2. 1 Psy280: Perception Prof. Anderson Department of Psychology Vision 6 Colour, depth and size

3. 2 Need for colour Some tasks are impossible without it Some tasks are impossible without it Can you find the word?

4. 3 COLOUR: What's it for? Identification / discrimination Identification / discrimination Detection (non-detection) Detection (non-detection) Detection Detection Potential mates, enemies, prey Potential mates, enemies, prey Camouflage Camouflage

5. 4 What are colours? Light varies in both intensity and wavelength Light varies in both intensity and wavelength Light of different wavelengths appear as different colours Light of different wavelengths appear as different colours

6. 5 COLOUR: 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 property Psychological property Interaction: physical light - nervous system Interaction: physical light - nervous system There are no color, just wavelengths… There are no color, just wavelengths…

7. 6 Newton’s dorm room experiment Light through prism Light through prism = rainbow Why? Why? Diff wavelengths have diff refractory properties Diff wavelengths have diff refractory properties Long (red) bent least, short (blue) most Long (red) bent least, short (blue) most

8. 7 COLOUR: ATTRIBUTES Isaac Newton (1666): “colour” of light.  White light (sunlight) = sum of components  Individual component = different colour exp.  Colour = wavelengths subtracted from light

9. 8 Redux: Do wavelengths have colour? “The Rays to speak properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour…” Newton “The Rays to speak properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour…” Newton Different sensory system would result in different rainbow Different sensory system would result in different rainbow

10. 9 SHORT 400-450nm violet 450-490nm blue MEDIUM 500-575nm green 575-590nm yellow LONG 590-620nm orange 620-700 red

11. 10 See objects = light reflected from them See objects = light reflected from them Reflectance curve Reflectance curve Achromatic colour: equal reflectance across wavelengths Achromatic colour: equal reflectance across wavelengths White, black, grey White, black, grey Chromatic colour: selective reflectance across wavelengths Chromatic colour: selective reflectance across wavelengths Spectral reflectance curves

12. 11 Different light sources have differing spectral composition Different light sources have differing spectral composition Sunlight: White Sunlight: White light bulb: Yellow/red light bulb: Yellow/red Not all light the same

13. 12 Additive and subtractive mixing Lights mix additively Lights mix additively more wavelengths = closer to white (like sunlight) more wavelengths = closer to white (like sunlight) Pigments mix subtractively Pigments mix subtractively more wavelengths = closer to black more wavelengths = closer to black Additive Subtractive B & Y commonly reflect green Y absorbs B B absorbs Y

14. 13 How many colours can we perceive? ~2,000,000= 200 hues x 500 brightness levels x 20 saturations levels ~2,000,000= 200 hues x 500 brightness levels x 20 saturations levels Hue Hue wavelength wavelength Brightness Brightness amplitude of wave = intensity amplitude of wave = intensity # of photons # of photons Saturation Saturation Degree of white Degree of white RED vs PINK RED vs PINK

15. 14 Trichromatic theory of colour perception 2 theories from the 1800s based on psychophysical data 2 theories from the 1800s based on psychophysical data Trichromatic theory of colour vision Trichromatic theory of colour vision Young and von Helmholtz Colour-matching experiments Colour-matching experiments Mix 3 pure lights (420, 560, 640) until matches another light (500nm) Mix 3 pure lights (420, 560, 640) until matches another light (500nm) Conclusions: able to duplicate colour by adjusting proportion Conclusions: able to duplicate colour by adjusting proportion

16. 15 Trichromatic theory of colour perception Trichromatic theory (cont’d) Trichromatic theory (cont’d) Colour vision depends on 3 receptor mechanisms with different spectral sensitivities Colour vision depends on 3 receptor mechanisms with different spectral sensitivities Particular wavelength stimulates 3 mechanisms to different degrees and pattern of activity in 3 mech = perception of colour Particular wavelength stimulates 3 mechanisms to different degrees and pattern of activity in 3 mech = perception of colour

17. 16 Trichromatic theory: Physiology Physiology – a century later… Physiology – a century later… 3 cone visual pigments with different absorption: 3 cone visual pigments with different absorption: Short: 419nm Short: 419nm Middle 531nm Middle 531nm Long: 558nm Long: 558nm

18. 17 Colour: Its all in the ratio Perception of colour depends upon ratio of excitation across receptors Perception of colour depends upon ratio of excitation across receptors

