OPT 404 - Color Vision PDF

OPT 404 - Visual Perception Colour Vision MCQ - Incorrect answer A) Duplicity theory of vision shows that there are 2 main classes of photoreceptors b) Light adaptation is faster than dark adaptation c) Dark adaptation is the slow recovery of sensitivity in the dark after bleaching the retina d) D...

OPT 404 - Visual Perception Colour Vision MCQ - Incorrect answer A) Duplicity theory of vision shows that there are 2 main classes of photoreceptors b) Light adaptation is faster than dark adaptation c) Dark adaptation is the slow recovery of sensitivity in the dark after bleaching the retina d) Dark adaptation depends only on the regeneration of rod pigment e) Rods operates mainly at mesopic and scotopic levels - Isaac Newton showed that the eye responds to visible light wavelengths between 360 - 760 nm to produce different colour sensations - White light or sunlight is composed to many different wavelengths which can be separated by a prism - If there were no observer, there would be no colour Colour Mixing - Additive colour mixture: - Mixing lights of different wavelengths - All wavelengths of the mixed light are reaching the eye - Subtractive colour mixture: - Mixing paints with different pigments - Additional pigments reflect fewer wavelengths Macula Lutea: - The macula is yellow in colour and therefore absorbs excess blue and UV that enters the eye and acts as a natural sunblock (analogous to sunglasses) - The colouring comes from the lutein and zeaxanthin, which are yellow xanthophyll carotenoids derived from the diet Rods and Cones - spectral sensitivity - Monochromatic light can be used to determine thresholds at different wavelengths Purkinje Effect/Shift - The tendency for the peak luminance sensitivity of the eye to shift towards the blue end of the colour spectrum at low illumination levels as part of the dark adaptation - Reds will appear darker compared to other colours as light levels decrease Spectral Cone Sensitivity - Rhodopsin for rods - Three types of iodopsins for cones - Three types of cone photoreceptors in retina, with peak sensitivity to long, medium and short wavelengths - L:M:S = 8:4:1 - Combination of the responses across all three cone types lead to colour vision/perception - Photopic luminosity function results form combination of the spectral sensitivity of all cones (or rods and cones in lower light levels) Cone Mosaic - Images of the cone mosaics for 12 subjects - Note the large variability of the rations in L and M cones Cone Distribution - 5-10% blue cones - In a ring around the edge of the fovea - Red and green ratio 2:1 (very variable from one person to another) (L:M:S = 8:4:1) - ‘Randomly’ mixed small patches or clusters Colour Matching experiments - In a colour matching experiment, the observer adjusts the amount of three wavelengths in one field (right) in order to match the colour of the single wavelength in the other field (left) - When the colours match, both fields are not physically the same but are perceived the same (pair of metamers) Results of Colour Matching - Colour matching experiments reveal that colours that are perceptually similar can be created by different wavelengths (Metamers) - Observers with normal colour vision need at least 3 wavelengths - Observers with colour deficiencies (Dichromacy) can match colours by using only 2 wavelengths Colour - Metamers - Two objects can appear the same colour even though their spectral power distributions differ - This is because the human visual system represents colour using only three cones reducing all colours into a code of three sensory outputs - Illuminant metameric failures can occur when two colour samples match under one light source but not another Cone spectral sensitivities or cone absorption spectra - The principle of univariance states that one and the same visual receptor cell can be excited by different combinations of wavelength and intensity, so that the brain cannot know the colour of a certain point of the retinal image - One individual photoreceptor type can therefore not differentiate between a change in wavelength and a change in intensity - The wavelength information can be extracted only by comparing the responses across different types of receptors How can we describe colours? - Dimensions of colours - Basic or primary colours are RED, YELLOW, GREEN, BLUE - Colour circle (newton) shows perceptual relationship among colours - Colours can defined as: - Lightness: intensity or luminance of colour - from dark to light - Saturation: purity of colour. - from grey to vivid colours - Hue: Colour itself - changes around the circle CIE chromaticity diagram (1931) - CIE - Committee Intenationale de l’Eclairage - Representation of all the chromaticities visible to the average person - 2D representation of hue and saturation - Saturation decreases towards W (neutral or white point) - Based on trichromatic colour matching data from large group - Pure spectral colours are along the perimeter of the curve (most saturated points, spectral locus) - Any additive mixture of any two primary wavelengths lies along the straight line that links both wavelengths - Complementary colours are located at each end of a straight line that go through W - Choose any three ‘primary’ colours R, G and B - Make any colour within the triangle by mixing different amounts of the primaries - For instance, if R, G and B are the primaries of your computer