this class, we often mention visual angle as a way ofmeasuring the size of a stimulus But that’s not the same thing as the size of an object It would be good to have size constancy where theperceived size of an object does not change with distance We don’t quite have that, but we are pretty closePurdue UniversityVisual angle How do we describe the size of visual stimuli? A larger image that is further way is exactly the same on the back ofthe eye!Purdue UniversitySize-distance scaling The size of the image on the retina depends onthe distance of the object We can compensate for distance S = K(R x D) S -> perceived size R -> size of the retinal image D -> perceived distance of the object K -> a constant to keep units validPurdue UniversitySize constancy Consider a snow man 15 feet away and 5 feet tall It produces a retinal image that has a visual angleof15 feet5 feet€ θ= 18.9€ θPurdue UniversitySize constancy The perceived size of the snowman would be S= K(R x D) S= K(18.9 degrees x 15 feet) S= K (283.8) If K = 0.0176 degrees-1 S= 5 feet Nothing interesting here, I just set the K term to give me theactual height of the snowmanProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySize constancy Consider a snow man 25 feet away and 5 feet tall It produces a smaller retinal image that has a visual angle of25 feet5 feet€ θ= 11.4€ θPurdue UniversitySize constancy The perceived size of the snowman would be S= K(RxD) S= K(11.4 degrees x 25 feet) S= K (285) If we keep K = 0.0176 degrees-1 S= 5.016 feet Small difference is due to rounding errors in the calculation We always get the same value for the size!Purdue UniversityEmmert’s Law The size-distance scaling idea explains an odd thing aboutafterimages They change apparent size depending on the depth of thesurface you look atPurdue UniversityColor afterimagePurdue UniversityEmmert’s Law The afterimage exists only on the retina Or for neural circuits that represent things in retinal coordinates So, it always has a fixed retinal size R = 23 degrees As you look at different places, the perceived depth changes On your desk D = 1 foot S = 0.0176 degrees-1 (23 degrees x 1 foot) = 0.4048 feet On the screen D = 60 feet S = 0.0176 degrees-1 (23 degrees x 60 feet) = 24.288 feetPurdue UniversityNo perceived depth What happens if youdo not have anyestimate of perceiveddepth? Then the perceivedsize is related only tothe visual angle of theimage As if the distancewas constant Then you cannotaccurately judgeperceived size The sun and themoon appear to beabout the same size 0.5 degreesProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityNo perceived depth In reality, thesun and moonare verydifferent insize And distance Sun is 93millionmiles away Moon is245,000miles awayPurdue UniversityPoor depth There is a similar effect when looking from the top of a tall building John Hancock building in Chicago We don’t have a good estimate of depth (we estimate it to be shorterthan it really is)Purdue UniversitySize-distance scaling You don’t have to “know”the distance of somethingfor it to contribute to yoursize percept It is not a consciouscalculation Your visual system doesit automatically Like it does foraccommodation orconvergence of the eyes This can lead to someillusionsPurdue UniversitySize illusion The horizontal lines arethe same size Easy to provePurdue UniversityCogLab version Method ofconstantstimuli Right linealways thesame Left linevaries fromtrial to trialPurdue UniversityCogLab version The line with wings was always 100 pixels If there was no illusion you would expect that proportion of 0.5would be at Size of line 100 pixelsProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityCogLab version The actual pixel size that leads to 0.5 proportion reportsindicates the perceived length of the line with wingsPurdue UniversityWhy the illusion? Size-distance scaling The outward wings indicate the line in between is further indepth Automatic process by your visual systemPurdue UniversityWhy the illusion? If two lines have the same retinal size and one is further away The further away line must have a larger physical sizePurdue UniversityPonzo illusion A similar effect explains the size illusion herePurdue UniversityIllusions Usually, applyingthis idea does notlead to an illusion The three figureshave the sameretinal size, butdifferent perceivedsizesPurdue UniversityOther effects Perceived size isintimately tied up withperceived depth, but it’snot the only issue Suppose you are abomber in an aircraft.Your mission is to blow upfuel tanks. You have onebomb left. Intelligence hastold you that the centertanks are full and theothers are empty. Whichone do you blow up?Prof. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityEbbinghaus illusion The (retinal) size of surrounding objects affects theperceived size of an objectPurdue UniversityJastrow Illusion Which object looks to be larger?Purdue UniversityJastrow Illusion Which object looks to be larger?Purdue UniversityJastrow Illusion It involves which part of an object is comparedto which part of the other objectPurdue UniversityMoon illusion Which of these drawings shows the moon sized properly?Purdue UniversityMoon illusion The moon seems to change size Very large when low on the horizonProf. Greg Francis6PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMoon illusion The moon seems to change size Smaller when up in the skyPurdue UniversityMoon illusion In reality, the moon is always the same, and so is it’s retinal image Actually there are small effects of atmosphere and the radius of the earth(extra distance) But these make the retinal image of the moon smaller on the horizon The sun and other objects show the same kind of size illusionPurdue UniversityExplanation When looking at the moon