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«Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences TUIJA JETSU Modeling color vision Publications of ...»

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The LGN consists of six layers that can be divided into two different parts: the top four layers are called parvocellular pathways and the bottom two layers magnocellular pathways. Distinction between magno- and parvocellular pahways already occurs at the ganglion cell level, and even though the precise distinction between these pathways is still under investigation, it has been suggested that the magnocellular pathways deliver the main part of all luminance-related signals, whereas the parvocellular layers are responsible for carrying the signals that result in color perception.Research by Schiller et. al. [75] shows that monkeys with lesions in the parvocellular layer lost their ability to detect color, which is a clear indication that the parvocellular layer plays a role in the color vision process.

Most of the neural fibers connect from the LGN to the primary visual cortex (V1). The visual cortex provides a complex and a greatly distorted map of the retinal image. V1 also seems to be the place where the information from the two eyes is brought together [49].

There is a lot of research (e.g. [14,21,28]) that shows that on the corDissertations in Forestry and Natural Sciences No 20 Tuija Jetsu: Modeling Color Vision tex there are a number of types of opponent cells that are excited by wavelengths at one end of the visible spectrum and inhibited by wavelengths at the other end of the spectrum. Opponent cells can also be found in the LGN [17]. A thorough summary of the color-coding in the different parts of the human color vision system has been done by Conway [13]. Opponent cells behave in a way that offers a foundation for the opponent-process theory of color vision [33].

1. Signals from the retinal ganglion cells continue to the optic nerve

2. After crossing in the optic chiasma, ganglion cell signals reach the LGN

- M-cells synapse in layers 1 & 2:

magnocellular layers (luminance signals)

- P-cells synapse in layers 3, 4, 5 & 6:

parvocellular layers (chromatic signals)

3. A LGN neuron also receives information from brain stem and visual cortex

- opponent cells on the LGN

4. LGN transfers information to the visual cortex

- opponent cells on the cortex

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18 Dissertations in Forestry and Natural Sciences No 20 3 Color Vision Deficiencies Even though most people have normal trichromatic color vision, about 8 % of males and less than 1 % of females have some level of color vision deficiency (CVD) [11]. The most severe version is a total lack of one or more types of cone cells in the eye. Dichromacy is a condition where one of three cone cell types is missing completely.

If two (long- and middle-wavelength) or all types of cone cells are missing, the defect is called monochromacy, and the color vision of the subject is limited to black, white and shades of gray. Instead of a total loss of a certain cone type, it is also possible that the sensitivity of a cone type has been shifted towards the sensitivity of another type (anomalous trichromacy).

The most common form of color vision deficiency is red-green color blindness, which results from the absence of either the long or middle wavelength sensitive visual photopigments [4]. Red-green blind subjects, as the name implies, have problems with differentiating between red and green hues. This type of color vision deciency can be divided into two sub-categories: protan and deutan defects, depending on the type of cones that are not working properly. A protan defect can more accurately be defined as protanopia if the long-wavelength cones are completely missing, and protanomaly if there has been a shift in pigment absorption to shorter wavelengths. A deutan defect can similarly be defined as deuteranopia if the middle-wavelength cones are completely missing, and deuteranomaly if there has been a shift to longer wavelengths. Because the red-green color vision deficiency is a typical case of X-linked recessive inheritance, this form of color deficiency is more common with men than with women. See Table 3.1 for more details.

A considerably rarer case of color vision deficiency is blueyellow or tritan color deficiency. Like protan and deutan defects, tritan defect can also have forms of tritanopia or tritanomaly, describing either a total or partial loss of short-wavelength cone pig

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ment. In the case of tritanomaly, the pigment absorption of shortwavelength cones is not shifted as in protanomaly and deuteranomaly but rather the pigment is partly missing [4, 49]. Tritan defects are inherited autosomally, so the probability of inheriting a tritan defect is equal for men and women. See Table 3.1 for more details.

Even though protan and deutan color vision deficiencies are generally referred to as red-green deficiencies, protan and deutan subjects only have problems in differentiating certain hues of red and green. The confused colors lie in a CIE 1931 x,y chromaticity diagram on so-called confusion lines that converge at a single point, the confusion point (Figure 3.1) [92]. The same principle holds also for tritan deficiency.

Color vision deficiencies are also common among other primates, especially among New World Monkeys [43,44]. For example, in the species of squirrel monkeys (Saimiri sciureus), some females have trichromatic color vision, whereas males are, in general, redgreen color blind. Until now, color vision deficiency has been considered to be an incurable condition. It was however shown by a group of researchers at the end of 2009 [59] that by using gene ther

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apy it is possible to cure color blindness in adult monkeys that had been color blind since birth. The experiments were conducted using a group of male squirrel monkeys, which were known to be redgreen color blind. In the experiments, a virus carrying the opsin for the third cone pigment type was added to the dichromatic retinas of the monkeys, providing a receptoral basis for trichromatic color vision. The researchers report that the treated monkeys’ improvement in color vision has remained stable for more than 2 years. This is a remarkable result that also proves the plasticity of the primate visual system in the sense that the rest of the visual system is able to adapt to the addition of the third receptor type.

3.1 COLOR VISION TESTS

There are various kinds of tests for recognizing the different types of color vision defects. A comprehensive description of different tests can be found, for example, from the book Diagnosis of Defective Colour Vision by Jennifer Birch [4], which has also been used as a general reference throughout this chapter. Table 3.2 shows the suitability of color vision tests for different purposes.

