Color vison, Color Blindness and its Evaluation

Mechanism, Phenomenon, Color blindness and its evaluation.

1. COLOR VISION DR. ANKITH NAIR

2. CONTENTS MECHANISM OF COLOR VISON NEUROPHYSIOLOGY OF COLOR VISION PHENOMENA ASSOCIATED WITH COLOR VISON COLOR TRIANGLES BASED ON PHOTOPIGMENTS AND THE COLOR METERIC NORMAL COLOR ATTRIBUTES COLOR BLINDNESS TESTS FOR COLOR VISON 2

3. INTRODUCTION Color Vision is the ability of eye to discriminate between colors excited by light of different wavelengths. It is a function of cones and is better appreciated in photopic vision Three types of cones – red, green and blue sensitive. Colors have three attributes Hue - The dominant spectral color is determined by the wavelength of that particular color. Red ( 723-647 nm), Green (575-492 nm), Blue (492-450 nm) Intensity – The brightness of the color depends upon the intensity of the component wavelength. Saturation – Refers to the degree of freedom from dilution with white. Estimated by measuring how much of a particular wavelength must be added to white before it is distinguishable from white. 3

4. 1 . MECHANISM

5. Theories of Color Vision Trichromatic theory (By Young and Helmholtz) Opponent color theory Granits dominator and modulator theory 5

6. Trichromatic theory There are three primary colors, red, green and blue and there are three types of cones with different pigments. Erythrolabe ( Porphyropsin – red) Chlorolabe (Lodopsin – green) Cyanolabe (Cyanopsin – blue) Sensation of any given color is determined by the relative frequency of impulses reaching the brain from each of the three cone systems. Color blindness is classified based on this theory. This theory fails, to explain the black sensation, as black is also considered a color. to explain how peripheral color blind zones perceive yellow, white or grey sensations. 6

7. Herring’s Opponent color theory Extension of trichromatic theory. According to this theory – There are four primary colors – blue , green , yellow and red . The photochemical substances give one sensation on breakdown and a different one on resynthesis. Complementary colors become antagonistic to its respective primary colors. 7 Photochemical substance Sensation 1 White – Black/ Luminance channel Breakdown – Resynthesis - White Black 2 Red- Green Channel Breakdown - Resynthesis - Red on Green off Green on Red off 3 Yellow – Blue channel Breakdown - Resynthesis - Yellow on Blue off Blue on Yellow off

8. Granits dominator and modulator theory Two types of ganglion cells, namely Dominator and Modulator cells. Dominators respond to the whole visual spectrum. They detect intensity of light but not the color. This is d/t ‘Y’ ganglion cells. Modulators are of three types responding to a narrow wavelength of light of blue, green and red-yellow. Hence, they are responsible for color vision. X ganglion cells are the modulators. 8

9. Photochemistry of color vision Cone pigments = 11 cis retinal + Photopsin. The 11 cis retinal is similar to rhodopsin, but the opsin part is different. The green and red sensitive cones are very similar in structure showing 96% homology of AA sequence. They show only around 43% homology with the opsin of blue sensitive cone pigment. All three have around 41% homology with the rod pigment rhodopsin. 9

10. 2 . NEUROPHYSIOLOGY

11. Genesis of visual signals in photoreceptors Photochemical changes in the cone pigment + a cascade of biochemical changes produce a visual signal in the form of cone receptor potential. Cone receptor potential has sharp onset and offset as opposed to rods with sharp onset but slow offset. 11

12. Processing and transmission of color vision signals in the retina The AP generated is transmitted by electronic conduction to other cells of retina across synapses of photoreceptors, bipolar and horizontal cells and then again across synapses of bipolar, ganglion and amacrine cells. 12

13. Neurophysiologic activities across each cells HORIZONTAL CELLS Amplitude of the response increased steadily with illumination. Luminosity (L) response – Hyperpolarizing with broad spectral function. Chromatic (c) response – Hyperpolarizing for a part then depolarizing for the remainder of the spectrum. This observation was the first evidence of opponent color coding in the vertebrate retina. BIPOLAR CELLS Show a centre – surround spatial pattern. Red light striking the centre causes hyperpolarizaton and green light cause depolarization in the surrounding. 13

14. GANGLION CELLS 3 distinct group of ganglion cells designated as W, X and Y cells, each serving a different function. Of these, X ganglion cells mediate color vision. When all 3 types of cones (RGB) stimulate the same ganglion cells, the resultant signal is white. Opponent color ganglion cell- Excited by one color cone and inhibited by the other. ( Red- Green) Double Opponent color cell – Have a system which is opponent for both color and space. It is concerned with ‘simultaneous color contrast’. 14 AMACRINE CELLS Exact role in color vision not clear. They may act as an automatic color control.

