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Chapter 5-8


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Matthias Niemeier

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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 - Maxwell’s 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 - Hering’s 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 the eye yellow Does Everyone See Colors the Same Way? – No - 8% of male population and 0.5% of female population have a form of color vision deficiency commonly known as “color blindness”, in which there is a malfunction in one or more of the genes coding the three cone photopigments - The genes that code for the M and L cone photopigments are on the X chromosome and guys only have one X, so if it is defective, the guy in question will have a problem - Different types of color blindness o Depends on type of cone affected o Type of defect; either the photopigment for that cone type is “anomalous” (different from the norm), or the cone type is missing altogether - Deuteranope: an individual who suffers from color blindness that is due to the absence of M-cones - Protanope: an individual who suffers from color blindness that is due to the absence of L-cones - Tritanope: an individual who suffers from color blindness that is due to the absence of S-cones - Color-anomalous: a better term for what is usually called “color-blind.” Most “color-blind” ppl can still make discriminations based on wavelength. Those discriminations are different from the norm – that is, anomalous. - Cone-monochromat: an individual with only one cone type. Cone monochromats are truly color blind. - Rod monochromat: an individual with no cones of any type. In addition to being truly color blind, rod monochromats are badly visually impaired in bright light (worse than cone monochromat) - Agnosia: failure to recognize objects in spite of the ability to see them; due to brain damage - Anomia: inability to name objects in spite of the ability to see and recognize them; due to brain damage Does Everyone See Colors the Same Way? – Maybe - Cultural relativism: in sensation and perception, the idea that basic perceptual experiences (e.g., color perception) may be determined in part by the cultural environment - Berlin and Kay – discovered that the various maps used in different cultures are actually rather similar; they found that the 11 basic terms in English are about as many as any group possesses - Rosch – color perception is not especially influenced by culture and language; blue and green are seen as categorically different, even if one’s language doesn’t employ color terms to express this difference - Learning new color subcategories produces increases in gray matter in parts of the brain implicated in color vision From the Color of Lights to a World of Color - Color contrast: a color perception effect which the color of one region induces the opponent color in a neighbouring region - Color assimilation: a color perception effect in which two colors bleed into each other, each taking on some of the chromatic quality of the other - Unrelated colors: a color that can be experienced in isolation - Related colors: a color, such as brown or gray, that is seen only in relation to other colors. For example, a “gray” patch in complete darkness appears white. Color Constancy - Color constancy: the tendency of a surface to appear the same color under a fairly wide range of illuminates - Color constancy is yet another difficult problem for the visual system to solved, the heart of the problem is that the illuminate: the light that illuminates a surface, is not constant - Spectral reflectance function: the percentage of a particular wavelength that is reflected from a surface - Spectral power distribution: the physical energy in a light as a function of wavelength The Problem with the Illuminant Physical Constraints Make Constancy Possible - Color constancy must be based on some information or assumptions that constrain the possible answers - Assumptions can be made about the illuminant - Assumptions can be made about surfaces - Reflectance: the percentage of light hitting a surface that is reflected and not absorbed into the surface. Typically reflectance is given as a function of wavelength - Assumptions can be made about the structure of the world - Bloj et al., experiment shows us how assumptions about the physics of the world influence the psychophysics of color perception o Folding a card inward o Folding a card to make it look like a roof - Vision is the nervous system’s best guess about what is happening in the world Color Vision in Animals - Two realms of behaviour where color vision is especially useful: eating and sex o Easier to forage for food - Many flowers have dramatic patterns that we can’t see because they are variations in the reflections of short wave-length (UV) light, which is outside our range - Honeybees use UV light to find the flower’s “black eye” - Vampire bats use infrared wavelengths beyond 700 nm to find where flood flows near the surface of the skin of its prey o Use of thermoreceptors in the snout - Dogs are dichromats - Chickens are tetrachromats (4) - Fireflies signal each other with bioluminescence – they make their own light CHAPTER 6: SPACE PERCEPTION AND BINOCULAR VISION - Ability to perceive and interact with the structure of space is one of the fundamental goals of the visual system - Realism: a philosophical position arguing that there is a real world to sense - Positivists: a philosophical position arguing that all we really have to go on is the evidence of the senses, so the world might be nothing more than an elaborate hallucination - Euclidean: referring to the geometry of the world, so named in honor of Euclid, the ancient Greek geometer of the third century BCE. In Euclidean geometry, parallel lines remain parallel as they are extended in space, objects maintain the same size and shape as they move around in space, the internal angles of a triangle always add to 180 degrees, and so forth. - The geometry of retinal images of that world is decidedly non-Euclidean - The geometry becomes non-Euclidean when the three dimensional world is projected onto the curved, two- dimensional surface of the retina - Having two eyes