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Sensation part 2

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Sensation & Perception Ch. 5 THE PERCEPTION OF COLOR Basic principles of color perception • Color is the result of the interaction of a physical stimulus with a particular nervous system • Reflected vs. emitted • The more the light is absorbed, the darker the surface will appear • Wavelength of reflected light determines cone response Properties of light Steps of color perception 1. Detection: wavelengths must be detected • 3 types of cones each with different photo-pigments, thus each having different sensitivity to light. All 3 together gives us the overall ability to detect wavelengths from 400nm-700nm o S-cones: short wavelength cones, less sensitive, more rare than the others (“blue”) o M-cones: medium wavelength cones (“green”) o L-cones: long wavelength cones (“red”) • 1 rod photoreceptor: detects in dimmer light and has a somewhat different sensitivity profile peaking at 500nm • Visual spectrum: blue left (400nm) red right (700nm) 2. Discrimination: we must be able to tell the difference between one wavelength and another • Unvariance: 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 o Explains the lack of color in dimly lit scenes. It is impossible to discriminate colors in dim light • Trichromatic Theory of Color Vision: theory that color is defined by the relative output of the three types of cones o The ability to discriminate one light from another is defined by our visual system • LGN: the 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 o Valois and others discovered LGN in Macaque monkeys to show that combinations of cone signals actually existed o Ganglion cells in the retina and LGN are maximally stimulated by spots of light; theses cells have receptive fields with a characteristic center-surround organization o Cone-opponent cell: a cell type—found in the retina, lateral geniculate nucleus, and visual cortex— that, in effect, subtracts one type of cone input from another  Opponent of (L-M), {(L+M)-S} & (L+M) color-opponent cells 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 and not to change dramatically as the viewing conditions change • 3 “numbers” defining color o Hue: the chromatic (colorful) aspect of color (red, blue, green, yellow)  Each point on the spectrum defines a different hue o Brightness: the perceptual consequence of the physical intensity of a light o Saturation: the chromatic strength of a hue. White has zero saturation, pin is more saturated, and red is fully saturated • Properties of light o 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  Analogous with the length, width, and height of a cube • Opponent Color Theory: theory that the 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 o Ewald Hering: some combinations of colors are illegal—reddish/green or bluish/yellow don’t exist o Hurvich & Jameson:  Hue cancelation: Taking one color, and attempting to determine how much of the opponent color of one of the starting color’s components must be added to eliminate any hint of that component form the starting color  Unique hues: any of the 4 colors that can be described with only a single color term: red, yellow, green, blue. Other colors can be described as compounds (reddish blue, reddish yellow) Afterimage: a visual image seen after the stimulus has been removed • Adapting stimulus: if you look at one color for a few seconds, a subsequently viewed achromatic region will appear to take on a color opposite to the original color Color in the Visual Cortex • Mismatch between responses of LGN cells and color perception • Achromatopsia: an inability to perceive colors that is caused by damage to the central nervous system o Loss of color vision after brain damage. Patients may be able to find boundaries between regions of different colors, but they cannot report what those colors might be • Double-opponent cells: 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. Color blindness • There is a consensus with color matching but we don’t all see it the same way, there are individual differences • With age, color changes slightly because the lens of the eye starts to turn yellow • Depends on cone type and if it is anomalous (only a few cones) or completely missing o Monochromat: missing cones entirely  Cone monochromat: just has one cone type (short or long or medium)—no color at all, just light and dark  Rod monochromat: no cones, only rods • Cannot see in the light, pretty much blind entirely at night, but can only see at night, but without high detail because there are no cones for that o ColorAnomalous: “color blind” can still make discriminations based on wavelength. Those discriminations are different form the norm—that is, anomalous  Deuteranope: missing the M-cone • Green/red colorblind but you can see light vs. dark  Protanope: missing L-cones • Green/red is gone  Tritanope: missing the S-cones • No blue/yellow • How to detect: o Use psychophysical methods to test it o Buy a kit with colored tiles and have them line the tiles up based on an order and see what mistakes they make and use that to figure out with colorblind they are o Ishahara plates: printings of different colored stimuli and ask them to identify a number or letter, and if they cant identify it, they cant see it Color in the environment—You rarely see one color by itself • Unrelated colors: colors on their own (in isolation) • Related colors: a color that is seen only in relation to other colors o Gray—in complete darkness appears white • Simultaneous perception • Kafka rings • White illusion • Gradient- simultaneous contrast • Color contrast: a color perception effect in which the color of one region induces the opponent color in a neighboring region Illumination: the light that illuminates a surface • What wavelength the surface reflects • Color constancy: the tendency for the perceived color to be consistent under different illumination o Shadows: luminance change without hue change looks like a shadow while luminance change with hue change looks less like a shadow. Shadows allow us to perceive constancy o Broadband light emitted o Knowledge of objects and their shape and how we are supposed to perceive them (strawberries are red.) o The colors we see depend on the surrounding objects o We can disregard specific types of illumination to keep our perceptions of color constant o We rely on knowledge of objects to determine what we perceive Color vision: • Allows us to pick food (is this meat good? Are these berries ripe?) • Insects and animals use color and color vision differently based on their needs • Humans and other primates are trichromats • Dogs are dichromats • Chickens and some other birds are tetrachromats CH. 6 DEPT PERCEPTION Depth Perception: the world that we see • Retinal image doesn’t necessarily match the object in the environment Monocular Cues: using only one eye to view your environment • Occlusion o Dynamic occlusion: deletion and accretion • Size/position o Images get smaller and textures get finer as they get farther away o Images that are higher in the picture are usually farther away o Knowing the size of an object can give clues to the size/distance of other objects o Familiar size • Arial perspective o Light is scattered by the atmosphere o More scatter means more distance • Linear perspective o
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