Foveal tritanopia

Measurements of sensitivity and color discrimination suggest that an area roughly 25’ in diameter in the central fovea lacks functioning blue-sensitive cones. Alternative explanations such as screening by macular pigment or Troxler fading are shown to be unable to account for the results. Evidence is presented that residual B cone-mediated color discrimination found in this and previous studies may be mediated by light scattered onto surrounding B cones. Scattered light can dramatically affect the color appearance of lights seen in the central fovea: a small fixated green tield appears blue when surrounded by a dim, inconspicuous violet corona.