15. Distribution of Color vision in retina Trichromatic color vision mechanism extends 20 to 30 degrees from the point of fixation. Peripheral to this, red and green become indistinguishable. In far periphery all color sense is lost, although cones are still found in this region. Centre most region is blue blind. So, when a red object is brought from the periphery to the centre. The individual first becomes aware of a colorless object in the periphery, then as salmon pink and finally red as it moves towards the centre. 15

16. Processing of Color signals in LGB All LGB neurons carry information from more than one cell. Color information from ganglion cells is relayed to parvocellular portion (layer 3-6) of the LGB. Spectrally non-opponent cells that give the same type of response to any monochromatic light make about 30% of all LGB neurons. Spectrally opponent are 60%. They are excited by some wavelengths and inhibited by others and thus appear to carry color information. Classified into 4 types. - Two types of cells with Red Green antagonism +R/-G and +G/-R. - Two types of cells with Blue and Yellow antagonism, +B/-Y and +Y/-B. 16

17. Analysis of color signals in the visual cortex Color information from the parvocellular LGB is relayed to layer 4c of the striate cortex (area 17). Information then passes to blobs in layers II and III. These are also centre-surround cells like ganglion and LGB cells. From the blobs, color information is relayed to thin strips in the visual association area and then to the Lingual and Fusiform gyri of occipital lobe which is the area concerned with color. 17

18. 3 . ASSOCIATED PHENOMENA

19. 19 Simultaneous color Contrast Perception of a particular colored spot against a colored background. E g – grey spot appears greenish in red surround. Successive color contrast Phenomenon of colored after images. The color of the after image tends to be near the complementary of the primary image Color consistency Human eye continues to perceive the color of an object even after the spectral composition of the light falling on it is markedly altered. Hierarchy of color coded cells Opponent color cells in ganglion cells (18) LGB (12,13) Double opponent cells in layer 4c of striate cortex (19,20) Blobs (21,22) The cells at one level of hierarchy converge to form the receptive field of the cells at the next higher level .

20. COLOR TRIANGLE BASED ON PHOTOPIGMENTS A three-dimensional graphic representation of all colors on basis of trichromacy of color mixture. The three axes represent various amounts of pigment absorption by each of these color pigments. A color is represented by a vector with origin at zero absorption and for its entire length represents a specific ratio of absorption of the three photopigments. Length of the vector identifies its brightness. The angle in the space represents the hue. The colors from green and red absorbing photopigments lie close to the floor of the color space. 20

21. THE COLOR-METRIC Developed to get around the problem of making sense that two people mean the same color when naming it. Purely evolved on a psychological basis and has no connection with the physiology of eye. The two main Color metric systems used internationally are The CIE color space system and The Munsell color system . 21

22. The CIE color space system Developed by the International Commission of Illumination (CIE), for precise identification of colors in textiles, paints, food coloring and soil types. The graph shows all possible colors, each defined by a pair of numerical co-ordinates or the chromaticity co-ordinate. Points on the curved border are pure spectral colors. (Rainbow) A straight line between any two points on the diagram shows all the colors that can be produced by mixing those colors. The color triangle is called the color gamut and the area inside it shows all possible colors that can be made from mixing the 3 colors in the corner. 22

23. The Munsell Color System The colors are represented in a cylinder in terms of hue, value and chroma (HVC) Hue dimension – Dominant spectral color located on the circumference of the cylinder. Value dimension – Lightness, indicated by moving up or down the cylinder. Chroma - Measured radially outward from the vertical axis. 23

24. COLOR BLINDNESS 24

25. Introduction Individual with normal color visions are known as Tri chro mates . In color blindness, faculty to appreciate one or more primary colors is either : Anomalous (defective) or, Anopic (absent) 25

26. Congenital vs Acquired 26 Present at birth. Begin after birth. Definitive severity and type. Fluctuating severity and type. Except for monochromatic type, there is no decreased VA or VF loss. Usually associated with decreased VA and VF loss. Easier to classify by type into precise categories of protan, deutan and tritan. Harder to classify and may be a combination of types. Affec ts both eyes equally. Frequently monocular. Most commonly protan or deutan and rarely tritan. Most commonly affect the short wave spectrum (blue-yellow defect ; Kollner’ rule Predominanly X linked, and therefore usually found in men. Results from aging, disease or drugs and affect both sexes equally.