is an evolutionary advantage – you can lose one and still see - Having two eyes enable you to see more of the world - Human visual field is limited to about 190 degrees from left to right, 110 degrees of which I covered by both eyes - Binocular visual fields give predator animals a better change to spot small, fast-moving objects in front of them that might provide dinner - Binocular: having two eyes - Binocular summation: the combination of signals from each eye in ways that make performance on many tasks better with both eyes than with either eye alone - Binocular disparity: the differences between the two retinal images of the same scene. Disparity is the basis for stereopsis, a vivid perception of the three-dimensionality of the world that is not available with monocular vision. - Monocular: with one eye - Stereopsis: the ability to use binocular disparity as a cue to depth - Stereopsis is not a necessary condition for depth perception or space perception - Stereopsis does add a richness to perception of the three dimensional world - Depth cues: information about the third dimension of visual space. Depth cues may be monocular or binocular - Monocular depth cues: a depth cue that is available even when the world is viewed with one eye alone - Binocular depth cue: a depth cue that relies on information from both eyes. Stereopsis is the primary example in humans, but convergence and the ability of two eyes to see more of an object than one eye sees are also binocular depth cues Monocular Cues to Three-Dimensional Space - M.C. Escher (artist) – master of the rules that govern our perception of space Occlusion - Occlusion: a cue to relative depth order in which, for example, one object obstructs the view of part of another object - Occlusion gives info about the relative position of objects - Occlusion is a nonmetrical depth cue: a depth cue that provides info about the depth order but not depth magnitude - Metrical depth cue: a depth cue that provides quantitative info about distance in the third dimension Size and Position Cues - Projective geometry: for purposes of studying perception of the three-dimensional world, the geometry that describes the transformations that occur when the three-dimensional world is projected onto a two-dimensional surface. For example, parallel lines do not converge in the real world, but they do in the two-dimensional projection of that world. - A shadow is a projection of an object onto a surface - Relative size: a comparison of size between items without knowing the absolute size of either one. - Texture gradient: a depth cue based on the geometric fact that items of the same size form smaller images when they are farther away. An array of items that change in size smoothly across the image will appear to form a surface titled in depth. - Relative height: as a depth cue, the observation that objects at different distances from the viewer on the ground plane will form images at different heights in the retinal image. Objects farther away will be seen as higher in the image. - For objects on the ground plane, objects that are more distant will be higher in the visual field - Familiar size: a depth cue based on knowledge of the typical size of objects like humans or pennies - Relative metrical depth cues: a depth cue that could specify, for example, that object A is twice as far away as object B without providing info about eh absolute distance to either A or B. - Absolute metrical depth cue: a depth cue that provides quantifiable info about the distance in the third dimension Aerial Perspective - Light is scattered by the atmosphere, and that more light is scattered when we look thru more atmosphere - Aerial perspective (or haze): a depth cue based on the implicit understanding that light is scattered by the atmosphere. More light is scattered when we look thru more atmosphere. Thus, more distant objects are subject to more scatter and appear farther, bluer, and less distinct. Linear Perspective - Linear perspective: a depth cue based on the fact that lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image - Vanishing point: apparent point at which parallel lines receding in depth converge - Linear perspective provides relative, but not absolute metrical depth info Pictorial Depth Cues and Pictures - Pictorial depth cue: a depth cue to distance or depth used by artists to depict three-dimensional depth in two- dimensional pictures - A realistic picture of photo is the result of projecting the three dimensional wolrd onto a two dimensional surface of film or canvas - Anamorphosis (or anamorphic projection): use of the rules of linear perspective to create a two-dimensional image so distorted that it looks correct only when viewed from a special angle or with a mirror that counters the distortion Motion Cues - Motion parallax: an important depth cue that is based on head movement. The geometric info obtained from an eye in two different positions at two different times is similar to the info from two eyes in different positions in the head at the same time. - Motion parallax provides relative metrical info about how car away objects are - Downside to motion parallax is that it works only if the head moves Accommodation and Convergence - Accommodation: the process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects) - Convergence: the ability of the two eyes to turn inward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typical on the fovea of each eye). Convergence reduces the disparity of that feature to zero (or nearly zero). - Divergence: the ability of the two eyes to turn outward, often used in order to place the two images of a feature in the world on corresponding locations in the two retinal images (typically on the fovea of each eye). Divergence reduces the disparity of that feature to zero (or nearly zero). - These cues are the only ones besides familiar size that can tell us the exact distance to an object Binocular Vision and Stereopsis - Corresponding retinal points: a geometric concept stating that points on the retina of each eye where the monocular retinal images of a single object are formed are at the same distance from the fovea in each eye. The two foveas are also corresponding points. - Vieth-Muller circle: the location of