1

Psy280: Psy280: PerceptionPerception

Prof. AndersonProf. Anderson

Department of PsychologyDepartment of Psychology

Vision 6Vision 6

Colour, depth and sizeColour, depth and size

2

Need for colourNeed for colour

Some tasks are impossible without itSome tasks are impossible without it

Can you find the word?Can you find the word?

3

CCOOLLOOUURR: What's it for?: What's it for? Identification / Identification /

discrimination discrimination

Detection (non-detection)Detection (non-detection) DetectionDetection

Potential mates, enemies, Potential mates, enemies, preyprey

CamouflageCamouflage

4

What are colours?What are colours? Light varies in both intensity and wavelengthLight varies in both intensity and wavelength Light of different wavelengths appear as different Light of different wavelengths appear as different

colourscolours

5

CCOOLLOOUURR: ATTRIBUTES: ATTRIBUTES

THIS IS NOT RED!It is 690nm

Colours don’t exist – they’re in our heads!Colours don’t exist – they’re in our heads! Psychological propertyPsychological property Interaction: physical light - nervous systemInteraction: physical light - nervous system There are no color, just wavelengths…There are no color, just wavelengths…

6

Newton’s dorm room Newton’s dorm room experimentexperiment

Light through prism Light through prism = rainbow= rainbow Why?Why?

Diff wavelengths have diff refractory Diff wavelengths have diff refractory propertiesproperties

Long (red) bent least, short (blue) mostLong (red) bent least, short (blue) most

7

CCOOLLOOUURR: ATTRIBUTES: ATTRIBUTESIsaac NewtonIsaac Newton (1666): “colour” of light. (1666): “colour” of light. White light (sunlight) = sum of White light (sunlight) = sum of

componentscomponents Individual component = different colour Individual component = different colour

exp.exp. Colour = wavelengths subtracted from Colour = wavelengths subtracted from

lightlight

8

Redux:Redux:Do wavelengths have Do wavelengths have

colour?colour? ““The Rays to speak properly are not coloured. In The Rays to speak properly are not coloured. In

them there is nothing else than a certain Power and them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Disposition to stir up a Sensation of this or that Colour…” NewtonColour…” Newton

Different sensory system would result in different Different sensory system would result in different rainbowrainbow

9

SHORTSHORT400-450nm violet400-450nm violet450-490nm blue450-490nm blue

MEDIUMMEDIUM500-575nm 500-575nm

greengreen575-590nm yellow575-590nm yellow

LONGLONG590-620nm 590-620nm

orangeorange620-700 red620-700 red

10

See objects = light reflected from themSee objects = light reflected from them Reflectance curveReflectance curve

Achromatic colour: equal reflectance across wavelengthsAchromatic colour: equal reflectance across wavelengths White, black, greyWhite, black, grey

Chromatic colour: selective reflectance across wavelengthsChromatic colour: selective reflectance across wavelengths

Spectral reflectance Spectral reflectance curvescurves

11

Different light sources have differing spectral composition Different light sources have differing spectral composition Sunlight: White Sunlight: White light bulb: Yellow/redlight bulb: Yellow/red

Not all light the sameNot all light the same

12

Additive and Additive and subtractive mixingsubtractive mixing

Lights mix additivelyLights mix additively more wavelengths = closer to white (like sunlight)more wavelengths = closer to white (like sunlight)

Pigments mix subtractivelyPigments mix subtractively more wavelengths = closer to blackmore wavelengths = closer to black

AdditiveAdditive SubtractiveSubtractive

B & Y commonly reflect greenB & Y commonly reflect green

Y absorbs BY absorbs BB absorbs YB absorbs Y

13

How many colours can How many colours can we perceive?we perceive?

