Optometry and Vision Sciences - Theses
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ItemCharacteristics of tritanopia and other researches into defective colour vision( [1964])The so called "red-green" forms of defective colour vision are relatively common and have been actively studied since the end of the nineteenth century. Helmholtz (1909) and other German physiologists and physicists made the first precise colorimetric-investigations of human vision. Since this "German period" the study of colour vision was pursued principally by Anglo-saxon physicists and physiologists. In 1935, Pitt published his monograph Characteristics of Dichromatic Yision and W. D. Wright published his Researches on Normal and Defective Colour Vision in 1947. Both these publications continue to form a foundation for current research in defective colour vision. Whilst our knowledge of "red-green" defective colour vision became remarkably accurate and detailed the so called "Blue-yellow" defects were neglected. Wright (1947) refers briefly to tritahopia and tritanomaly commenting that "the amount of reliable information about tritanopia is meagre" (p297) and that "no tritanomalous observers have yet been discovered who could make tests at the colorimeter" (p320). The comparitive rarity of these blue-yellow colour vision defects was the principal reason for the neglect. Wright (1947) at the time suggested the incidences of tritanopia and tritanomaly were both one in a million but warned that these figures "are necessarily very approximate" (p303). However Schmidt (1943) had screened 21,000 visitors to an exhibition and found one case of tritanomaly. The rarity of these two defects is to some extent the product of the kind of screening tests employed. The Ishihara plates make no attempt to test for "blue-yellow" defects and the Nagel anomaloscope uses the Rayleigh equation. An attempt to modify the Nagel anomaloscope to provide a BG equation was not successful (Jaeger 1955). The Stilling pseudo-iso chromatic plates included some plates for the detection of blue-yellow defects, but the Ishihara suplanted this test in many countries. The identification of blue-yellow defects in mass and routine colour vision testing was made practicable by Farnsworth. Farnsworth (1943) produced his Farnsworth-Munsell 100 Hue test and has devised single pseudoisomatic plates which have been shown to be a very effective means of identifying this class of colour vision defect (Jaeger, 1955). One of these plates was used by Wright (1952) in a mass survey which yielded 17 confirmed cases of tritanopia. On the basis of this survey Wright calculated that the incidence of tritanopia was between 1 in 13,000 and 1 in 65,000, the higher freauency being the more likely. This is a much higher incidence than the 1 in 1,000,000 that had suggested previously, but much lower than the 2 in 100 of red-green dichromats among men.
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ItemAn investigation of the blue-sensitive mechanisms of human colour vision( 1971)The characteristics of the blue-sensitive process of human colour vision have been explored by means of the two-colour increment threshold technique of W. S. Stiles and the measurement of retinal directional sensitivity. The objectives were a) to find points of difference between the three independent blue-sensitive mechanisms π1, π and π3 which might lead to reasonable hypotheses concerning their physiological basis and functional role, and b) to establish whether the unique threshold behaviour of the blue process at the fovea is due to an absence of blue cones as Wald (1967) has concluded. It has been found that the π3 mechanism has similar characteristics to π1; it is absent in tritanopia, exhibits extensive complete spatial summation and has high directional sensitivity. The π2 mechanism appears to be less directionally sensitive. Both π1 and π3 have been demonstrated at the central fixation area of the fovea. On the basis of these results and evidence in the literature a theory of the blue-sensitive mechanisms is proposed.