Movements of the eyes, which are normally conjugate in that the two eyes move together in an integrated manner, and are controlled by various complex neural systems terminating on the motoneurons of the eye muscle nuclei, thereby enabling the simultaneous control of twelve muscles through six cranial nerves. There are five types of eye movements
* optokinetic reflex movements; saccadic or smooth-pursuit movements that stabilize the retinal image when the whole visual field moves relative to the head
* saccadic movements: voluntary, conjugated movements that change the optical axis of the eyes from one point of fixation to another. When awake, saccades are performed at round several per minute
* smooth-pursuit movements: movements performed when following a moving object to keep the image stationary on the fovea of the retina. More often than not, they involve coordinated movements of the head, but without conjugate movements eye movements in the opposite direction. This does not happen as the vestibulo-ocular reflex is inhibited during voluntary smooth-pursuit movements
* vestibulo-ocular movements: saccadic and smooth-pursuit movements elicited by movements of the head relative to the visual field
* vergence movements: movements that change the visual axes of the eyes relative to each other when the point of fixation moves away from or toward the eyes so that the image is maintained on the corresponding points of the retina. They are necessary for binocular fusion, and thus stereoscopic vision. Moreover, they are accompanied by accommodation and pupillary constriction.
Finally, it is worth remembering that eye movements make their first appearance around mid-gestation in human prenatal life (see figure below).
