Chapter 5 Notes: The Perception of Colour PDF

Chapter 5 THE PERCEPTION OF COLOUR What colour do we perceive when we see a lemon? On the perception of colour…. “Among our sensations it is difficult not to confuse what comes from the part of objects with what comes from the part of our senses. Supposing this, one clearly sees that it is not eas...

Chapter 5 THE PERCEPTION OF COLOUR What colour do we perceive when we see a lemon? On the perception of colour…. “Among our sensations it is difficult not to confuse what comes from the part of objects with what comes from the part of our senses. Supposing this, one clearly sees that it is not easy to say much about colors, and that all one can expect in such a difficult subject is to give some general rules and to derive from them consequences that can be of some use in the arts and satisfy somewhat the natural desire we have to render account of everything that appears to us.” -Edme Mariotte, 1717 What is colour? Color is not a physical property but a psychophysical property Most of the light we see is reflected. We see only part of the electromagnetic spectrum, between 400 and 700 nm A single sheet of paper is 100,000nm thick Three steps to colour perception 1. Detection: Wavelengths of light must be detected in the first place. 2. Discrimination: We must be able to tell the difference between one wavelength (or mixture of wavelengths) and another. 3. Appearance: We want to assign perceived colors to lights and surfaces in the world and have those perceived colors be stable over time, regardless of different lighting conditions. Step 1: Colour detection Three types of cone photoreceptors: S-cones detect short wavelengths (blue range) – peak at 420nm M-cones detect medium wavelengths (green range) – peak at 535nm L-cones detect long wavelengths (red range) – peak at 565nm The difference between the types of cones lies in their photopigments Step 1: Colour detection A few terms to know: Spectral sensitivity: The sensitivity of a cell or other device to different wavelengths on the electromagnetic spectrum Photopic: Light intensities that are bright enough to stimulate the cone receptors and bright enough to saturate the rod receptors (maximal response rate) Sunlight and indoor lighting Scotopic: Light intensities that are bright enough to stimulate rod receptors but too dim to stimulate the cone receptors Moonlight and dark room at night Step 1: Colour detection More accurate to refer to the three cones as “short,” “medium,” and “long” rather than “blue,” “green,” and “red,” each responds to a variety of wavelengths The L-cone’s peak sensitivity is 565 nm, which corresponds to yellow, not red! Step 1: Colour Detection S-Cones are most rare – about 5-10% of the total cone population L-Cones are most common – about 2 L-Cones for every M-Cone in the average retina 4-5 million cones total One rod type – peaks at about 500nm Step 2: Colour discrimination How do photoreceptors respond to light? Response curve: Shows the receptor’s response to light of different wavelengths held at a constant intensity E.G. 625nm light produces a response midway between baseline and peak response Step 2: Colour discrimination The principle of univariance: An infinite set of different wavelength and intensity combinations can elicit exactly the same response from a single type of photoreceptor. Therefore, one type of photoreceptor cannot make color discriminations based on wavelength Step 2: Colour discrimination It get worse! We could use a light at 535nm (the peak of this photoreceptor) and reduce it’s intensity until it produced the same level of response as light at 450nm or 625nm Thus, any mix of wavelengths at the right intensity can give the same response Step 2: Colour discrimination Rods are sensitive to scotopic light levels. All rods contain the same photopigment molecule: rhodopsin Therefore, all rods have the same sensitivity to different wavelengths of light Consequently, rods obey the principle of univariance and cannot sense differences in color Under scotopic conditions, only rods are active, so that is why the world seems drained of color  How do we solve the problem of univariance? Step 2: Colour discrimination Trichromacy (trichromatic theory of color vision): the color of any light is defined in our visual system by the relationships of three numbers The outputs of three receptor types (3 cones) Also known as the Young-Helmholtz theory Step 2: Colour discrimination SOLVING THE PROBLEM OF UNIVARIANCE Step 2: Colour discrimination Trichromacy also solves the problem of intensity changes in univariation With one receptor type different intensities of one wavelength can produce the same response as stronger or weaker intensities of other wavelengths With three receptor types, different intensities produce different response sizes, but pattern between of responses between the three receptors remains the same Step 2: Colour discrimination SOLVING THE PROBLEM OF UNIVARIANCE More Cones, More Colours Snakes only have two cone types – dichromatic vision Can only see blue and green Can also see some ultraviolet wavelengths More Cones, More Colours BUT…. Snakes have a second set of ”vision” organs called vision pits They sense infrared wavelengths Use body heat to find prey Snake vision pits sense infrared heat energy More Cones, More Colours Not all animals have three cones Pigeons have four cone types – one allows them to see ultraviolet light Mantis Shrimp have as many as 16 different types of cones More Cones, More Colours More Cones, More Colours 16 cones allow for more complex colour vision See UV wavelengths well below what even pigeons can sense Also detect polarized light Lots of wavelengths… lots and lots We rarely experience a single wavelength when sensing light Nearly every light and surface we observe is reflecting a range of wavelengths How do we deal with multiple wavelengths? Step 2: Colour discrimination Metamers: Different mixtures of wavelengths that look identical; more generally, any pair of stimuli that are perceived as identical in spite of physical differences These two light sources produce the same response – but the sources are different Visual system knows only what it is told by the cones