Eye movements

The eye movements either stabilize gaze by keeping the eyes fixated on an object during the head rotation, or shift gaze therefore the fovea is brought to bear on an object, or track a moving object. There are five kinds of eye movements, each controlled by a separate neural system, bring these aims.

The gaze stabilization is controlled by the vestibulo-ocular and optokinetic systems. Fast head rotation, detected by the semi-circular ducts gives input for vestibulo-ocular reflexes (VOR), while optokinetic reflexes based on visual input to monitor slow head rotations. For both the systems their output causes conjugate eye movements in the reverse direction to the head rotation, therefore retinal images do not shift.

The three systems organize gaze shift. The saccadic system produces extremely quick eye movements, saccades that move the gaze from one point in the visual field to the other, bringing new aims onto the fovea. The smooth pursuit system allows gaze to follow a moving target, therefore its image remains on the fovea. Lastly, for animals with binocular vision, the vergence system permits the eyes to move in reverse directions; either both converge or both diverge, and hence both eyes can remain directed towards an object as it gets closer or recedes.The outcome of all the five eye motor systems is through oculomotor neurons in the brainstem, the axons of which run in three pairs of cranial nerves to the skeletal muscles which move the eyes.