Screening tests are used to discover whether or not a person has a color vision deficiency. Grading tests aim at defining the severity of the deficiency. Classifying tests are used to identify the type of the deficiency (protan, deutan, tritan) and diagnostic tests to differentiate between dichromats and anomalous trichromats. There are also numerous tests that measure occupational suitability rather than really define the type of the subject’s color vision deficiency. Commonly used tests in clinical practice include arrangement tests, like Farnsworth-Munsell 100-Hue test and Farnsworth D15 test [23, 24], pseudoisochromatic plates (American Optical Society (HRR) plates, Ishihara plates [32, 42]), anomaloscopes, like Nagel anomaloscope [90] and lantern tests, like Holmes-Wright lanterns [35].

A spectral anomaloscope (Figure 3.2(b)) is a standard reference test for determining normal or abnormal red-green color vision and for diagnosing the exact type of red-green color deficiency. In

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Table 3.2: Suitability of color vision tests for different tasks according to J.

Birch [4] Numbering used in the table: 0 (not suitable) - 3 (excellent suitability) Task A: Precise colour matching (Spectral anomaloscopes) Task B: Identification of figure (Pseudoisochromatic plates) Task C: Arrangement of hues in sequence (Hue discrimination) Task D: Colour naming (Lanterns)

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anomaloscope testing, the subject is shown a circle that consists of two halves, test and target stimuli. The subject is asked to manipulate the red/green mixture ratio and the yellow luminance of the test stimulus by using two control wheels until the test stimulus matches the target stimulus in both color and brightness. Four separate matches are made.

Pseudoisochromatic plates (Figure 3.2(c)) are designed in a way that individual elements of the plates are spots or patches of color which vary in size and lightness. The design exploits isochromatic color confusions so that elements in the plates can be seen by people with normal color vision but cannot be seen by color deficient 23 Dissertations in Forestry and Natural Sciences No 20 Tuija Jetsu: Modeling Color Vision people. It is possible to design plates so that figures either vanish, appear or transform when the results of normal color vision subjects and color deficient subjects are compared.

Arrangement, i.e. hue discrimination, tests (Figure 3.2(a)) are usually meant to be used for identifying people with significant color deficiency, who are likely to experience practical difficulties in specific occupations. Arrangement tests consist of color samples that the subject arranges in a natural order according to hue, lightness or saturation. Based on the errors made during the process, it is usually possible to define the type and severity of the color deficiency.

Lantern tests (Figure 3.2(d)) are often used when testing people interested in careers in maritime, military, aviation and transport services. In these fields, recognition of colored light signals correctly is considered to be important for safety. Colors are either shown in pairs or singly in order to demonstrate the different kinds of light signals and the subject has to name the color(s) present. It should, though, be kept in mind that color naming is not an ideal method for identifying color deficiency, because when dealing with only small number of colors, it is possible to get reasonable results also with lucky guesses.

3.2 DIFFERENCES BETWEEN INDIVIDUALS

A lot of research has been carried out in an attempt to explain the differences in color vision between individuals (for example [2, 3, 19, 34, 36, 46, 51–53, 58, 65, 74, 86, 91], just to mention a few more recent papers). Because color vision deficiency is linked to the Xchromosome, it is genetically possible for women to be carriers of the color deficiency gene without being color vision deficient themselves. Findings in the field of molecular genetics have revealed that instead of three retinal visual pigments, for the female it is actually genetically possible to have four different photopigments on the retina [19,52,65]. The effect of the fourth photopigment on color vision has not really been noticed in the conventional color vision

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tests, and it is yet partly unknown how it really affects color vision.

Jameson et.al. [46] have found that women with four-photopigment genotypes perceive significantly more chromatic appearances than either male or female trichromat controls. Bimler et.al.

[3] have also examined the differences in color experience between trichromat male and female subjects and concluded that when differentiating between colors, males placed significantly less weight on a red-green axis, and more on lightness. However, they soundly state that the differences in behavior can be caused by a number of reasons, ranging from retinal performance to patterns of socialization. On the other hand, Hood et.al. [36] report that the chromatic discrimination along a red-green axis was impaired in the case of carriers of deutan deficiencies but was normal in the case of carriers of protan deficiencies. Also, results from Bimler et.al. [2] support the fact that the color space of heterozygous women would be compressed in a red-green dimension. Because of a low number of subjects, they were unable to provide reasonable separate results for protan and deutan carriers.

In addition to individual differences in sensor photopigments, there are also large inter-subject variations in the distribution of cone cells in the retina. The estimates of the relative numbers of cone cells sensitive to long (L) wavelengths and middle (M) wavelengths depend a lot on the experimental method in use [53]. A recently developed technique for measuring cone distribution from a living eye involves high-resolution adaptive-optics imaging combined with retinal densitometry [34, 74, 91]. Also an adaptive-optics scanning laser ophthalmoscope has been used to measure the packing of the cones on the retina [9, 10]. Kremers et.al. [53] mention in their summary article of different methods for defining the cone distribution that the L/M cone ratio in normal eyes has previously been found to vary between 10:1 and 1:3. Large individual variation is also shown in the results of Roorda and Williams [74,91] and

Hofer et.al. [34], where the ratio of L to M cones varied from 1.1:1

to 16.5:1 within normal subjects. Hofer et.al. also found a protan carrier with normal color vision who had an even more extreme

26 Dissertations in Forestry and Natural Sciences No 20

Color Vision Deficiencies L:M ratio (0.37:1). All subjects in these two experiments had nearly identical S-cone densities. In the case of color vision deficient subjects, it has also been shown by adaptive-optics imaging [6] that in some cases there is nothing in the place of the deficient cone type, and in other cases cones have been replaced by another type.



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