~2,000,000= 200 hues x 500 brightness ~2,000,000= 200 hues x 500 brightness levels x 20 saturations levelslevels x 20 saturations levels

Hue Hue wavelengthwavelength

Brightness Brightness amplitude of wave = intensityamplitude of wave = intensity # of photons# of photons

SaturationSaturation Degree of whiteDegree of white REDRED vs vs PINKPINK

14

Trichromatic theory of Trichromatic theory of colour perceptioncolour perception

2 theories from the 1800s based on 2 theories from the 1800s based on psychophysical datapsychophysical data

Trichromatic theory of colour vision Trichromatic theory of colour vision

Young and von HelmholtzYoung and von Helmholtz Colour-matching experimentsColour-matching experiments

Mix 3 Mix 3 pure pure lights (420, 560, 640) lights (420, 560, 640) until matches another light (500nm)until matches another light (500nm)

Conclusions: able to duplicate colour Conclusions: able to duplicate colour by adjusting proportionby adjusting proportion

15

Trichromatic theory of Trichromatic theory of colour perceptioncolour perception

Trichromatic theory (cont’d)Trichromatic theory (cont’d)

Colour vision depends on 3 receptor Colour vision depends on 3 receptor mechanisms with different spectral mechanisms with different spectral sensitivitiessensitivities

Particular wavelength stimulates 3 Particular wavelength stimulates 3 mechanisms to different degrees and mechanisms to different degrees and pattern of activity in 3 mech = pattern of activity in 3 mech = perception of colourperception of colour

16

Trichromatic theory: Trichromatic theory: PhysiologyPhysiology

Physiology – a century later…Physiology – a century later… 3 cone visual pigments with different 3 cone visual pigments with different

absorption:absorption: Short: 419nmShort: 419nm Middle 531nmMiddle 531nm Long: 558nm Long: 558nm

17

Colour: Its all in the ratioColour: Its all in the ratio

Perception of colour depends upon ratio of excitation Perception of colour depends upon ratio of excitation across receptorsacross receptors

18

MetamersMetamers

Lights that are physically different can look Lights that are physically different can look identicalidentical

How so?How so? Ratio of excitation across receptors is =Ratio of excitation across receptors is = Same colour despite different wavelengthsSame colour despite different wavelengths

Explains colour-matching experimentExplains colour-matching experiment Although both lights have different wavelengths, Although both lights have different wavelengths,

they perceptually look the samethey perceptually look the same Metamers look the same because generate same Metamers look the same because generate same

activation responses in 3 types of cone receptorsactivation responses in 3 types of cone receptors

19

Principle of univariancePrinciple of univariance Do we need 3 receptors?Do we need 3 receptors? What about 1?What about 1?

NO, not possible due to NO, not possible due to principle of univarianceprinciple of univariance

Varying intensity (# of Varying intensity (# of photons) can allow to have photons) can allow to have same # of isomerized same # of isomerized molecules of pigmentsmolecules of pigments

This is why we don’t see This is why we don’t see colour in dim light, because colour in dim light, because rely on one ROD pigmentrely on one ROD pigment

What about 2?What about 2? YES but fewer colours (see YES but fewer colours (see

text)text) More confusion btwn More confusion btwn

colourscolours

20

Opponent process theory of Opponent process theory of colourcolour

Ewald HeringEwald Hering Opposing responses generated by blue Opposing responses generated by blue

and yellow and by red and green.and yellow and by red and green. Phenomenological observationsPhenomenological observations

AfterimagesAfterimages Simultaneous color contrastSimultaneous color contrast Can’t picture reddish-green or bluish-yellowCan’t picture reddish-green or bluish-yellow Colour-blind: red+green; blue-yellowColour-blind: red+green; blue-yellow

21

AfterimagesAfterimages

22

Afterimages and Afterimages and simultaneous colour simultaneous colour

contrastcontrast Colour oppositesColour opposites

23

Opponent process: Opponent process: Colour appearanceColour appearance

Rating of Rating of colour colour experience for experience for different different wavelengthswavelengths

Little co-Little co-occurrenceoccurrence

Reddish-greenReddish-green Bluish-yellowBluish-yellow

24

Opponent-theoryOpponent-theory

3 mechanisms: respond in opposite 3 mechanisms: respond in opposite ways to intensity and wavelengthways to intensity and wavelength Black (-) | white (+)Black (-) | white (+) Red (+) | Green (-)Red (+) | Green (-) Blue (-) | Yellow (+)Blue (-) | Yellow (+)

25

Physiology: Opponent Physiology: Opponent neurons in retina and LGNneurons in retina and LGN

Signals from cones are transformed early.

