Lateral-eyed afoveate animals use the subcortical accessory optic system to generate accurate responses to full-field optokinetic input. When humans rotate their eyes to pursue a moving target, the visual world sweeps across their retinas, creating a contraversive optokinetic stimulus. Humans have developed a cortical foveal pursuit system that suppresses the perception of this full-field optokinetic motion during active pursuit. When foveal vision is slow to develop in infancy, this phylogenetically old optokinetic system, which is normally operative in the first 2 months of human life, continues to be ontogenetically expressed. Hypothetically, the incursion on cortical pursuit of the antagonistic motion stimulus from this subcortical optokinetic system facilitates development of the unstable oscillatory activity of the eyes that characterizes infantile nystagmus.
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