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PSYB51H3 (306)
Chapter 5-8


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Matthias Niemeier

CHAPTER 5: THE PERCEPTION OF COLOR - Color is not a physical property of things in the world, it is a creation of the mind - Blood looks red because we look at it with our particular visual systems Basic Principles of Color Perception - Apparent color of a bit of the world is correlated with the wavelengths of the light rays reaching the eye from that bit of the world - Some wavelengths are absorbed by the surfaces they hit, the more light that is absorbed, that darker the surface will appear - Color of a surface depends on the mix of wavelengths that reach the eye from the surface - Color is the result of the interaction of physical stimulus with a particular nervous system Three Steps to Color Perception - Detection: wavelengths must be detected - Discrimination: must be able to tell the different between one wavelength and another - Appearance: want to assign perceived colors to lights and surfaces in the world. Want perceived colors to go with the object and not to change dramatically as the viewing conditions change. Step 1: Color Detection - S-cone: a cone that is preferentially sensitive to short wavelengths; known as blue cone - M-cone: a cone that is preferentially sensitive to short wavelengths, known as green cone - L-cone: a cone that is preferentially sensitive to long wavelengths, known as red cone - Cones work at daylight (phototopic) light levels - Rods work in dimmer (scotopic) light levels Step 2: Color Discrimination The Problem of Univariance - Problem of univariance: the fact that an infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make color discriminations based on wavelength - Univariance explains the lack of color in dimly lit scenes - Dim light stimulates only the rods, and the output of that single variety of photoreceptor does not permit color vision The Trichromatic Solution - Trichromatic theory of color vision: the theory that the color of any light is defined in our visual system by the relationships of three numbers the outputs of three receptor types now known to be the three cones. Also known as the Young-Hemholtz theory. Metamers - Metamers: different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of physical differences - Nervous system knows only what the cones tell it - Two quick warnings: o Mixing wavelengths does not change the physical wavelengths; color mixture is a mental event, not a change in the physics of light o For mixture of a red and green light to look perfectly yellow, we would have to have just the right red and just the right green; other mixes might look a bit more reddish or a bit more greenish - All the light reaching the retina from one patch in the visual field will be converted into three numbers by the three cone types The History of Trichromatic Theory - Schnapf et al., recorded the activity of single photoreceptors - Nathans et al., found the genes that code for the different photopigments - Isaac Newton a prism would break up sunlight into the spectrum of hues, and a second prism would put the spectrum back together in white - Maxwells color matching experiment a color is presented on the left, and on the right, the observer adjusts a mixture of the three lights to match the color on the left A Brief Digression into Lights, Filters, and Finger Paints - Additive color mixture: a mixture of lights. If light A and light B are both reflected from a surface to the eye, in the perception of color the effects of those two lights add together - Finger paint looks a particular color because it absorbs some wavelengths, subtracting them from the white light falling on a surface covered with the pigment - 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. - Georges Seurat and other Postimpressionist artists of the late 10 century experimented with Pointillism, a style of painting that involved creating many hues by placing small spots of just a few colors in different textures From Retina to Brain: Repackaging the Information - Cones in the retina are the neural substrate for detection of lights - To tell difference between different lights, the nervous system will look at differences in the activities of the three cone types - Nervous system computes two differences: (L M) and ([L + M] S) - Combining L and M signals is a pretty good measure of the intensity of light Cone-Opponent Cells in the Retina and LGN - Cone signals exist in the lateral geniculate nucleus - LGN: a structure in the thalamus, part of the midbrain, that receives input from the retinal ganglion cells and has input and output connections to the visual cortex - Cone-opponent cell: a cell type found in the retina, LGN, and visual cortex that, in effect, subtracts one type of cone input from another - (L M) cones are cone-opponent cells Step 3: Color Appearance Three Numbers, Many Colors - Because we have exactly 3 different types of cone photoreceptors, the light reaching any part of the retina will be translated into 3 responses, one for each local population of cones - If light rays reflecting off two surfaces produce the same set of cone responses, the two surfaces must and will appear to be exactly the same color - Working with just three numbers can discriminate the surfaces of more than 2 million different colors - Can describe each of the colors in the spectrum by their wavelength - Going beyond the spectrum, we have the 3D color space: the three dimensional space, established because color perception is based on the outputs of three cone types, that describes the set of all colors - Achromatic: referring to any color that lacks a chromatic (hue) component. Black, white, or gray. - Hue: the chromatic (colourful) aspect of color (red, blue, green, yellow, and so on). - Saturation: the chromatic strength of a hue. White has zero saturation, pink is more saturated, and red is fully Saturated - Brightness: the perceptual consequence of the physical intensity of a light - Nonspectral hues colors part of the hue strip that are not present in the spectrum Opponent Colors - Herings opponent color theory: the theory that perception of color is based on the output of three mechanisms, each of them resulting from an opponency between two colors: red-green, blue-yellow, and black- white - Unique hues: any of four colors that can be described with only a single color term: red, yellow, green, blue. Other colors (e.g., purple or orange) can be described as compounds (reddish blue, reddish yellow) Detection light is differentially absorbed by three photopigments in the cones Discrimination differences are taken between cone types, creating cone-opponent mechanisms, important for wavelength discriminations Appearance further recombination of the signals creates color-opponent processes that support the color-opponent nature of color appearance Color in the Visual Cortex - Cells in the LGN seem to be cone-opponent cells, so the transformations that produce the color-opponent processes that support color appearance are likely to be found in the visual cortex - Double-opponent cell: a cell type, found in the visual cortex, in which one region is excited by one cone type, combination of cones, or color and inhibited by the opponent cones or color. Another adjacent region would be inhibited by the first input and excited by the second - Single-opponent cell: another way to refer to cone-opponent cells, in order to differentiate them from double- opponent cells - Achromatopsia: an inability to perceive colors that is caused by damage to the CNS o Patients may be able to find boundaries between regions of different colors, but the cannot report what those colors might be o Experience of colour seems specifically impaired Adaptation and Afterimages - Adaptation can be color-specific - Afterimages: a visual image seen after the stimulus has been removed - Adapting stimulus: a stimulus whose removal produces a change in visual perception or sensitivity - Negative afterimage: an afterimage whose polarity is the opposite of the original stimulus. Light stimuli produce dark negative afterimages. Colors are complementary; for example, red produces green, and yellow produces blue - Adapted processes behave less vigorously than unadapted processes - Neutral point: the point at which an opponent color mechanism is generating no signal. If red-green and blue- yellow mechanisms are at their neutral points, a stimulus will appear achromatic. (The black-white process has no neutral point.) Does Everyone See Colors the Same Way? Does Everyone See Colors the Same Way? Yes - Among different ppl unique green can vary from at least 495 to 530 nm - Some of these differences will be due to factors like age, which turns the lens of
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