M retinal ganglion cells are achromatic • dark - light

P retinal ganglion: centre / surround are sensitive to different wavelengths of light

• red – green • blue - yellow

26

Architecture of opponent Architecture of opponent cellscells

Dual process Dual process theorytheory

L + M – L + M –

S+ A- (sum M&L)S+ A- (sum M&L)

27

Colour and lightness Colour and lightness constancyconstancy

Pure wavelength information Pure wavelength information insufficient to explain colour insufficient to explain colour perceptionperception

Luminance insufficient to Luminance insufficient to explain lightnessexplain lightness

28

Wavelengths and colour Wavelengths and colour perceptionperception

V1 Selective for the wavelength of light However, precise wavelength of light

often bears little relationship to the perceived colour

V4 Neurons behave as if they are responding

to colours as seen by human observers

29

10 minute break10 minute break

30

DepthDepth

Of feeling? Knowledge?Of feeling? Knowledge? SpaceSpace

3D world —>2D projection on retina3D world —>2D projection on retina—> 3D perception—> 3D perception Need to “reconstruct” 3D worldNeed to “reconstruct” 3D world

31

Flatland: Flatland: A romance of many A romance of many

dimensionsdimensions Edwin Abbott (1884)Edwin Abbott (1884)

A point, a line, a cubeA point, a line, a cube

32

How do we reconstruct How do we reconstruct depth?depth?

3 3 sources of informationsources of information Extraretinal oculomotor cuesExtraretinal oculomotor cues

Physiological/muscular feedbackPhysiological/muscular feedback Monocular cuesMonocular cues

Pictorial Pictorial Can be recovered from one eyeCan be recovered from one eye Lots of themLots of them

BinocularBinocular DisparityDisparity 2 eyes, 2 views of the world2 eyes, 2 views of the world

33

OculomotorOculomotor

Afferent feedback from Afferent feedback from bodybody

VergenceVergence ““Convergence”Convergence” Degree of crossing as eyes Degree of crossing as eyes

fixatefixate Near vs farNear vs far

AccomodationAccomodation Stretching of lens to focus Stretching of lens to focus

light on retinalight on retina

34

Monocular depthMonocular depth

Are 2 eyes better Are 2 eyes better than 1?than 1? YesYes

Are 3 eyes better Are 3 eyes better than 2?than 2?

Not many one eyed Not many one eyed or three eyed or three eyed creaturescreatures

Nonetheless, can see Nonetheless, can see depth with 1 eyedepth with 1 eye

35

Monocular cues: Monocular cues: Linear perspectiveLinear perspective

Parallel lines converge with distanceParallel lines converge with distance Converge at vanishing point Converge at vanishing point

(horizon)(horizon)

36

Monocular cues: Monocular cues: familiarity and relative familiarity and relative

sizesize 2 objects are of 2 objects are of

equal size equal size (familiarity)(familiarity)

Smaller retinal Smaller retinal projection—>further projection—>further awayaway

World: Same size Retina: Different size

37

Monocular cues: Monocular cues: Relative height and Relative height and

shadowsshadows

38

Monocular cues: Monocular cues: OcclusionOcclusion

Layers of depth stretching out to Layers of depth stretching out to horizonhorizon

39

Monocular cues: Monocular cues: Atmospheric blur and Atmospheric blur and

depth of focusdepth of focus BlurrinessBlurriness

HazeHaze

Depth of Depth of focusfocus In front and In front and

behind of behind of fixationfixation

40

Monocular cues: Monocular cues: Combine to form depthCombine to form depth

Occlusion, relative height, and Occlusion, relative height, and shadowsshadows

Impossible: Conflicting cues

41

Monocular cues: Monocular cues: Dynamics cuesDynamics cues

Motion parallaxMotion parallax Velocity = distance/timeVelocity = distance/time

Km/hourKm/hour As observer movesAs observer moves

Objects closer move fasterObjects closer move faster Greater distance across retinaGreater distance across retina

