Eye movement abnormalities in rod monochromatism and blue-cone monochromatism.

Eye movements were recorded with electro-oculography in seven patients with rod monochromatism (RM) and five with blue-cone monochromatism (BCM). The continuous horizontal nystagmus was similar in both groups. However, three patients with BCM demonstrated an intermittent, high-frequency nystagmus, in addition to the continuous nystagmus. The most striking differences between the groups were present during monocular smooth pursuit and optokinetic tracking. Patients with RM demonstrated better tracking when targets moved in the temporal-to-nasal (T-N) direction in the visual field of the viewing eye, than when targets moved in the nasal-to-temporal (N-T) direction. The velocity of optokinetic nystagmus (OKN) increased gradually over several seconds during T-N target movement. Patients with BCM did not show a directional asymmetry or a slow build-up of OKN during monocular tracking. Differences in eye movement abnormalities can be useful in differentiating these two forms of congenital color blindness from other forms of congenital nystagmus and from each other.