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Chapter 3iii.pdf

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Christopher Teeter

Colour Perception▯ - Colour is important for detecting patterns and objects amongst a background▯ - Example, a red berry stands out against a background of green leaves▯ - Colour vision adds something distinctive and important to simple brightness perception▯ - Perception of colour allows us to distinguish features of objects when borders and contours do not▯ - Processing of colour begins in the retina where 3 cone types respond maximally to 3 different wavelengths of light; red, green, blue▯ - Our colour vision is trichromatic▯ Colour Constancy: Our colour vision corrects for the variation in the overall illumination so that the object’s colour appears about the same (Banana still looks yellow on a cloudy day, dimly lit▯ ▯ Theories of Colour Vision▯ - Thomas Young first proposed that our perception of colour must depend upon the existence of 3 colour receptors and each of which is sensitive to a different wavelengths of light▯ - Hermann von Helmhotlz added to this theory in the 1850s to propose that there were - receptors sensitive to red (long), green (medium) and blue (short) wavelengths of light▯ According to the Young-Helmhotlz theory of trichromatic colour vision, a short wavelength of light (blue) will strongly activate the short wavelength receptors, but only a weak activation of the other receptors, giving rise to the perception of blue▯ - A yellow light will produce a more intermediate response of the medium and short wavelength receptors — the brain interprets what colour is visible based on the relative strength of the 3 cone types▯ Simultaneous Contrast: Two colours, side by side, interact with one another and change our perception of them (Seeing a yellow X in a blue square looks more brighter than an X in a white square, but they are the same colour yellow)▯ ▯ - Hering drew attention to the fact that combinations of red, green and blue can usually describe our perception of colour▯ - For instance, you might be able to describe a colour as bluish-green▯ - However, some colour combinations seem to be impossible, no one ever speaks of a greenish-red colour or a yellowy-blue▯ - Based on these observations, Hering proposed the opponent-process theory▯ - Similar to the antagonistic venter-surround receptive field organization of the retina, these opponent channels would respond in opposing directions to pairs of colours▯ - Example: A reg-green cell would increase its activity as a result of stimulation with red (R) light and would decrease in response to green (G) light +R-G▯ - Example: A blue-yellow cell would signal +B-Y▯ - Although there is no yellow cone, yellow channels are made up from the both medium (green) and long (red) wavelength sensitive cones▯ - We don’t see colours like “reddish-green” because the simultaneous activation of the red cell and the green cells in a +R-G channel because the positive response of red would be cancelled out by the negative response of green, yielding no relative difference in the activation of this channel▯ ▯ Explaining afterimage with colour opponent channels▯ - While staring at a red patch, you are stimulating the cells that are coding for the colour red, but not stimulating cells that code for the colour green▯ - The rel
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