Lecture 4 What does color do for us? Colour Perception • The world of color is a world of yellow daffodils, painted window shutters, orange-red sunsets. We pick favourite colours and react emotionally to color (purple with rage, green with envy). • Few mammals except primates have colour vision, so other than creating aesthetic appearance and mood what does it do for us? • The images on the right suggest that evolution of color vision was probably related to the advantages it provides in finding food. What is colour? Why does the sky look blue? • One way to interpret Newton’s result is that the perception of • In his room at Cambridge color occurs when some University, Isaac Newton wavelengths are subtracted from placed a prism so that white light. sunlight shining through a hole in the shutter of his • This occurs naturally when light window entered the prism. from the sun is scattered by particles of air in the atmosphereto • He discovered that the prism create the perception of blue sky split the sunlight into a and yellow sun. spectrum of colours. • Short wavelenght light (blue) is • The visible spectrum ranges scattered more than long from 400 nm (violet) to 700 wavelength light (red). This effect nm (red). becomes most pronounced when the light has to travel further (sunset) Why are objects the color they are? Mixing paint versus mixing colour • The colour of an object depends on • The artist can never perfectly wavelengths contained in the duplicate the effects of light incident light and which of these because pigments and light do wavelengths it reflects. not combine in the same way. • The red apple contains colourant • When painters mix blue and particles (pigments) that absorb yellow to produce green, they green and blue light and allow red are subtracting blue and yellow to be reflected, whereas the yellow from the total spectrum of banana contains colourant particles light. that absorb blue light reflecting red and green. • In contrast, when different coloured lights combine, the • Light that is reflected at the effect is additive making more interface of a surface has the same of the spectrum visible. color as the incident light. The trichromacy theory of colour vision Physiology of trichromatic theory • Support for the trichromacy theory • Young and Helmholtz realised was established when it was that the brain could not have a discovered that there were three receptor for every colour. types of cone in the human retina that respond to different • Consequently, he proposed that wavelengths of light. our perception of colour was based on the relative activation – red (long wavelength) of three receptors (red, green – green (middle wavelength) and blue). – blue (short wavelength) • Support for this theory comes color matching experiments in which any colour can be produced by a combination of the three primary colours. Colour blindness Opponent-process theory of color vision • Although trichromatic theory • Acquired or congenital colour explains a number of color vision blindness can result from failure phenomena, there are some to make one of the cone perceptions that it cannot explain. pigments. • These color perceptions were • People with normal vision are demonstrated by Hering and were known as trichromats invorporated into the opponent- process theory of color vision. • People with only two cones are known as dichromats. • He proposed that are perception of – protanopia (long wavelength) color was based on the relative – deuteranopia (middle wavelength) amount of colour in the scene – tritanopia (short wavelength) – red versus green – blue versus yellow – dark versus light Afterimages • Afterimages support the idea of opponent processes. – Prolonged viewing of red gives an after-effect of green and vice versa. – In contrast, prolonged viewing of blue gives rise to an after-effect of yellow and vice versa. The physiology of opponent-process theory Colour constancy • When looking at the color of an object • Evidence for the opponent-process one is usually confident that the color theory of color vision came from the is: grass is green, London buses are discovery of opponent neurons in the red etc. retina and lateral geniculate nucleus. • However, the colour reflected by an • Signals from cones are transformed object depends on the illumination. early in the visual pathway. – A white shirt in room illumination reflects more long wavelength light – M retinal ganglion cells are achromatic compared to outside. However, in both • dark - light circumstances we perceive the shirt to be white. – However, P retinal ganglion cells have an opponent organisation in which the centre and surround of the receptive • This phenomenon known as color field are sensitive to different constancy and is determined by wavelengths of light comparing the wavelength of light • red - green reflected from a single object to the • blue - yellow wavelengths of light in the surround. The importance of context in colour perception Is there a color area in the brain? • The importance of context in • V1 color perception is shown in this example where the tiles – Blobs in primary visual cortex are selective for the in the centre of both patches wavelength of light. are identical in terms of color However, the wavelength of and brightness light often bears little relationship to the • However, the perceptual perceived colour. system assumes that they must be different color • V4 because one of the tiles is in – However in area V4 neurons shade. behave as if they are responding to colours as seen by human observers Achromatopsia How color affects taste • Lesions to areas such as V4 • The appearance of food result in a condition known as influences its desirability and achromatopsia. research also indicates that peoples ability to identify • People with achromatopsia are tastes and smells depends to unable to distinguish between some degree on color. different colours (hues). • For example, when asked to • They often report that colours distinguish tastes of beer, become bland reflecting dirty subjects identified only 70% shades of grey. when the beer was colourless and accuracy dropped to 25% when the beer was coloured red!.