Lecture 6 - Chapter 7 Colour Vision

1. Lecture 6 - Chapter 7 Colour Vision Stimulus (what is colour?) The neural code for colour Theories of colour vision Disorders of colour vision Colour constancy Lightness constancy

3. Functions of colour vision: Detection of objects - ie. Picking berries Discrimination of objects - ie. Making sure you pick ripe berries.

6. Other Functions of colour vision: Grouping of objects - Gestalt grouping Recognitionof familiar objects Tanaka & Presnell, 1999

7. Describing colour experience • We can describe all the colours we can discriminate using just four words : red, yellow, green and blue. • If presented with many colours and asked to describe them, they can’t do it if one of the 4 colours is omitted. • So, red, yellow, blue and green are considered to be basic colours. • The basic colours can be arranged in a circle, and the ordering matches the ordering of the colours in the visible spectrum.

8. Colour circle

9. Visible spectrum • We can discriminate up to 200 hues across the length of the spectrum. • More colours can be made by changing saturation, the amount of white. • If you have more white in a colour, it is less saturated, and if you take white away it becomes more saturated.

10. Saturation more white is added as you move to the centre of the circle. Hue= chromatic colour, it changes as you move around the circle. There are about 1 million discriminable colours

11. Wavelength and colour perception

12. What makes an object appear coloured? Selective Reflection Reflectance is the percentage of light falling on an object that is reflected from it. Selective reflection is the reflection of different amounts of different wavelengths.

14. Selective Reflectance - Glacial ice

16. Selective transmission • Some objects are not ‘solid’ and allow light to pass through. • Some of these selectively transmit certain wavelengths. • For example, translucent objects (clear liquids) transmit light

17. Types of Colour… Chromatic - blue, red, green etc. these are seen when an object selectively reflects or transmits some wavelengths. Achromatic - white, black, shades of grey. These are seen when all wavelengths are reflected or transmitted equally.

18. Theories of colour vision: 1. Trichromatic Theory (Young & Helmholtz) Colour matching experiments Any colour can be matched using 3 wavelengths

19. Colour vision depends on 3 receptors with different spectral sensitivities. The pattern of activity across 3 receptors codes colour

20. Physiological Support for Young/Helmholtz 1. Identification of three types of cone pigments (70 yrs later)

21. Physiological support for Young/Helmholtz 2. Can predict colour perception based on pattern of activity in cones.

22. Colour mixing Additive - Add lights Subtractive - Mixing paint

23. Mixing light - Additive mixing

24. + = All that is reflected by the combination is M Blue pigment only reflects S + M Yellow pigment reflects M and some L Mixing paints - Subtractive mixing We can only perceive wavelengths that reach the eye

25. Metamers • If two combinations of wavelengths produce the same neural pattern, the observer will experience the same percept.

26. Are three receptors necessary? • Colour vision is possible with only 2 types of receptors, but not with just one type. • If you only had one type of receptor, then it would respond to light, but changing the wavelength would not change it’s response. • One receptor type couldn’t signal the difference in wavelength, it would only signal a difference in intensity. • This is called the principle of univariance. • When there is only one receptor/pigment you can make any two wavelengths create the same response, by adjusting their intensities.

27. Principle of Univariance

28. Principle of Univariance

29. Principle of Univariance

30. Trichromats • People with three cone types are called trichromats. • People with only two cone types are called dichromats. • Dichromats see colour, but not the full range experienced by trichromats. • Monochromats have only one cone type (or no cones). • There is a condition called unilateral dichromat, where only the cones in one eye are deficient. This allows the person to make comparisons that aren’t typically possible.

31. Colour Deficiencies Monocromatism - can match any colour using one other Extremely rare condition, usually lacking any cones. These individuals are truly colourblind, and see in shades of grey. This has implications for their vision during the daytime (sensitivity to glare). If they only have Rods, then their resolution is poor.

32. Dichromatism • Dichromats usually experience colour. • There are three main types of dichromatism: • Protanopia - perceive blues and yellows, with a neutral point (grey) around 492nm. Long wavelength cones missing. • Deuteranopia - similar to protanopia, but the neutral point is at 498 nm. Missing medium wavelength cones. • Tritanopia - can perceive green and red with a neutral point around 570nm. Likely missing short wavelength cones.

33. Colour deficiencies

34. Colour deficiencies

35. Triteranopia Protanopia Deuteranopia original

36. Red-green colour deficient people will see only spots in the 4 images on the right

37. Colour normal will see a 5, r-g deficient will see a 2

40. Theories of colour vision continued: • 2. Opponent-Process Theory (Hering) • Based on phenomenological observations of colour pairings • Afterimages • Simultaneous colour contrast • Colour descriptions Demo

41. Afterimages

43. Simultaneous contrast

45. Hering’s theory: Three mechanisms Each responds in opposite ways to different wavelengths or intensities of light. W+ B- Y+ B- R+ G-

46. Physiological Support for Hering R. DeValois found colour opponent cells in LGN These cells were organized: R+G- / R-G+, Y+B- / B+Y Many of these opponent cells were later identified in monkey cortex.

47. Opponent cells in LGN

48. Which model is correct BothThey just describe different levels of processing

49. Trichromatic and opponent processing combined

50. Colour vision in the cortex Is there a special area for colour in the cortex? Electrophysiology and Neuropsychology suggested ‘yes’ (see description of cerebral achromatopsia). Recent single-unit recording and fMRI studies say ‘no’.