If the response to a mixture of two (red and green) wavelengths produces the same response as a single wavelength (yellow) then they must look identical A few warnings about mixing wavelengths… Mixing wavelengths does not change them Think of it like mixing sugar and sand – they do not change, they just mix 500nm and 600nm wavelengths mixed are still 500nm and 600nm It is not the average of 500nm and 600nm It is not the sum of 500nm and 600nm It is two separate wavelengths that we sense at once and mix in our visual system To get a mixture of red and green that perfectly matches a single yellow wavelength, it needs to be a perfect mixture – off by a nanometer on the red or the green and it will not look identical Step 2: Colour discrimination Let’s mix things up…. From Maxwell to Helmholtz and Young…. Maxwell’s light mixing experiment demonstrated that three primary colours were needed to create all colours of visible light Two were insufficient to create all values Four did not add any new values Helmholtz and Young both independently took this information and drew the inference that our visual system must work within these limits Step 2: Colour discrimination Addition and subtraction: Additive colour mixing: A mixture of lights. If two lights, A and B, are both reflected off a surface to the eye, the addition of those two lights creates the perception of a single colour. Subtractive colour mixing: A mixture of pigments. If two pigments, A and B, mix, some of the light shining on the surface where they are mixed will be subtracted by A, and some by B. The remaining light that is reflected gives rise to the perception of a single colour. Did you know… Paints look the color they do because when white light hits them, they absorb all wavelengths of light EXCEPT the color they appear to be A blue shirt looks blue because the medium and long wavelengths of light are absorbed by the shirt but the short wavelengths bounce off of it This is why white clothing tends to be cooler than black clothing on a hot, sunny day Step 2: Colour discrimination We can create subtractive effects with light by passing light through filters Filters absorb some wavelengths of light We can combine filters to absorb even more wavelengths Step 2: Colour discrimination From Cones to Perception… Individual cones do not send separate signals to the brain L and M cones are sensitive to similar wavelengths – likely branches from a common cone ancestor in evolution Signals an importance in detecting subtle differences in these wavelengths Differences between S cones and L or M cones are easier to detect Cone-opponent cell: A neuron whose output is based on a difference between sets of cones In LGN there are cone-opponent cells with center-surround organization Cone-opponent cells Example: excitatory center cell (dominated by L cone input) with an inhibitory surround (dominated by M cone input) L - M cell Output is modulated by input: Detects a small point of light, sends signal about the spatial position of light (luminance) without colour information Detects a large patch of light, sends signal about the wavelength composition (chrominance) without spatial information Step 2 : Colour discrimination Cone-Opponent Outputs (L - M) and (M – L) [L + M] – S and S – [L + M] L and M cones have similar S cones very different sensitivity sensitivities than L or M cones Difference between activation of As M and L coensprovide nearly these two cones reveals the same info, they are combined information about red vs green to compare with S cones Difference between S and combined M and L output reveals information about blue vs yellow Step 3: Colour appearance Even if we do not know colour names, we can tell two different colours apart We could call both of these coral or pink, but we know that they are not the same colour Before we can talk about what colours look like we need to create some common language Step 3: Colour appearance Colour space: a three- dimensional space that describes the set of all possible colours analogous to 3D space based on the output of the three cone types HSB color space: Defined by hue, saturation, and brightness.  Step 3: Colour appearance We can think of colours as degrees of red, green, and blue or as degrees of hue saturation and brightness Hue: The chromatic (color) aspect of light Saturation: The chromatic strength of a hue – number of colour specks Brightness: The distance from black in color space – amount of light reflected per colour speck Step 3: Colour appearance Nonspectral colors: Some colors that we see do not correspond to a single wavelength of light Purple and magenta are only perceived when both S- and L-cones are stimulated but M-cones are not Step 3: Colour appearance Ewald Hering suggested an alternative to trichromatic colour theory Opponent colour theory: the perception of colour is based on the output of three mechanisms, each resulting from the opponency between two colours Red-green Blue-yellow Black-white Step 3: Colour appearance the brain perceives color in terms of opposing pairs of colors antagonistic processing - meaning that only one cone in each pair can signal the brain at a time. limits perception by inhibiting one while activating the other  Step 3: Colour appearance Hue cancellation experiments Start with a color, such as bluish green The goal is to end up with pure green with no hints of blue or yellow Shine some yellow light to cancel out the blue light Adjust the intensity of the yellow light until there is no sign of either blue or yellow in the green patch The amount of the cancellation colour (yellow) used indicates the strength of the cancelled (blue) hue Step 3: Colour appearance We can use the hue cancellation paradigm to determine the wavelengths of unique hues Unique hue: Any of four colors that can be described with only a single color term: red, yellow, green, blue. Steps 1, 2 and 3 in summary Step 1: Detection S-, M-, and L-cones detect light Each cone responds to a different range of wavelengths of light Step 2: Discrimination Cone-opponent mechanisms discriminate wavelengths [L – M] and [M – L] compute something like red vs. green [L + M] – S and S – [L + M] compute something like blue vs. yellow Step 3: Appearance