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).
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.