Objects further move slowerObjects further move slower E.g. looking out a train E.g. looking out a train

windowwindow

42

Why have two eyes?Why have two eyes? Not just more = betterNot just more = better

Shared field of view (FOV)Shared field of view (FOV) 2 overlapping but distinct visions of the world2 overlapping but distinct visions of the world Sacrifice: 360 degree FOVSacrifice: 360 degree FOV Gain: depth through horizontal disparityGain: depth through horizontal disparity

Predators (overlap) vs prey (larger FOV)Predators (overlap) vs prey (larger FOV)

Binocular cues: Binocular cues: StereopsisStereopsis

No overlap

Substantial overlap

43

Stare at your thumbStare at your thumb

One eye at a timeOne eye at a time Thumb moves side by Thumb moves side by

sideside Horizontal disparity Horizontal disparity

2 very different 2 very different perspectives on worldperspectives on world

Vertical disparity?Vertical disparity?

44

HoropterHoropter Fixate on an objectFixate on an object An imaginary sphere that An imaginary sphere that

defines corresponding defines corresponding points on the retinaspoints on the retinas

Zero disparityZero disparity Uncrossed disparityUncrossed disparity

Nasal of foveaNasal of fovea Further in depthFurther in depth

Crossed disparityCrossed disparity Temporal of foveaTemporal of fovea Closer in depthCloser in depth

Horopter: Horopter: An isodepth sphereAn isodepth sphere

RetinasUncrossed

Crossed

Fixation/

zero disparity

45

Remember? LGN retinal layers

Organization of LGN: Retinotopy

6 representations of retina in register

46

How do we know How do we know steropsis produces depth steropsis produces depth

perception?perception? Depth perception may depend solely Depth perception may depend solely

on “knowledge”on “knowledge” Monocular cuesMonocular cues

Occlusion, familiarity etc.Occlusion, familiarity etc.

Retinal disparity vs knowledgeRetinal disparity vs knowledge Depth without awareness of form?Depth without awareness of form?

47

Random dot stereogramsRandom dot stereograms StereoscopeStereoscope

WheatstoneWheatstone

Crossed

StereoscopeStereoscope L & R eye shown L & R eye shown

separate imagesseparate images Random dots with Random dots with

invisible invisible disparitiesdisparities

Disparity alone Disparity alone can result in depthcan result in depth

48

Magic eye: Magic eye: AutostereogramsAutostereograms

49

3D movies: Anaglyphs3D movies: Anaglyphs

Color filters project Color filters project different images to different images to each eyeeach eye

50

Disparity representations Disparity representations in the brainin the brain

Can’t happen at the ganglion cell Can’t happen at the ganglion cell layerlayer

V1 ocular dominance columnsV1 ocular dominance columns V1 has neurons tuned to retinal V1 has neurons tuned to retinal

disparitiesdisparities

51

Part 2: Perceiving SizePart 2: Perceiving Size Not as simple as size of stimulus on retinaNot as simple as size of stimulus on retina Visual angle: retinal projection depends on Visual angle: retinal projection depends on

distancedistance Different physical Different physical sizesize Same retinalSame retinalProjectionProjection

Bigger stimulus further awayBigger stimulus further away = closer smaller stimulus= closer smaller stimulus

52

Size constancySize constancy

Perception of size remains constantPerception of size remains constant Despite different visual angle/retinal Despite different visual angle/retinal

sizesize

53

Size distance scalingSize distance scaling

Perceived size = Perceived size = retinal imageretinal image sizesize X X distance from objectdistance from object

Without depth informationWithout depth information Perceived size = Perceived size = retinal image sizeretinal image size

54

Emmert’s lawEmmert’s law

Perceived size of an after image Perceived size of an after image depends on depth perceptiondepends on depth perception

55

Size-depth illusionsSize-depth illusions Moon appears larger on the horizon than Moon appears larger on the horizon than

the skythe sky Same retinal sizeSame retinal size Difference in magnitude Difference in magnitude

estimationestimation Horizon provides depth cuesHorizon provides depth cues Sky does notSky does not

Appear flattenedAppear flattened

56

The endThe end