Prof. Greg Francis
Size perception Size constancy
PSY 310 Our visual system 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 retina 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 afterimage The size-distance scaling idea explains an odd thing about afterimages
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 accommodation 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? Ponzo 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