19. 18 Metamers Lights that are physically different can look identical Lights that are physically different can look identical How so? How so? Ratio of excitation across receptors is = Ratio of excitation across receptors is = Same colour despite different wavelengths Same colour despite different wavelengths Explains colour-matching experiment Explains colour-matching experiment Although both lights have different wavelengths, they perceptually look the same Although both lights have different wavelengths, they perceptually look the same Metamers look the same because generate same activation responses in 3 types of cone receptors Metamers look the same because generate same activation responses in 3 types of cone receptors

20. 19 Principle of univariance Do we need 3 receptors? Do we need 3 receptors? What about 1? What about 1? NO, not possible due to principle of univariance NO, not possible due to principle of univariance Varying intensity (# of photons) can allow to have same # of isomerized molecules of pigments Varying intensity (# of photons) can allow to have same # of isomerized molecules of pigments This is why we don’t see colour in dim light, because rely on one ROD pigment This is why we don’t see colour in dim light, because rely on one ROD pigment What about 2? What about 2? YES but fewer colours (see text) YES but fewer colours (see text) More confusion btwn colours More confusion btwn colours

21. 20 Opponent process theory of colour Ewald Hering Ewald Hering Opposing responses generated by blue and yellow and by red and green. Opposing responses generated by blue and yellow and by red and green. Phenomenological observations Phenomenological observations Afterimages Afterimages Simultaneous color contrast Simultaneous color contrast Can’t picture reddish-green or bluish-yellow Can’t picture reddish-green or bluish-yellow Colour-blind: red+green; blue-yellow Colour-blind: red+green; blue-yellow

22. 21 Afterimages

23. 22 Afterimages and simultaneous colour contrast Colour opposites Colour opposites

24. 23 Opponent process: Colour appearance Rating of colour experience for different wavelengths Rating of colour experience for different wavelengths Little co-occurrence Reddish-green Reddish-green Bluish-yellow Bluish-yellow

25. 24 Opponent-theory 3 mechanisms: respond in opposite ways to intensity and wavelength 3 mechanisms: respond in opposite ways to intensity and wavelength Black (-) | white (+) Black (-) | white (+) Red (+) | Green (-) Red (+) | Green (-) Blue (-) | Yellow (+) Blue (-) | Yellow (+)

26. 25 Physiology: Opponent neurons 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

27. 26 Architecture of opponent cells Dual process theory Dual process theory L + M – S+ A- (sum M&L)

28. 27 Colour and lightness constancy Pure wavelength information insufficient to explain colour perception Pure wavelength information insufficient to explain colour perception Luminance insufficient to explain lightness Luminance insufficient to explain lightness

29. 28 Wavelengths and colour perception 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

30. 29 10 minute break

31. 30 Depth Of feeling? Knowledge? Of feeling? Knowledge? Space Space 3D world —>2D projection on retina—> 3D perception 3D world —>2D projection on retina—> 3D perception Need to “reconstruct” 3D world Need to “reconstruct” 3D world

32. 31 Flatland: A romance of many dimensions Edwin Abbott (1884) Edwin Abbott (1884) A point, a line, a cube A point, a line, a cube

33. 32 How do we reconstruct depth? 3 sources of information 3 sources of information Extraretinal oculomotor cues Extraretinal oculomotor cues Physiological/muscular feedback Physiological/muscular feedback Monocular cues Monocular cues Pictorial Pictorial Can be recovered from one eye Can be recovered from one eye Lots of them Lots of them Binocular Binocular Disparity Disparity 2 eyes, 2 views of the world 2 eyes, 2 views of the world

34. 33 Oculomotor Afferent feedback from body Afferent feedback from body Vergence Vergence “Convergence” “Convergence” Degree of crossing as eyes fixate Degree of crossing as eyes fixate Near vs far Near vs far Accomodation Accomodation Stretching of lens to focus light on retina Stretching of lens to focus light on retina

35. 34 Monocular depth Are 2 eyes better than 1? Are 2 eyes better than 1? Yes Yes Are 3 eyes better than 2? Are 3 eyes better than 2? Not many one eyed or three eyed creatures Not many one eyed or three eyed creatures Nonetheless, can see depth with 1 eye Nonetheless, can see depth with 1 eye

36. 35 Monocular cues: Linear perspective Parallel lines converge with distance Parallel lines converge with distance Converge at vanishing point (horizon) Converge at vanishing point (horizon)

37. 36 Monocular cues: familiarity and relative size 2 objects are of equal size (familiarity) 2 objects are of equal size (familiarity) Smaller retinal projection—>further away Smaller retinal projection—>further away World: Same size Retina: Different size