Further transformations of the signals create final color-opponent appearance Where do we process colour? V1 – basic colour detection (colour is present) V2 – colour contrast processing V4 – primary colour processing, particularly colour constancy Achromatopsia: have intact vision be cannot process colour information – do not experience colour vision Can see boundaries between colours but cannot name the colours  Still not sure what blobs in the V1 hypercolumns do Evidence that they respond to colour rather than orientation in some Remember the cases Globs: blob-like areas in monkey cortex that respond more strongly to Blobs? colour than the surrounding cells Theory: blobs might be the start of the integration process where orientation, line, and light start to combine with colour Individual differences n colour perception Some colour names are more commonly used than others Different cultures describe colours differently Cultural relativism: basic perceptual experiences (e.g., color perception) may be determined in part by the cultural environment Cultural differences English – 11 basic colour names Basic colour terms matter Two – light vs dark Three – light, dark, one chomatic value Additional chromatic values added in a predictlable order Basic terms evolve and change Colour category boundaries It’s easier to remember which of two colours you saw when the colour choices are in two separate categories than when they are both in the same category Even when the different in hue is the same We recall colours partly by giving them names What if your culture doesn't have a name for that? Dani of New Guinea Two colour terms: light and dark When asked to complete the colour memory task, showed the same category memory effect Suggests that same colour boundaries exist even if the names do not Genetic differences in colour perception Genetic Differences in Color Perception About 8% of males and 0.5% of females have some form of color vision deficiency Color-anomalous: usually called “color blindness,” individuals can still make discriminations based on wavelength. Those discriminations are just different from the norm Colour genes are coded on the x- chromsome Do we all see colour the same? Qualia: private conscious experience of sensation or perception You can talk about your qualia with others, but no one can experience your qualia Your experience of “red” is unique to you  Several types of color-blind/color-anamolous people Deuteranope: Due to absence of M-cones. Protanope: Due to absence of L-cones. Tritanope: Due to absence of S-cones.  Genetic differences Tetrachromat: some women have an extra cone type (total of 4) allowing for the perception of extra colours Cone monochromat: Has only one cone type; truly color-blind. Rod monochromat: Has no cones of any type; truly color-blind and very visually impaired in bright light. Agnosia: Inability to recognize something Colour agnosia – can see the colours but cannot recognize them Anomia: Inability to name objects or colors in spite of the ability to see and recognize them Colour anomia – can see the colours and can recognize them but cannot name them (e.g. it’s the same colour as the sky or the ocean)  Synaesthesia A perceptual experience that is elicited by a stimulus that does not typically produce that experience, while the stimulus that does normally produce that experience is absent – experienced by approximately 4% of people Grapheme – colour synaesthesia: letters, numbers, or words have specific colour associations E.G. B is Q is L is P is Chromesthesia: link between colours and sound Can be musical notes associated with colours Can be everyday sounds associated with colours Colour-taste synaesthesia: certain colours evoke specific taste sensation Not “red is cherry” Tends to be vague sensations of taste vs specific flavours What Causes Synaesthesia? Not Synaesthesia We all experience certain colour associations with things that are not inherently coloured Some near-universal colour associations: Temperature Emotion Time Number values How do we know synaesthesia is real? Specificity - Consistency - the same shades/experiences are response every time not vague but specific Colour Constancy The tendency of a surface to appear the same colour under a variety of illumination conditions Colour Color contrast: A color perception effect in which the color of one region induces the opponent color in a neighboring region. Constancy Colour Color assimilation: two colors bleed into each other, each taking on some of the chromatic quality of the other. Constancy Remember Adaptation? Afterimages: A visual image seen after a stimulus has been removed Negative afterimage: An afterimage whose polarity is the opposite of the original stimulus Light stimuli produce dark negative afterimages Colors are complementary. Red produces green afterimages and blue produces yellow afterimages (and vice versa) This is a way to see opponent colors in action Colour Constancy How do we keep colour constant? Physical constraints make constancy possible Intelligent guesses about the illuminant o Most illuminants are “broadband” Assumptions about surfaces o Most surfaces are “broadband” How does the illuminant interact with the surface? The problem of illuminants People perceived the dress differently, depending on their assumptions about the color of the illuminant o Assume yellow illuminant = Black & Blue o Assume blue illuminant = White & Gold We use visual cues to make assumptions about lighting to process colour in images When we fail to make the correct assumptions, our perceptions of the colours can be very wrong Why do we have colour vision? Food Easier to find riper berries with colour vision The flavour of food is affected by colour Sex Flower colours entice bees and other pollinators Colourful patterns in feathers and scales provide sexual signals in many species Food White wine dyed to look like rosé tastes more like rosé than white White plates make colours appear more vibrant and true, making foods taste sweeter and more appetizing Sex Why is red sexy? Red = danger Danger = increased heart rate and respiration In the absence of danger, increased heart rate and respiration is interpreted as love or sexual desire