Prof. Greg Francis

Size perception Size constancy

PSY 310  Our is useful for identifying the properties of objects in the world Greg Francis  Surface (color, texture)  Location (depth)  Size

 In this class, we often mention visual angle as a way of measuring 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 the perceived size of an object does not change with distance Lecture 22  We don’t quite have that, but we are pretty close Why the cars look like toys. Purdue University Purdue University

Visual angle Size-distance scaling

 How do we describe the size of visual stimuli?  The size of the image on the depends on  A larger image that is further way is exactly the same on the back of the distance of the object the eye!

 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 valid Purdue University Purdue University

Size constancy Size constancy

 Consider a snow man 15 feet away and 5 feet tall  The perceived size of the snowman would be

 S= K(R x D)  It produces a retinal image that has a visual angle  S= K(18.9 degrees x 15 feet) of θ =18.9  S= K (283.8)

-1  If K = 0.0176 degrees θ€ 5 feet  S= 5 feet

15 feet  Nothing interesting here, I just set the K term to give me the actual height of the snowman €

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PSY 310: Sensory and Perceptual Processes 1 Prof. Greg Francis

Size constancy Size constancy

 Consider a snow man 25 feet away and 5 feet tall  The perceived size of the snowman would be

 It produces a smaller retinal image that has a visual angle of  S= K(RxD)

 S= K(11.4 degrees x 25 feet) θ =11.4  S= K (285)

-1 5 feet  If we keep K = 0.0176 degrees θ €  S= 5.016 feet  Small difference is due to rounding errors in the calculation

25 feet  We always get the same value for the size! €

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Emmert’s Law Color  The size-distance scaling idea explains an odd thing about

 They change apparent size depending on the depth of the surface you look at

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Emmert’s Law No perceived depth

 What happens if you  The afterimage exists only on the retina do not have any  Or for neural circuits that represent things in retinal coordinates estimate of perceived depth?  So, it always has a fixed retinal size  R = 23 degrees  Then the perceived size is related only to  As you look at different places, the perceived depth changes the visual angle of the

 On your desk D = 1 foot image  S = 0.0176 degrees-1 (23 degrees x 1 foot) = 0.4048 feet  As if the distance was constant  On the screen D = 60 feet  Then you cannot  S = 0.0176 degrees-1 (23 degrees x 60 feet) = 24.288 feet accurately judge perceived size

 The sun and the moon appear to be about the same size  0.5 degrees Purdue University Purdue University

PSY 310: Sensory and Perceptual Processes 2 Prof. Greg Francis

No perceived depth Poor depth

 In reality, the  There is a similar effect when looking from the top of a tall building sun and moon are very  John Hancock building in Chicago

different in  We don’t have a good estimate of depth (we estimate it to be shorter size than it really is)

 And distance  Sun is 93 million miles away  Moon is 245,000 miles away

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Size-distance scaling Size illusion

 You don’t have to “know”  The horizontal lines are the distance of something the same size for it to contribute to your  Easy to prove size percept

 It is not a conscious calculation

 Your visual system does it automatically  Like it does for or convergence of the eyes

 This can lead to some illusions

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CogLab version CogLab version

 Method of  The line with wings was always 100 pixels constant  If there was no illusion you would expect that proportion of 0.5 stimuli would be at Size of line 100 pixels  Right line always the same

 Left line varies from trial to trial

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PSY 310: Sensory and Perceptual Processes 3 Prof. Greg Francis

CogLab version Why the illusion?

 Size-distance scaling  The actual pixel size that leads to 0.5 proportion reports  The outward wings indicate the line in between is further in indicates the perceived length of the line with wings depth  Automatic process by your visual system

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Why the illusion?

 If two lines have the same retinal size and one is further away  A similar effect explains the size illusion here

 The further away line must have a larger physical size

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Illusions Other effects

 Usually, applying  Perceived size is this idea does not intimately tied up with perceived depth, but it’s lead to an illusion not the only issue

 The three figures  Suppose you are a have the same bomber in an aircraft. retinal size, but Your mission is to blow up different perceived fuel tanks. You have one bomb left. Intelligence has sizes told you that the center tanks are full and the others are empty. Which one do you blow up?

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PSY 310: Sensory and Perceptual Processes 4 Prof. Greg Francis

Ebbinghaus illusion Jastrow Illusion

 The (retinal) size of surrounding objects affects the  Which object looks to be larger? perceived size of an object

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Jastrow Illusion Jastrow Illusion

 Which object looks to be larger?  It involves which part of an object is compared to which part of the other object

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Moon illusion Moon illusion

 Which of these drawings shows the moon sized properly?  The moon seems to change size

 Very large when low on the horizon

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PSY 310: Sensory and Perceptual Processes 5 Prof. Greg Francis

Moon illusion Moon illusion

 The moon seems to change size  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  Smaller when up in the sky (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 illusion

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Explanation Explanation

 When the moon is on the horizon there are some cues to distance  When looking at the moon in an empty sky, there are no of the horizon elements cues to the distance of the moon  Linear perspective  No disparity  Occlusion  No occlusion  Atmospheric perspective  No linear perspective  Relative size

 We default to a standard  The moon is behind all objects

 Eye convergence  So it must be further than them

 Which contributes to  Which leads to certain perceived size eye convergence

 Possibly also to a further distance

 So the moon must be bigger! Purdue University Purdue University

Tricky Other interesting size illusions

 There’s still no real agreement on the reason for the moon illusion  The two half discs are identical in size and shape

 In particular, the horizon moon often seems closer than the moon in the sky  You would expect the opposite

 There may be multiple effects going on

 Ponzo illusion

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PSY 310: Sensory and Perceptual Processes 6 Prof. Greg Francis

Other interesting size illusions Conclusions

 Vertical horizontal illusion  Size constancy  We do a pretty good job overall

 Size-distance scaling

 Emmert’s law

 Illusions

 Moon illusion

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Next time

 Review for exam 2.

 Bring your questions.

 Then Exam 2 on Friday

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PSY 310: Sensory and Perceptual Processes 7