L4 Colour Perception - Study Notes PDF

3318 L4 colour perception colour is NOT a physical property of the world - The percept of colour in the absence of any physical stimulation tells us that colour perception is the product of network activity. - visible light - Color is not a physical property but a psychophysical pr...

3318 L4 colour perception colour is NOT a physical property of the world - The percept of colour in the absence of any physical stimulation tells us that colour perception is the product of network activity. - visible light - Color is not a physical property but a psychophysical property - "There is no red in a 700 nm light, just as there is no pain in the hooves of a kicking horse." Steven Shevell (2003) - Most of the light we see is reflected - Typical light sources: Sun, light bulb, fire - We see only part of the electromagnetic spectrum, between 400 and 700 nm Basic principles of colour perception - 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 colours to lights and surfaces in the world and have those perceived colours be stable over time, regardless of different lighting conditions step 1: colour detection - Photopic: Light intensities that are bright enough to stimulate the cone receptors and bright enough to "saturate" the rod receptors - Sunlight and bright indoor lighting are both photopic lighting conditions - Scotopic: Light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors - Moonlight and extremely dim indoor lighting are both scotopic lighting conditions - colour vision with 3 cone receptors - Three types of cone photoreceptors - S-cones detect short wavelengths - M-cones detect medium wavelengths - L-cones detect long wavelengths - More accurate to refer to them as "short," "medium," and "long" rather than "blue," "green," and "red" since they each respond to a variety of wavelengths - The L-cone's peak sensitivity is 565 nm, which corresponds to yellow, not red! - the output of these aren't independent - trichromatic theory - Trichromacy: The theory that the colour 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--Helmholtz theory - Metamers: its all about output - Metamers: Different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of physical differences step 2: colour discrimination - why more than 1? - The problem of univariance: An infinite set of different wavelength--intensity combinations can elicit exactly the same response from a single type of photoreceptor - Therefore, one type of photoreceptor cannot make colour discriminations based on wavelength - scotopic vision: rods - Rods are sensitive to scotopic light levels. - All rods contain the same photopigment molecule: Rhodopsin - All rods have the same sensitivity to various wavelengths of light - Therefore, rods suffer from the problem of univariance and cannot sense differences in colour - Under scotopic conditions, only rods are active, so that is why the world seems drained of colour - colour discrimination - With three cone types, we can tell the difference between lights of different wavelengths - Under photopic conditions, the S-, M-, and L-cones are all active - comparing cone responses - Lateral geniculate nucleus (LGN) has cells that are maximally stimulated by spots of light - Visual pathway stops in LGN on the way from retina to visual cortex (remember this from last week) - LGN cells have receptive fields with centre--surround organization - Cone-opponent cell: A neuron whose output is based on a difference between sets of cones - In LGN there are cone-opponent cells with centre--surround organization: e.g. - Just like we talked about in lecture 2 - in the LGN and cortex, the cone responses are differentiated - leads to cone-opponent cells with centre-surround organisation SUMMARY: detection and discrimination - Detection - 3 cone types detect a range of wavelengths - Short, medium and long wavelengths - Discrimination - Metamers show that the cone output is important; even if the physical wavelengths represented, are different - Retina and LGN contain cells to repackage that into cone-opponent difference signals - This output is all the brain has to work with: and yet we can discriminate between \>2 million colours. appearance - representing colour - organising the dimensions of colour - Colour space: A three-dimensional space that describes all colours. There are several possible colour spaces - RGB colour space: Defined by the outputs of long, medium, and short wavelength lights - HSB colour space: Defined by hue, saturation, and brightness - Hue: The chromatic (colour) aspect of light - Saturation: The chromatic strength of a hue - Brightness: The distance from black in colour space - These are implementations or conceptualisations that work with trichromatic vision - the photoshop analogy - 3 cone opponent channels: red/green, blue/yellow, and black/white - Colours are not independent, but are instead opponent - Opponent colour theory: The theory that perception of colour is based on the output of three mechanisms or channels, - each of them based on an opponency between two colours: - Red--green - blue--yellow - black--white - Some LGN cells are excited by L-cone onset in centre, inhibited by M-cone onsets in their surround (and vice-versa) - Red versus green - Other cells are excited by S-cone onset in centre, inhibited by (L + M)-cone onsets in their surround (and vice-versa) - Blue versus yellow - So, these excitatory and inhibitory wirings lead to opponency in colour combinations - opponent colours - Ewald Hering (1834--1918) noticed that some colour combinations are legal while others are illegal - We can have bluish green, reddish yellow (orange), or bluish red (purple) - We cannot have reddish green or bluish yellow - Adaptation and after-images - We have already talked about how adaptation can be used to reveal the mechanisms underpinning perception. - Its also true for colour - 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 - Colours are complementary. Red produces green afterimages and blue produces yellow afterimages (and vice-versa) - This is a way to see opponent colours in action - colour deficiency: cone missing/range shifted/culture - Does everyone see colours the same way? - Does everyone see colours the same way? --- Yes - General agreement on colours - Some variation due to age (lens turns yellow) - Does everyone see colours the same way? --- No - About 8% of male population, 0.5% of female population has some form of colour vision deficiency: Colour blindness - Colour blindness - Several types of colour-blind people: - Deuteranope: Due to absence of M-cones (red-green deficit) - Protanope: Due to absence of L-cones - Tritanope: Due to absence of S-cones (blue-yellow deficit) - Several types of colour-blind people: - Colour-anomalous: Have two types of cones (typically L- and M-cones) which are so similar that they can't make discriminations based on them - Cone monochromat: Have only one cone type; truly colour-blind - Rod monochromat: Have no cones of any type; truly colour-blind and badly visually impaired in bright light - Testing colour vision: Ishihara plates - Red-green the most common deficiency - Generally due to deficiency of M or L cones. - 10 times more likely in males. - Different answer (left) nothing (right) - colour in context: colour contrast and colour assimilation - Colours very rarely appear in isolation. Usually, many colours are present in a scene - When many colours are present, they can influence each other - Colour contrast: A colour perception effect in which the colour of one region induces the opponent colour in a neighbouring region - Colour assimilation: A colour perception effect in which two colours bleed into each other, each taking on some of the chromatic quality of the other - The curious case of cross-wiring - Synaesthesia: When one stimulus evokes the experience of another stimulus that is not present. - Example: letters appearing to have colours (grapheme-colour synaesthesia) or sounds having tastes - About 4-5% of the population experiences synaesthesia - Music stars (e.g. Billie Eilish, Pharrell Williams) - colour constancy: lighting source - Colour constancy: The tendency of a surface to appear the same colour under a fairly wide range of illuminants - To achieve colour constancy, we must discount the illuminant and determine what the true colour of a surface is regardless of how it appears - Illuminant: The light that illuminates a surface - E.g. Sun, lights above us in this lecture theatre - The visual system must take note of the lighting source, the illuminant - We cannot always discount the illuminate, and we cannot all do it equally well - black and blue vs white and gold dress