Wavelength discrimination deteriorates with illumination in blue cone monochromats

Two types of incomplete congenital achromats were studied: one type (blue cone monochromats) has a conspicuous short wavelength cone mechanism, and the other type (deutan incomplete achromats) has a conspicuous long wavelength cone mechanism. The photoreceptor mechanisms were inferred from color matches and from test action spectra measured on rod-saturating backgrounds of different wavelengths. Interestingly, the illumination-dependency of color discrimination (for 5 degrees bipartite fields that were centrally fixated) differed between the two patient types, even though rhodopsin photoreceptors were common to both. As illumination level increased, the ability to discriminate wavelength differences deteriorated for the blue cone monochromats, whereas, for the deutan achromats, wavelength discrimination remained relatively constant even near 100,000 scotopic trolands. The performance decrement in the blue cone monochromats was probably not associated with rod saturation, as the field action spectrum to cause a just-noticeable-difference (jnd) decrement in discrimination was poorly fitted by a rhodopsin action spectrum. In addition, the blue cone monochromats had rhodopsin photoreceptors that did not saturate in bright illuminations. The authors hypothesize that the deterioration of wavelength discrimination at high illuminations is not an abnormality of blue cone monochromacy. Rather, it may be a property of the normal color mechanism through which signals from the short wavelength cones pass.