THE OCULOMOTOR BEHAVIOUR OF HUMAN ALBINOS

Horizontal and vertical eye movements were recorded in 16 human albinos with a scleral search coil technique. Spontaneous nystagmus, responses to target steps, voluntary pursuit and optokinetic nystagmus (OKN) were assessed, including the effects of selective stimulation of the nasal or temporal halves of the retina. The results suggest a subdivision of albinos into three classes of oculomotor behaviour. Class I (n = 11) is characterized by vigorous spontaneous nystagmus (of the pendular unidirectional jerk or bidirectional jerk type), the absence of true horizontal OKN but the presence of the ability to control the direction of gaze in an imprecise way. In Class II (n = 2) there is a vigorous unidirectional jerk nystagmus which reverses in direction spontaneously or as a result of visual stimulation. Moving stimuli typically elicited inverted pursuit, the smooth eye movements having a direction opposite to that of the stimulus movement. Class III (n = 3) is characterized by very little or no spontaneous nystagmus and virtually normal oculomotor responses. Only pursuit of motion in the temporal direction, projected onto the temporal half retina, was defective. In all three classes, vertical eye movements were disturbed much less than horizontal. Anomalous visual projections (confirmed in all subjects by asymmetrical monocular visual evoked cortical potentials) are a likely basic cause underlying the oculomotor instability, but the large intersubject differences show that the eventual consequences of misrouting and secondary adaptations can vary widely among subjects.