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Chapter 4

Ch. 4 notes - Colour Perception.doc

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Christopher Teeter

Colour and Depth Perception • Perception of colour allows us to distinguish features of objects when borders and contours don’t • Processing of colour begins at the retina – 3 types of cones respond best to 3 diff. wavelengths of light: red, blue, and green – colour vision is trichromatic (3 kinds of cones) • Colour perception Is not an absolute process – there is an abstracting process that occurs • Colour is a constant property – the wavelength of light the objects reflects is its colour • Composition of wavelength is determined by its reflectance and the wavelength composition of the illuminating light (ex. incandescent bulbs have yellow light and fluorescent bulbs have blue light) – large variation in the wavelengths of light across these conditions • Banana still looks yellow on a cloudy day/ in a dimly lit room/under a fluorescent bulb – our colour vision corrects for the variation in the overall illumination is that an object’s colour appears about the same - called colour constancy Variations of Colour Vision • Humans, bees, and macaque monkeys are trichromats (3 types of cones) – bees are sensitive to UV light • Some animals (ex. goldfish, pigeons, and ducks) have eyes that contain 4 types of retinal receptors • Other animals (ex. rabbits, squirrels, and cats) are dichromats (2 types of cones) • Some are monochromats (lack chromatic vision entirely) – resolve objects based on differences in brightness and contrast only Colour Mixing • There are 2 ways we can combine colours together: 1. Additive (mixing lights) – process of overlapping red, green, and blue lights together – the combination of different mixtures will produce new colours of intermediate wavelength/white light if all 3 are combined 2. Subtractive (mixing pigments) – combining pigments of colours together (ex. paints/ink) – pigments reflect some wavelengths, but absorb all others (ex. red paint reflects long wavelengths and absorbs med. and short) – if all 3 colours are mixed together, each is absorbed/‘subtracted’ and it results in black (inkjet printers can produce this effect) Theories of Colour Vision • Human can match any colour by mixing light made up of wavelengths of the 3 primary colours (red, green and blue) • Thomas Young (1802) – our perception of colour must depend on the existence of 3 colour receptors and each is sensitive to a diff. wavelength of light • Herman von Helmholtz (1850s) – added that there were receptors sensitive to red (long), green (med.), and blue (short) wavelengths of light • Young-Helmholtz theory – a short wavelength (blue) will strongly activate the short wavelength receptors, but weakly activate the other receptors, giving rise to the perception of blue • A yellow light will produce a more immediate response of the med and short wavelength receptors • Brain interprets what colour is visible based on the relative strength of the 3 cone types • In support of this theory, researchers have been able to identify 3 diff cone visual pigments – each is most sensitive to a different wavelength of light • Trichromatic theory of colour perception is unable to explain the after image (if you stare at 4 coloured squares for 1 min then 4 white squares right after, the white ones appear to be also coloured and the pattern of colours flips left/right) • Ewald Hering – first to describe this visual illusion – noticed that 2 colours side by side interact with one another and change our perception of them – simultaneous contrast • Ex. perceive the yellow X on a black background to be brighter than the yellow X on the white background (Figure 2, pg. 122) because of the contrast • Also drew attention to the fact that combinations of red, green and blue can usually describe our perceptions of colour (ex. greenish-blue) – some combinations (ex. reddish-green) don’t ever occur – red/green and blue/yellow are mutually exclusive to one another • Opponent-process theory – our perception of colour relies on 3 mutually antagonistic mechanisms that process the info received by cones and recode them into a signal in relation to pairs of colours – form different colour channels  Similar to the antagonistic center-surround receptive field organization of the retina, these opponent channels would respond in opposing directions to pairs of colours  One set of neurons encodes black-white differences, corresponding to the intensity/luminance differences in the field  Another set responds to red and green colour differences  A third set responds to yellow and blue colour differences  In the retina, we can think of these channels with the perspective of the retinal ganglion cell receptive field organization º A red-green cell would inc. activity with stimulation from red light (R) and dec. activity in response to green light (G) – it can be said to signal +R-G/other cells signal the opposite in the presence of green and the absence of red (+G-R) º A blue-yellow cell would signal +B-Y/+Y-B – there are no yellow cones so yellow channels are made up of the responsive properties of med (green) and long (red) wavelength sensitive cones  We perceive colour when there is a relative diff in the activity that is biased towards one of the colour cells  We don’t see colours like reddish-green because the simultaneous activation of the red and green cells in a +R-G channel would result in the positive response to red canceling out with the negative response to green and no diff in the activation of this channel would be produced • We can explain the perception of the colour afterimages with our understanding of colour opponent channels – when you are staring at the red patch, you are stimulating the cells that are coding for red, but not the ones coding for green – relative difference in activation gives
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