Lecture 5- The Perception of Color and – Motion
We see A LOT of colours—millions of them
Basic Principles of Colour Perception
• 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.
• 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
– 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
– 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
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
Neurophysiological support for the Opponent Colour Theory:
– LGN has colour-opponent cells: neurons whose output is based on a difference between sets of
Evidence for colourprocessesafterLGN
– e.g., L-M cell: red/green Actually: red – bluish green
Coloursystemin V2: thin stripes
Zeki1993: humanV4=„colour area‟
Achromatopsia: An inability to perceive colors that is due to damage to the central nervous
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
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
> 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?
• 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 orange= brown
Illuminants and reflectance
– can an