Ocular movements, or movements of the eye, are caused and controlled by six extraocular muscles attached to each eyeball. Movement occurs when some of these muscles contract and others are relaxed. The different types of ocular movements are saccades, smooth pursuit, and vergence. The vestibulo-ocular reflex (VOR)is often also considered to be a type of ocular movement.
Saccades, or saccadic ocular movements, are extremely fast eye movements. They are so quick, in fact, that they are considered to be the fastest movement that the human body is capable of. This movement happens voluntarily when a person shifts his gaze from one object in his field of vision to another. During these ocular movements, both eyes shift together, in the same direction.
Involuntary saccades occur during a sleep cycle commonly known as rapid eye movement, or REM sleep. Fixation maintenance is another form of involuntary saccades, in which the eyes vibrate from side to side to help maintain focus. Even when a person believes that he is holding completely still, this movement still happens. Because of this ocular movement, the eyes are almost always moving.
When compared to saccades movements, smooth pursuit eye movements are typically slower and much less jerky. These ocular movements occur when a person is trying to watch a moving object. During this type of movement, the head is generally kept still, and only the eyes move.
Vergence ocular movements occur when a person switches focus from an object at one distance to an object at another distance. Convergence eye movements happen when a person shifts his focus from an object far away to an object that is up close. Divergence occurs when a person switches focus from an object that is close up to an object that is far away. A simple way to demonstrate this would be to have a person focus on a small object, then slowly move it closer to his face and then farther away.
The vestibulo-ocular reflex is an involuntary eye movement. This usually happens when a person is moving his head, but also trying to keep an object in focus. Simply put, the eyes move to correspond with head movement. Typically, the eyes will move in the opposite direction that the head moves. For example, if someone were watching something in front of him and he turns his head slightly to the right, his eyes would move to the left to keep the object in focus.
Because a person's head is almost always moving to some degree, VOR is considered to be a very important reflex. It allows individuals to clearly fixate on objects. Individuals with a poor vestibulo-ocular reflex, due to an injury or other problem, often have trouble focusing on items such as print or fine details.