The retina in each eye is composed of photoreceptors, each designed to detect specific visual stimuli, for example, light, darkness, or color. To detect objects in the environment, the eyes move to place the object on the fovea, the part of the eye with the highest acuity. Eye movements also keep the object on the retina despite movement of the object or head. There are five types of eye movements: saccades, vergence, pursuit, vestibulo-ocular reflex and optokinetic response, according to the University of Western Ontario.
Saccades
If an object appears to one side, for example during reading, the eyes rotate in tandem to foveate the object. Saccades are used to place an object in front of the fovea in order to see the object clearly. Saccades are very fast minute eye movements; in fact, it is impossible to produce a slow saccade, according to Brown University.
Vergence
Vergence eye movements are created when a person looks from a far object to a near object and vise versa. Convergence occurs when a person looks from a far object to a near object, for example, when a person looks at their nose. Divergence occurs when looking near to far; this movement type rotates the eyes out toward the sides. Vergence movements are slower than saccadic eye movements. Also, during saccades, both eyes move in the same direction; however, during vergence, the eyes move in opposite directions, according to the University of Western Ontario.
Pursuit
When an object is moving, pursuit eye movement maintains the object on the fovea. Pursuit movements are smooth compared to rapid saccades. This type of movement is only generated when watching a moving object. They are not produced voluntarily, according to the University of Western Ontario.
Vestibulo-Ocular Reflex
When the head is moved from side to side while watching a stationary object, a reflexive eye movement called the vestibulo-ocular reflex, or VOR, is created. The VOR creates involuntary eye movements that rotate the eyes in the opposite direction to head motion to keep the object foveated, according to Dartmouth University.
Optokinetic Response
The optokinetic response, or OKR, generates eye moves that maintain gaze on a moving object. The response can be triggered when an area of the viewed environment slips on a large portion of the retina. It produces the sense of “self motion;" for example, a person could feel like they are moving when sitting in a stopped car next to another car that begins to move, according to the University of Western Ontario.
