3/23/2005 © Dr. Zachary Wartell 1 Depth and Size Perception.

3/23/2005 © Dr. Zachary Wartell 1

Depth and Size Perception

3/23/2005 © Dr. Zachary Wartell 2

Human Depth Perception

• Cue Theory – brain learns to identify info. in retinal image that is correlated with depth– oculomotor cues– pictorial cues– movement-produced cues– binocular disparity cues

3/23/2005 © Dr. Zachary Wartell 3

Oculomotor Cues: Accommodation

●susp. lig. kept tight by eye fluid which stretches and thins lens (bend light less)

●cil. mus. (donut shape) contracts allows lens to spring into its natural rounder shape (bend light more)

suspensory ligments

ciliary muscle

3/23/2005 © Dr. Zachary Wartell 4

Oculomotor Cues: Vergence

physical box

eyes

fixation point

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Oculomotor Cues: Vergence

physical box

eyes

divergence

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Oculomotor Cues: Vergence

physical box

eyes

convergence

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Pictorial/Monocular Depth Cues

• overlap/occlusion• size in field of view• height in field of view• atmospheric perspective• familiar size• linear perspective• texture gradiant

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Overlap/occlusion

• only relative or “ordinal” (psy.) information (partial order - math)

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Size in field of view

• larger size causes an object to appear closer• Ames balloon experiment

– inflating illuminated balloons in darkened room

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Height in field of view

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Atmospheric/aerial Perspective

• farther objects less distinct, color may be less saturated

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Familiar Size

• knowledge of actual size influences distance judgement

• Epstein (1965)– photo of 3 dime, quarter and nickel at same

size as quarter– viewed by one eye at same distance– darkened room, photo lit by spotlight– judged smaller coin to be closer– binocular viewing foils illusion

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Linear Perspective

• Leon Battisa Alberti (1435) – principles of perspective drawing

• Leonardo da Vinci – drawing on plane of glass

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Texture Gradient

• elements that are equally spaced appear closer and closer together in distance

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ab

Movement-Produced Cue: Motion Parallax

• looking out of car – near objects “move” faster and blur, far objects “move” slower

A B

a'b'

Δa

Δb

ab

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• related to motion parallax and overlap

Movement-Produced Cue: Deletion & Accretion

Deletion

Accretion

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Stereopsis

• Wheatstone (1838) – stereoscope showed that differences alone in left/right eye image yields depth

• “stereopsis” – impression of depth due to two different image on retina

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Corresponding Points

• locations on retina connecting to same part of visual cortex

a'b'

ab

f' f

[Goldstein,44]

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Corresponding Retinal Points

• locations on retina connecting to same part of visual cortex – roughly equivalent to aligning retina atop one another

a'b'

ab

f' f

left right

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Horopter

A

B

bl

br

cl

crC

horopteral

bl

cl

cr

ar

br

• A is fixation point• horopter points have

correspondingretinal images

• horopter dependenton fixation pt.

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Noncorresponding (disparate) retinal points

AB

bl

br

cl

cr

C

horopteral

bl

cl

crar

br

retinal disparity:

θb= θbl-θbr < 0 θc= θcl-θcr > 0

θ<0 θ>0

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Crossed (<0) versus Uncrossed (>0) disparity

ABC

horopter

uncrossedcrossed

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Corresponding (Image) Points

• correspondence problem – how does brain determine what left eye image point should be matched with what right eye image point?

(Al Ar)

(Al Cr)

(Cl Cr)

(Ar Cl)

al

cl

crar

?

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Random-Dot Stereogram - Julesz (1971)

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Al, Bl

F

FlFr

BA

Br Ar

Rear View

Top ViewHoropter

Zone of SingleVision

Panum’s fusion area

Al, Bl Fl Br Ar

Fr

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Local vs Global Stereopsis

• local stereopsis – simple stimuli (1 line), small FOV, image correspondence use local info

• global stereopsis – complex stimuli, large FOV, image correspondence needs global info.

• Important: experimental psychophysical results in local & global case can differ

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Types of stereopsis

• no stereopsis – diplopia / diplopic images, perceived depth at fixation plane or undefined

• patent/quantative stereopsis – magnitude, direction, maybe diplopic, small disparities

• latent/qualitative stereopsis - only direction, always diplopic

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Human Size Perception

• Visual Angle

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Law of Size Constancy

• humans correctly perceive an object’s physical size no matter what its distance from us and no matter what the size of image on retina is

• Holway and Boring (1941)

hallways

subject

targetcircle

test circles

1o

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Holway and Boring (1941)

10 50 100

10

20

30

distance to test circle (ft)

size

of t

arge

t circ

le (

in)

vis. angle

idealall cues

1 eye

+peep hole

+drapes

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Emmert’s Law

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References

• E. Bruce Goldstein. Sensation and Perception, 4th Edition. Brooks/Cole Publishing Company, Pacific Grove.