38. 37 Monocular cues: Relative height and shadows

39. 38 Monocular cues: Occlusion Layers of depth stretching out to horizon Layers of depth stretching out to horizon

40. 39 Monocular cues: Atmospheric blur and depth of focus Blurriness Blurriness Haze Haze Depth of focus Depth of focus In front and behind of fixation In front and behind of fixation

41. 40 Monocular cues: Combine to form depth Occlusion, relative height, and shadows Occlusion, relative height, and shadows Impossible: Conflicting cues

42. 41 Monocular cues: Dynamics cues Motion parallax Motion parallax Velocity = distance/time Velocity = distance/time Km/hour Km/hour As observer moves As observer moves Objects closer move faster Objects closer move faster Greater distance across retina Greater distance across retina Objects further move slower Objects further move slower E.g. looking out a train window E.g. looking out a train window

43. 42 Why have two eyes? Why have two eyes? Not just more = better Not just more = better Shared field of view (FOV) Shared field of view (FOV) 2 overlapping but distinct visions of the world 2 overlapping but distinct visions of the world Sacrifice: 360 degree FOV Sacrifice: 360 degree FOV Gain: depth through horizontal disparity Gain: depth through horizontal disparity Predators (overlap) vs prey (larger FOV) Predators (overlap) vs prey (larger FOV) Binocular cues: Stereopsis No overlap Substantial overlap

44. 43 Stare at your thumb One eye at a time One eye at a time Thumb moves side by side Thumb moves side by side Horizontal disparity Horizontal disparity 2 very different perspectives on world 2 very different perspectives on world Vertical disparity? Vertical disparity?

45. 44 Horopter Horopter Fixate on an object Fixate on an object An imaginary sphere that defines corresponding points on the retinas An imaginary sphere that defines corresponding points on the retinas Zero disparity Zero disparity Uncrossed disparity Uncrossed disparity Nasal of fovea Nasal of fovea Further in depth Further in depth Crossed disparity Crossed disparity Temporal of fovea Temporal of fovea Closer in depth Closer in depth Horopter: An isodepth sphere Retinas Uncrossed Crossed Fixation/ zero disparity

46. 45 Remember? LGN retinal layers Organization of LGN: Retinotopy 6 representations of retina in register

47. 46 How do we know steropsis produces depth perception? Depth perception may depend solely on “knowledge” Depth perception may depend solely on “knowledge” Monocular cues Monocular cues Occlusion, familiarity etc. Occlusion, familiarity etc. Retinal disparity vs knowledge Retinal disparity vs knowledge Depth without awareness of form? Depth without awareness of form?

48. 47 Random dot stereograms Stereoscope Stereoscope Wheatstone Wheatstone Crossed Stereoscope Stereoscope L & R eye shown separate images L & R eye shown separate images Random dots with invisible disparities Random dots with invisible disparities Disparity alone can result in depth Disparity alone can result in depth

49. 48 Magic eye: Autostereograms

50. 49 3D movies: Anaglyphs Color filters project different images to each eye Color filters project different images to each eye

51. 50 Disparity representations in the brain Can’t happen at the ganglion cell layer Can’t happen at the ganglion cell layer V1 ocular dominance columns V1 ocular dominance columns V1 has neurons tuned to retinal disparities V1 has neurons tuned to retinal disparities

52. 51 Part 2: Perceiving Size Not as simple as size of stimulus on retina Not as simple as size of stimulus on retina Visual angle: retinal projection depends on distance Visual angle: retinal projection depends on distance Different physical Different physicalsize Same retinal Same retinalProjection Bigger stimulus further away Bigger stimulus further away = closer smaller stimulus = closer smaller stimulus

53. 52 Size constancy Perception of size remains constant Perception of size remains constant Despite different visual angle/retinal size Despite different visual angle/retinal size

54. 53 Size distance scaling Perceived size = retinal image size X distance from object Perceived size = retinal image size X distance from object Without depth information Without depth information Perceived size = retinal image size Perceived size = retinal image size

55. 54 Emmert’s law Perceived size of an after image depends on depth perception Perceived size of an after image depends on depth perception

56. 55 Size-depth illusions Moon appears larger on the horizon than the sky Moon appears larger on the horizon than the sky Same retinal size Same retinal size Difference in magnitude Difference in magnitudeestimation Horizon provides depth cues Horizon provides depth cues Sky does not Sky does not Appear flattened Appear flattened

57. 56 The end