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V Diaconu; The Red-Green Mechanism in Dichromats . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3798.
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Introduction: It is well known that dichromat red-green color deficiencies represent X-linked genetic disorders in which one of the L or the M cone receptors could not be expressed in the retina. The anomaloscope test shows that red-green dichromats can not differentiate yellow from red or green. Using pseudo isochromatic plates most dichromat subjects show residual red-green color discrimination and are categorized as having a mild red green color defect. Purpose: In the present study we examine the increment spectral sensitivity function of two deuteranopes and two protanopes for the detection of circular stimuli (1.3 deg) presented for 200 msec on a 15 deg white and also different specific color adaptation backgrounds. A mathematical model presented by Diaconu and Faubert (IOVS 2001) was used to derive the chromatic and achromatic mechanism parameters from the sensitivity curves. Subjects: Subjects were two deuteranopes and two protanopes. Two other normal trichromat observers where tested for control. All subjects were evaluated for congenital colour defects using the HRR pseudoisochromatic plates and the Nagel Anomaloscope. With the Nagel Anomaloscope the subjects could match red and green against yellow without difficulty. Protanopes required very low intensities of yellow to match red. The HRR plates categorize the subjects as having strong R-G defect but they could identify correctly the last two red-green diagnostic plates. Method: The capacity to detect wavelengths between 480 nm and 640 nm at 10 nm intervals was evaluated with a standard spectral sensitivity setup mounted on an optical bench, consisting of a Xenon arc lamp with a monochromator. The same setup was used to evaluate the unique yellow spectral locus for each subject, necessary to compute the chromatic and achromatic parameters from the sensitivity. Results: Increment spectral sensitivity curves for protanope and deuteranope subjects are similar to the M- and L-cone fundamentals, respectively. They show a yellow spectral locus 578nm with 5 nm standard deviation comparable with a normal yellow spectral locus 577nm with 1.2 nm standard deviation. The chromatic and achromatic parameters show a red green mechanism enclosed by a strong luminance or yellow mechanism represented by M and L cone sensitivity for protanopes and deuteranopes respectively. Conclusion: Though the dichromats match reds and greens to yellows they may still experience sensation of red, yellow or green through a weak red-green mechanism.
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