Lanthony's new color test. IV. Neutral zone and neutral grays.

The classification of neutral grays is helpful for the differential diagnosis of color vision defects, especially when anomaloscopic examination is not possible. In congenital color vision defects the PR, RP region of the spectrum appears 'dark' to protan-defective subjects and 'light' to deutan-defective subjects. Acquired color vision defects have a common base type, the type III blue-yellow defect. The defect develops depending on the site of the primary lesion and also on the fixation mode. Classification of neutral grays may help in differentiating optic nerve diseases from retinal diseases; when visual functions become mediated by rods, sensitivity shifts and the BV, B region of the spectrum will appear relatively 'light' to the patient.