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HE Smithson, P Sumner, JD Mollon; How to Find a Tritan Confusion Line . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2955.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Modern experiments on colour vision often seek to isolate one of the cardinal directions of colour space. By exploiting transient tritanopia we have developed a new psychophysical method to locate the tritan line for an individual observer and for a specific position in the visual field. To validate this method we determine first that our estimate of the tritan line is not critically dependent on the chromaticity chosen for the adapting field, and second that our estimate varies with retinal eccentricity, as predicted from the distribution of macular pigment. Methods: A violet probe stimulus that is visible in the presence of an adapting yellow field may be invisible for several seconds after the yellow field has been turned off. We adopt the working hypothesis that this «transient tritanopia» reflects polarization of the S-opponent mechanism [Pugh & Mollon, 1979, Vision Research, 19, 293-312], and suggest that the threshold for a detecting a chromatic change will be maximally elevated for the axis of colour space that offers no modulation of the L/M-opponent mechanism. Stimuli were presented on a calibrated CRT. Subjects were first required to view an adapting field for 2 minutes. On each trial, chromatic sensitivity was probed 400 ms after an abrupt transition, from the top-up adaptation field, to equal energy white. Candidate tritan vectors were chosen from either side of the theoretical tritan axis, and the vector exhibiting maximum loss of sensitivity under conditions of transient tritanopia was identified as the tritan vector. Data were compared to models that assume selective desensitization of the S-opponent mechanism. Results: The maximal loss of sensitivity was confined to a narrow range of hue angles, and the position of the maximum was not systematically affected by the L:M ratio of the adapting field. In addition, our estimate of the tritan line changed as expected when we changed the eccentricity of the targets: our observers resembled more closely the CIE 10° observer when detecting stimuli at ≷3° eccentricity, and the Judd 2° observer when detecting stimuli presented around 1°. Individual differences were also identified. Conclusion: We propose transient tritanopia as an effective means of locating an individual's tritan confusion line for a specific position in the visual field. The method has practical advantages. For example, it can be carried out using a CRT without the need to construct a uniform auxiliary field, and luminance noise may be used so that test stimuli need not be precisely equiluminant.
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