monitor, then the triangle represents the ‘gamut’; of all colours the monitor can produce Trichromatic Theory of Colour Vision - Proposed by T. Young and H. Con Helmholtz - Three different receptor mechanisms are responsible for colour vision - Behavioural evidence: - Colour matching experiments - Observers adjust the amount of three wavelengths in a comparison field to match a test field of one wavelength (colour matching) Retinal Organisation of Colour: - Colour detected by three cone classes of receptors - S CONE - short wavelength pigment absorbing maximally at 420 nm - M CONE - middle wavelength pigment absorbing maximally at 530nm - L CONE - long wavelength pigment absorbing maximal at 565nm - Combinations of the responses across all three cone types lead to perception of all colours Hering’s Colour Opponent Theory - Proposed by Hering in the 19th Century - Colour Vision is caused by the opposing responses generated by blue and yellow and by green and red - At the level of retinal ganglion cells, some cells show colour opponency - Behavioural Evidence: - Colour afterimages and simultaneous colour contrast - Types of Colour blinds are red/green and blue yellow Two stages of Colour processing: Three channels: 1 luminance and 2 chromatic Three channels (Colour opponent theory) - One achromatic black - white channel - Two colour channels with four primary colour mechanisms: - One compares responses of RED and GREEN - The other compares responses of YELLOW and BLUE - If R > G, the colour has a red tint - If R < G, the colour has a green tint - If R = G, the colour contains neither red nor green (grey) - If Y > B, the colour has a yellow tint - If Y < B, the colour has a blue tint - If Y = B, the colour contains neither yellow nor blue (grey) - What happens at the LGN? - Parvocellular neurone carry information regarding form and colour - LGN receptive fields are similar to retinal ganglion cells receptive fields Cortical Processing (V1, V2, V4) - Cells in V1 and V2 maintain circular colour opponent receptive fields - Some cells in V1 and V2 are double opponent eg centre L and M respond respectively to ON and OFF and the opposotive for the surround or any other opponent combinations - Cells in V4, the ventral pathway, are also selectively responsive to colour Colour Deficiencies: - Result from a reduced sensitivity to specific wavelengths of light, and they are often inherited - Red-Green Colour deficiency: This is the most prevalent type of colour blindness. People with red-green colour deficiencies have difficulty distinguishing between red and green hues. There are two main subtypes: 1. Protanomaly: Reduced sensitivity to red light. 2. Deuteranomaly: Reduced sensitivity to green light 2.Blue-Yellow Colour deficiency : People with blue-yellow colour deficiencies struggle to differentiate between blue and green hues, as well as between yellow and red hues. Tritanomaly: Reduced sensitivity to blue light. 3. Monochromacy: In extreme cases, individuals may have only one type of cone cell functioning, resulting in complete colour blindness. This condition is known as monochromacy and can be either total (seeing the world in shades of grey) or specific to one colour Types of colour deficiencies: Congenital (Genetic/Hereditary) - Red/Green: Protan/Deutan defects - Yellow/Blue: Tritan Defects Acquired - Eye disease, intoxication, certain chemicals (eg lead, mercury), injury to eye (retinal disease or optic neuritis) or brain injuries - Normally monocular defects Prevalence and Genetics Congenital Protan/Deutan (red/green) colour vision effects - Inherited as a recessive X chromosome linked trait - Females have XX, males have XY - males have a greater chance - 8% in Caucasian males (1 in 12) - 0.4 females (1 in 20) - 15% Caucasian females carry colour vision deficiency genes - Most common defect is Deuteranomaly (5% of all affected males) Congenital Tritan Effects: - Not X-chromosome linked, inherited as an autosomal trait (chromosome 7) - Equal numbers of males and females affected - Rare, prevalence of 1 in 13000 (Tritanopia) and 1 in 500 (Tritanomalous) Dichromacy and Anomalous Trichomacy Colour Deficiencies - Anomalous trichromacy - Three cone types - one has altered sensitivity - Dichromacy - two cone types - Monochromacy - One cone type (S only) - Blue cone monochromacy, No cone types, only rods: rods monochromacy Protan - Missing or altered L Deutan - Missing or altered M Tritan - Missing or altered S Red/Green Colour deficiencies: - PROTAN - L cone deficiency - Protanomalous - altered sensitivity - Protanope - Missing L cones Blue/Yellow Colour Deficiency - Tritan - S cone deficiency - Tritanope - Missing S cones - Protan: red cone sensitivity shifted towards green cone (shorter wavelengths) - Deutan: green cone senstivity shifted towards red cone (longer wavelengths) - Tritan: blue cone sensitivity shifted towards the green cone (longer wavelengths) Ishihara - pseudo chromatic plates - Colour Vision assessment Key points to consider for all colour vision tests: - Illumination - Results are only valid if test is administered under the recommended illumination conditions - Refractive correction - perform at end of routine with appropriate refraction in place - Viewing distance - Needs to be exact to ensure observations are being made with the central rod free retinal area - Presentation time - Short (few seconds) to limit the effects of colour adaptation