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Matthias Neimier

Lecture 5- The Perception of Color and – Motion We see A LOT of colours—millions of them Basic Principles of Colour Perception
 • Trichromacy
 • Opponent Processes
 • Does Everyone See Colours the Same Way? (some see green as blue and vice versa) • From the Colour of Lights to a World of Colour  Colour: Not a physical property but rather a psychophysical property – Most of the light we see is reflected
 – Typical light sources: Sun, light bulb; emit a broad spectrum of wavelengths – 400–700 nm  Greenfilter: nose and stars appear~ equally bright but actually are very different in colour.  Problem of univariance: An infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor – One type of photoreceptor cannot make colour discriminations based on wavelength Only the response of a receptor will tell us something about what light we are looking at. However, ... • the out put of one cone is completely ambiguous • There aren‟t red/green/blue cones!  Scotopic: Referring to dim light levels at or below the level of bright moonlight – Rods are sensitive to scotopic light levels – All rods contain same type of photopigment 
 molecule: Rhodopsin – All rods have same sensitivity to wavelength, making it impossible to discriminate light Newton: prisms breakup sunlight into spectral components (and put it back again). (Colour not a physical property, must be a form of sensation colour is something we make up ) Young-Helmholtz(-Maxwell) theory: theory of trichromatic colour vision. Colour vision is based on 3 photoreceptors sensitive to particular ranges of wavelengths  Maxwell‟s colour-matching technique (whenever you have a lightsource with some sort of hue, a particular wave length, taking 3 diff light sources and superimposing them onto one another, you‟ll always be able to get it to a blue light)  Cone photo receptors: Three varieties – S-cones: Cones that are preferentially sensitive to 
 short wavelengths, 440 nm (blue cones) – M-cones: Cones that are preferentially sensitive to middle wavelengths, 535 nm (green cones) – L-cones: Cones that are preferentially sensitive to long wavelengths, 565 nm (red cones)  With three cone types we can tell the difference between lights of different wavelengths  We seldom see one wavelength at a time.  How do cones respond to a broad range of wavelengths?  (Let‟s ignore S-cones for now.)  Red and green light if mixed together in the right proportion will stimulate L- and M-cones the same as yellow light, i.e.,it looks like yellow light  Metamers: any pair of stimuli that are perceived as identical in spite of physical differences. In terms of light: different mixtures of wavelengths that look identical (one light source broken apart is just one colour)  Additive colour mixture: A mixture of lights. If light A and light B are both reflected from a surface to the eye, in the perception of colour, the effects of those two lights add together  This is what happens when mixing light with different colours.  But what happens if we mix differently coloured paints? Red + green = ? It looks yellow to us, because the responses of the m and l cones are just the same. Mixing coloured light and paint together, it becomes brighter. If you take out colour, it becomes darker.  Subtractive colour mixture: A mixture of pigments. If pigments A and B mix, some of the light shining on the surface will be subtracted by A, and some by B. Only the remainder contributes to the perception of colour  Colourspace is 3D because we have three cones – Hue: Chromatic aspect of color
 – Saturation: Chromatic strength of a hue
 – Brightness: Distance from black in colour space  Non-spectral hues: hues that don‟t exist as pure forms of light but only as mixtures of different wavelengths  ex. Magenta is not a pure colour: it‟s a mix of blue and orange  420nm+ 680nm stimulates L-and S-cones but not M-cones  Hering‟s idea about some colours being “illegal”, (e.g., reddish green, or bluish yellow)  Opponent colour theory: The theory that perception of colour is based on the output of three mechanisms, each of them on an opponency between two colours; red–green, blue– yellow, and black–white  In the dark, we have rods  Psychophysical support for the Opponent Colour Theory  Afterimage: A visual image seen after the stimulus has been removed (ex. green spots on a white background you have the afterimage that is red) Negative Afterimage: [picture] > A black and white castle looks like a bit of green, after looking at the green picture before that Opponent Processes  Neurophysiological support for the Opponent Colour Theory: – LGN has colour-opponent cells: neurons whose output is based on a difference between sets of cones  Evidence for colourprocessesafterLGN – e.g., L-M cell: red/green Actually: red – bluish green  ColoursysteminV1:blobs
  Coloursystemin V2: thin stripes  Zeki1993: humanV4=„colour area‟
  Achromatopsia: An inability to perceive colors that is due to damage to the central nervous system  Does everyone see colours the sameway?–Y..ES! – General agreement on colours
 – Same metameres.
 – Some variation due to age (the corneas turn a little bit yellow); so as we age, we see things more yellow-ish Does everyone see colors the same way? –NO! – About 8% of male population, 0.5% of female population have some form of colour vision deficiency: Colour blindness – Ishihara test  Two types of colour blind people
 – Cone monochromat: Only one cone type; truly colour-blind – Rod monochromat: No cones of any type; truly colour-blind, badly visually impaired in bright light > They will be overwhelmed with so much light. They are normal during night, but During daylight, they have to wear sunglasses , because it‟s just too bright for them  3 types of colour-anomalous people: 1. Deuteranope: no M-cones
 2. Protanope: no L-cones
 3. Tritanope: no S-cones  Does every one see colours the same way? –Maybe • Various cultures describe colours differently • English: 11 colour terms (strict definition) • Other languages have different numbers, e.g. 2/3 – Idea of cultural relativism Brown light is actually orange light!  the brown circle turned orange once the background turned dark Dark orange= brown  Colour constancy
  Illuminants and reflectance – can an
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