27. Congenital Color Blindness X linked recessive inherited condition. 8% males vs 0.4% females affected in Caucasian societies. Broadly classified into two types :- Dyschromatopsia – color confusion d/t deficiency of mechanism to perceive colors. Achromatopsia – Rare condition presenting as cone monochromatism or rod monochromatism. 27

28. 28 Anomalous trichromatism Dyschromatopsia Congential Color Blindness Achromatopsia Dichromatism Cone monochromatism Rod Monochromatism

29. Dyschromatopsia Anomalous trichromatism – The mechanism to appreciate all three primary colors present but is defective for one or two of them. May be of the following types:- Protanomolous – defective red appreciation. Deuter-anomalous – defective green appreciation. Tritanomolous – defective blue color appreciation. 29 Dichromatic color vision – Faculty to perceive one of the three colors is absent. They are called dichromates. May be of the following types :- Protanopia – Complete red color defect. Deuteranopia – Complete green defect. Tritanopia – Absent blue color appreciation. Red Green deficiency is more common (Protanomaly / anopia , Deuteranomaly / anopia ) and is a source of danger in occupations such as drivers, sailors and traffic police.

30. Achromatopsia Cone monochromatism – Presence of only one primary color and thus the person is truly color blind. VA is usually 6/12 or higher. Rod monochromatism – May be complete or incomplete. Autosomal recessive trait. It is characterized by – Total color blindness Day blindness Nystagmus Usually normal fundus. 30

31. 31

32. Inheritence of color blindness Tritanomaly and Tritanopia are rare and show no sexual selectivity. Color blindness is present in males if the X chromosome has the abnormal gene, but females only show the disease when both x chromosomes are affected. Females with one abnormal gene are carriers. The common occurrence of Red and Green defect is probably due to their genes being located near each other in a head to tail tandem array on the q arm of X chromosome and prone to recombination. The blue cone is encoded by the gene on chromosome 7. 32

33. Acquired color blindness Categories of acquired defects include changes in Pre receptor, receptor and post receptor mechanisms that affect the perception of light stimuli. May follow damage to macula or optic nerve and is associated with central scotoma or decreased VA. Blue- Yellow impairment is seen in retinal lesions such as CSR, macular edema and shallow retinal detachment. Red Green deficiency seen in ON lesions such as optic neuritis, Leber’s optic atrophy and compression of the optic nerve. Acquired blue color defect occurs in old age due to increased sclerosis of lens owing to physical absorption of the blue rays by increased amber colored pigment in the nucleus. 33

34. __TESTS FOR COLOR VISION 34

35. Objectives in testing color vision The exact nature of the defect. Whether the subject is likely to be a source of danger to the community. 35

36. The Lantern test Subject names various colors shown by a lantern and is judged by the mistakes. It depends upon the size of the apertures of the lantern and the nature and intensity of the light source. 36

37. The Farnsworth Munsell 100 hue test Identifies hue discrimination by an error score. Greater the score, poorer the color vision. Patient with optic neuropathy display a characteristic pattern in this test. 37

38. Isochromatic Charts 38 Consist of colored lithographic plates with bold numerals represented in dots of same size but of tints which are most readily confused with those of the figures by the color defective people. Tests are also included in which the numbers can be read by the color defective persons but not by the trichromats. Originally Stilling’s test, now replaced by Ishihara’s plates which largely identifies Red Green anomalies. The Hardy- Rand- Ritler test identifies blue yellow anomalies.

39. Anomaloscopes A bright disc is seen on looking down a telescope, divided into two halves by a horizontal line. In the Nagel anomaloscope, one half is illuminated by light of the sodium line of the spectrum ( yellow) and has to be matched by a mixture of red (lithium line) and green (thallium line) in the other half. Amounts of Red and Green in the mixture can be varied by turning knobs. Pickford Nicolson anomaloscope is similarly used for blue yellow anomalies. 39

40. T HANK YOU! 40