Are there two types of deuteranopes?

1. The colour vision of a Type I deuteranope who fulfils both of Willmer's criteria (normal foveal luminosity curve, two cone mechanisms in the central fovea revealed by a small 10' test flash) has been studied.2. Spectral sensitivity curves (at threshold) on bright red or green backgrounds are identical in the red-green range.3. Heterochromatic brightness-match luminosity curves measured after bright red or green bleaches are identical in the red-green range.4. Study of prereceptor light losses show normal colour of the ocular media; spectral reflexion coefficient measurements reveal no evidence of macular pigment.5. Luminosity curves measured through a filter which artificially replaces the missing macular pigment is identical to the deuteranopic (Type II) curve. Lack of macular pigment explains the ;normal' luminosity curve.6. Red and violet backgrounds raise the thresholds for 10' red and violet tests by different amounts because two cone (the red and the blue) mechanisms are concerned.7. Reducing the size of the test to 4' eliminates the contribution of the blue cone mechanism to threshold. Now only the red mechanism determines the threshold.8. It is concluded that this subject has only a single red-green cone pigment, normal erythrolabe, like other (Type II) deuteranopes.