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Lecture 3

Psyc 2220 - Readings Week 3.docx

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York University
PSYC 2220
Richard Murray

Readings Week 3 – Chapter 5 Colour Basic Principles of Colour Perception: -the colour of a surface depends on the mix of wavelengths that reach the eye from the surface. Three Steps to Colour Perception: 1.Detection: Wavelengths must be detected. 2.Discrimination: We must be able to tell the difference between one wave-length and another. 3.Appearance- we want to assign perceived colors to lights and surfaces in the world. Moreover, we want those perceived colors to go with the object (blood is red) and not to change dramatically as the viewing conditions change (blood should remain red in sun and shadow, for example). STEP 1 -3 types of cone photoreceptors detect the wave lengths. (S-cones 420nm, M cones 535nm, l cones 565nm). -these three allow us to detetct from 400nm – 700nm. Step 2 : -Problem of Univariance: the fact that an infinite set of different wave-length-intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make colour discrimintaions based on wave-lengths. -Univariance explains the lack of colour in dimly lit scenes. The same mix of wavelengths that produces color perception during the day remains present on that moonlight night, but we fail to see colours because dim light stimulates only rods, and the output of that single variety of photoreceptors does not permit color vision. The Trichromatic Solution -we detect differences between wavelengths because we have more than one kind of cone photoreceptor. -Trichromacy- wavelengths from 420-660 produce unique set of three responses from the three cones. The combines signal is used as our basis for colour. -Trichromatic theory of vision. Metamers: A mixture of different wavelengths that look identical. -Almost every light and every surface that we see is emitting or reflecting a wide range of wavelengths. -All the light reaching the retina from one patch in the visual field will be converted into three numbers by the three cone types. If those numbers are sufficiently different from the numbers in another pacth, you will be able to discriminate those patches. If not, those patches will be metamers. They will look identical, even is the wavelengths are physically different. History of Trichromatic Theory: -Newton knew colour was a mental event. -Young and Helmholtz deduced that three different color mechanisms must limit the human experience of colour. -123 Additive colour matching: a mixture of lights. An additive mixture of blue and yellow looks white. It includes a mix of wavelengths that stimulate the three cones roughly equally. Subtractive color 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 color. From Retina to Brain: -To tell the difference between different lights, the nervous system will look at differences in the activities of the three cone types. This work begins in the Retina. Three signals will occur. (L-M) , (L+M)-S), (L+M) Cone-Opponent Cells in the Retina and LGN -An antagonoistic relationship characterizes color. Some of the retinal and LGN ganglion cells are excited by the L-cone onset in their centre and inhibited by m-cone onsets in their surround. These (L-M) cells are one type of cone-opponent cell, so named because different sources of chromatic information are pitted against each other. Step 3 : COLOUR APPEARANCE Three Numbers, Many Colours: Colourspace- the three dimensional space, established because color perception is based on the outputs of three cone types, that describes the set of all colors. R G B- -Hue- chromatic aspect of a light. Saturation –dimension corresponds to the amout of hue present in a light. (pure white has zero saturation, bloodred is saturated). Brightness is the perceptual consequence of the physical intensity of light. Opponent Color Theory: -Herring unlike Young and Helmholtz thichromatic theory with 3 basic colour (red, blue and green) is called opponent colour theory and has 4 basic colours in two opponent pairs: red vs green, and blue vs yellow. -Leo Hurvich and Dorothea Jameson revised herings idea and developed a way to demonstrate this opponency. They called in hue cancellation. Unique hue – any of the four colours that can be described with only a single color term: red, yellow, green, blue. Other colours have to be described as compounds. Color in the Visual Cortex: -Cells in the LGN are cone-opponent cells, so the transformation that produce the color-opponent process that support colour appearances are likely to be found in the visual cortex. Double opponent cells are first found in the visual cortex. -a single opponent cell conveys information about the color of a broad area. A double opponent cell in contrast conveys information about chromatic edges. So it helps you know where the colour has changed. Adapatation and Afterimages: -the illusory color that is seen after staring at the orginal in the negative afterimage. Does Everyone See Colours the Same way? -colour blindness, in which there is a malfunction in one commonly known as “colour blindness”, in which there is a malfunction in one or more of the genes coding the three cone photopigments. Does everyone see colours the same way. Cultural relativism –meaning each group was free to create its own linguistic map of color space. But Rosch found that Dani’s performance on such tasks reflected the same color boundaries, even when their language did not recognize the distinction between the two colours. From the Color of Lights to a World of Color Unrelated color: a color that can be experienced in isolation Related color: a color, such as brown or gray, that is se
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