Psy280: Perception

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

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

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

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

5. COLOUR: ATTRIBUTES THIS IS NOT RED! It is 690nm • Colours don’t exist – they’re in our heads! • Psychological property • Interaction: physical light - nervous system • There are no color, just wavelengths…

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

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

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 • Different sensory system would result in different rainbow

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

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

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

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

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

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

15. Trichromatic theory of colour perception • Trichromatic theory (cont’d) • 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

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

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

18. Metamers • Lights that are physically different can look identical • How so? • Ratio of excitation across receptors is = • Same colour despite different wavelengths • Explains colour-matching experiment • 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

19. Principle of univariance • Do we need 3 receptors? • What about 1? • NO, not possible due to principle of univariance • 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 • What about 2? • YES but fewer colours (see text) • More confusion btwn colours

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

21. Afterimages

22. Afterimages and simultaneous colour contrast • Colour opposites

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

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

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

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

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

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

29. 10 minute break

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

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

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

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

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

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

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

37. Monocular cues: Relative height and shadows

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

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

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

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

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

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

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

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

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

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

48. Magic eye: Autostereograms

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

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