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Optokinetic Nystagmus (OKN)

The OKN is based on the fact that the eyes tend to follow or track the motion of one element at a time in a steadily moving display. As the tracked element moves out of sight, the eyes will "snap back" to fixate and follow another one. This subcortically-mediated function is termed optokinetic nystagmus (OKN) and is present at about five days after birth. This response, thought to be controlled by the tectopulvinar system, can be used as an objective measure of an infant's ability to see detail in a moving stimulus, such as a bar grating. The absence of an OKN suggests that the infant doesn't perceive the bars in the grating as separate elements. By varying the characteristics of the display such as its contrast or fineness of a grating, the researcher can measure an infant's spatial vision (e.g. acuity or contrast sensitivity).

The infant's eyes will only track a moving target to a certain distance, at which point their eyes snap back to fixate on a new target.