objects whose images fall on geometrically corresponding points in the two retinas. If life were simple, this circle would be the horopter, but life is not simple. - Horopter: the location of objects whose images lie on corresponding points. The surface of zero disparity. - Diplopia: double vision. If visible in both eyes, stimuli falling outside of Panum’s fusional area will appear diplopic. - Panum’s fusional area: the region of space, in front of and behind the horopter, within which binocular single vision is possible - Crossed disparity: the sign of disparity created by objects in front of the plane of fixation (the horopter). The term crossed is used because images of objects located in front of the horopter appear to be displaced to the let in the right eye, and to the right in the left eye. - Uncrossed disparity: the sign of disparity created by objects behind the plane of fixation (the horopter). The term uncrossed is used because the images of objects located behind the horopter will appear to be displaced to the right in the right eye, and to the left in the left eye. Stereoscopes and Stereograms - Sir Charles Wheatstone invented the stereoscope: a device for simultaneously presenting one image to one eye and another image to the other eye. Stereoscopes can be used to present dichotic stimuli for stereopsis and binocular rivalry - Free fusion: the technique of converging (crossing) or diverging the eyes in order to view a stereogram without a stereoscope - About 3-5% of the population lack stereoscopic depth perception – a condition known as stereoblindness: an inability to make use of binocular disparity as a depth cue. This term is typically used to describe individuals with vision in both eyes. Someone who has lost one (or both) eyes is not typically described as stereoblind. - Steroblindness is usually a secondary effect of childhood visual disorders like strabismus Random Dot Stereograms - Bela Julesz theorized that stereopsis might be used to discover objects and surfaces in the world - Julesz thought stereopsis might help reveal camouflaged objects - Random dot stereograms: a stereogram made of a large number (often in the thousands) of randomly placed dots. Random dot stereograms contain no monocular cues to depth. Stimuli visible stereoscopically in random dot stereograms are Cyclopean stimuli - Cyclopean: referring to stimuli that are defined by binocular disparity alone. Named after the one-eyed Cyclops of Homer’s Odyssey. Stereo Movies, TV, and Video Games - 3D movies make use of stereoscopic photography - Earliest 3D glasses had a red and blue/green lens, acted as filters - Modern 3D movies use special glasses that contain liquid crystals Using Stereopsis - Military use - Have a better look inside the body - Stereoscopic displays are beginning to be used in radiology Stereoscopic Correspondence - Correspondence problem: in binocular vision, the problem of figuring out which bit of the image in the left eye should be matched with which bit in the right eye. The problem is particularly vexing when the images consist of thousands of similar features, like dots in random dot stereograms - Uniqueness constraint: in stereopsis, the observation that a feature in the world is represented exactly once in each retinal image. This constraint simplifies the correspondence problem. - Continuity constraint: in stereopsis, the observation that, except at the edges of objects, neighbouring points in the world lie at similar distances from the viewer. This is one of several constraints that have been proposed to help solve the correspondence problem The Physiological Basis of Stereopsis - Input from the two eyes must converge onto the same cell - Convergence doesn’t happen until the striate cortex - A binocular neuron has two receptive fields, one in each eye - Most binocular neurons respond best when the retinal images are on corresponding points in the two retinas, thereby providing a neural basis for the horopter - Some binocular neurons respond best when similar images occupy slightly different positions on the retinas of the two eyes, basically these neurons are tuned to a particular binocular disparity - Nonmetrical stereopsis might tell you that a feature lies in front of or behind the plane of fixation - Metrical stereopsis is also possible Combining Depth Cues The Bayesian Approach Revisited - Bayesian approach: a way of formalizing the idea that our perception is a combination of the current stimulus and our knowledge about the conditions of the world – what is and is not likely to occur. The Bayesian approach is stated mathematically as Bayes’ theorem – P(A|O) = (P(A) x P(O|A)/P(O) – which enables us to calculate the probability (P) that the world is in a particular state (A) given a particular observation (O). - Ideal observer: a theoretical observer with complete access to the best available info and the ability to combine different sources of info in the optimal manner. It can be useful to compare human performance to that of an ideal observer Illusions and the Construction of Space - A guess is wrong whenever we see a two dimensional picture and see it as a three dimensional - We make a plausible guess about the three dimensional world that is being represented in the two dimensional picture - “Ponzo illusion” – in one of five pairs of horizontal lines, only two lines are the same length Binocular Rivalry and Suppression - Binocular rivalry: the competition between the two eyes for control of visual perception, which is evident when completely different stimuli are presented to the two eyes. - the more interesting of the two stimuli is likely to be dominant - binocular rivalry is never completely won by either eye - if object is within Panum’s area, we fuse its two images into a single stereoscopic perception - if it is outside Panum’s area, we normally suppress one of the copies - We don’t see rivalry because we aren’t looking, we’re focused on the object on our fovea, plus the peripheral vision is blurry - Great feature of rivalry is that it dissociates the stimulus on the retina from the stimulus that you see Development of Binocular Vision and Stereopsis - John McKee: o 1. As children get older, they get bet
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