Visual Perception

Cecilia R. AragonIEOR 170

UC Berkeley

Spring 2006

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Acknowledgments

• Thanks to slides and publications by Pat Hanrahan, Christopher Healey, Maneesh Agrawala, and Lawrence Anderson-Huang.

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Visual perception

• Structure of the Retina• Preattentive Processing • Detection • Estimating Magnitude• Change Blindness• Multiple Attributes• Gestalt

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Visual perception and psychophysics

Psychophysics is concerned with establishing quantitative relations between physical stimulation and perceptual events.

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Structure of the Retina

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Structure of the Retina

• The retina is not a camera!• Network of photo-receptor

cells (rods and cones) andtheir connections

[Anderson-Huang, L. http://www.physics.utoledo.edu/~lsa/

_color/18_retina.htm]

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Photo-transduction

• When a photon enters a receptor cell (e.g. a rod or cone), it is absorbed by a molecule called 11-cis-retinal and convertedto trans form.

• The different shapecauses it to ultimatelyreduce the electricalconductivity of thephoto-receptor cell.

[Anderson-Huang, L. http://www.physics.utoledo.edu/~lsa/_color/18_retina.htm]

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Electric currents from photo-receptors

• Photo-receptors generate an electrical current in the dark.

• Light shuts off the current.• Each doubling of light causes

roughly the same reduction of current (3 picoAmps for cones, 6 for rods).

• Rods more sensitive, recover more slowly.

• Cones recover faster, overshoot.

• Geometrical response in scaling laws of perception. [Anderson-Huang, L. http://www.physics

.utoledo.edu/~lsa/_color/18_retina.htm]

Preattentive Processing

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How many 5’s?

385720939823728196837293827

382912358383492730122894839

909020102032893759273091428

938309762965817431869241024

[Slide adapted from Joanna McGrenere http://www.cs.ubc.ca/~joanna/ ]

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How many 5’s?

385720939823728196837293827

382912358383492730122894839

909020102032893759273091428

938309762965817431869241024

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Preattentive Processing

• Certain basic visual properties are detected immediately by low-level visual system

• “Pop-out” vs. serial search• Tasks that can be performed in less than 200 to

250 milliseconds on a complex display• Eye movements take at least 200 msec to

initiate

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Color (hue) is preattentive

• Detection of red circle in group of blue circles is preattentive

[image from Healey 2005]

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Form (curvature) is preattentive

• Curved form “pops out” of display

[image from Healey 2005]

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Conjunction of attributes

• Conjunction target generally cannot be detected preattentively (red circle in sea of red square and blue circle distractors)

[image from Healey 2005]

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Healey appleton preattentive processing

http://www.csc.ncsu.edu/faculty/healey/PP/index.html

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Preattentive Visual Features

line orientationlengthwidthsizecurvaturenumberterminatorsintersection

closurecolor (hue)intensityflickerdirection of motionstereoscopic depth3D depth cues

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Cockpit dials

• Detection of a slanted line in a sea of vertical lines is preattentive

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Detection

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Just-Noticeable Difference

• Which is brighter?

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Just-Noticeable Difference

• Which is brighter?

(130, 130, 130) (140, 140, 140)

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Weber’s Law

• In the 1830’s, Weber made measurements of the just-noticeable differences (JNDs) in the perception of weight and other sensations.

• He found that for a range of stimuli, the ratio of the JND ΔS to the initial stimulus S was relatively constant:

ΔS / S = k

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Weber’s Law

• Ratios more important than magnitude in stimulus detection

• For example: we detect the presence of a change from 100 cm to 101 cm with the same probability as we detect the presence of a change from 1 to 1.01 cm, even though the discrepancy is 1 cm in the first case and only .01 cm in the second.

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Weber’s Law

• Most continuous variations in magnitude are perceived as discrete steps

• Examples: contour maps, font sizes

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Estimating Magnitude

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Stevens’ Power Law

• Compare area of circles:

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Stevens’ Power Law

s(x) = axb

s is the sensation

x is the intensity of the attribute

a is a multiplicative constant

b is the power

b > 1: overestimate

b < 1: underestimate[graph from Wilkinson 99]

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Stevens’ Power Law

[Stevens 1961]

Sensation ExponentBrightness 0.33

Smell 0.55 (Coffee)

Loudness 0.6

Temperature 1.0 (Cold)

Taste 1.3 (Salt)

Heaviness 1.45

Electric Shock 3.5

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Stevens’ Power Law

Experimental results for b:

Length.9 to 1.1

Area .6 to .9

Volume .5 to .8

Heuristic: b ~ 1/sqrt(dimensionality)

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Stevens’ Power Law

• Apparent magnitude scaling

[Cartography: Thematic Map Design, p. 170, Dent, 96]

S = 0.98A0.87

[J. J. Flannery, The relative effectiveness of some graduated point symbols in the presentation of quantitative data, Canadian Geographer, 8(2), pp. 96-109, 1971] [slide from Pat Hanrahan]

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Relative Magnitude Estimation

Most accurate

Least accurate

Position (common) scale

Position (non-aligned) scale

Length

Slope

Angle

Area

Volume

Color (hue/saturation/value)

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Change Blindness

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Change Blindness

• An interruption in what is being seen causes us to miss significant changes that occur in the scene during the interruption.

• Demo from Ron Rensink: http://www.psych.ubc.ca/~rensink/flicker/

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Possible Causes of Change Blindness

[Simons, D. J. (2000), Current approaches to change blindness, Visual Cognition, 7, 1-16. ]

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Multiple Visual Attributes

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The Game of Set

• Color• Symbol• Number• Shading

A set is 3 cards such that each feature is EITHER the same on each card OR is different on each card.

[Set applet by Adrien Treuille, http://www.cs.washington.edu/homes/treuille/resc/set/]

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Multiple Visual Attributes

• Integral vs. separable Integral dimensions

two or more attributes of an object are perceived holistically (e.g.width and height of rectangle).

Separable dimensions

judged separately, or through analytic processing (e.g.

diameter and color of ball).

• Separable dimensions are orthogonal.• For example, position is highly separable from

color. In contrast, red and green hue perceptions tend to interfere with each other.

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Integral vs. Separable Dimensions

Integral

Separable[Ware 2000]

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Gestalt

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Gestalt Principles

• figure/ground• proximity• similarity• symmetry• connectedness• continuity• closure• common fate• transparency

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Examples

Figure/Ground

[http://www.aber.ac.uk/media/Modules/MC10220/visper07.html]

ProximityConnectedness

[from Ware 2004]

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Conclusion

What is currently known about visual perception can aid the design process.

Understanding low-level mechanisms of the visual processing system and using that knowledge can result in improved displays.