There are a variety of muscles that surround the eye: the lateral, medial, superior and inferior rectus muscles and the superior and inferior oblique muscles of the eye. These muscles work together to move the eye in specific ways which allow us to follow stimuli and interact with the world around us. The goal of these movements is to reposition the fovea, a specific part of the eye dense with photoreceptors, to ensure focus on a specific stimuli, target, or object.
Saccades are referred to as “ballistic” eye movements that can range from short to long. These movements are abrupt and change the point of fixation. Saccades are used when reading and when observing a room. Interestingly, these movements can be both voluntary and involuntary, depending on the situation. Moreover, saccades are the eye movement type associated with rapid eye movement (REM) sleep. Saccades are carried out via predictions made by the brain on where the target should be next. In response to this prediction, the extraocular muscles pull on the eye to re-aim the fovea at the intended target. If there is a change in velocity, or some other unexpected issue that results in an altered final position of the object, a second saccade will be made to correct the error after the first one has been carried out.
Unlike the ballistic nature of saccades, smooth pursuit movements involve a continuous moving of the eye to realign the fovea with the target. Whereas saccades are intended to be used with faster stimuli, smooth pursuit movements are normally reserved for those that are slower. Saccades can be done without a stimulus (e.g., when simply glancing around a new room); however, smooth pursuit movements normally have to have a stimulus present. Only certain trained individuals are capable of making this eye movement without an object to follow.
Vergence movements of the eye are performed when a stimulus is either moving closer or further away from oneself. Unlike the previously mentioned eye movement types, vergence eye movements are disconjugate. This means that the eyes move independently of one another, i.e, they do not have to move in the same direction. The two types of vergence eye movements are convergent and divergent. Convergent movements are used when an object gets closer and divergent eye movements are used when a stimulus gets further away.
Vestibular-ocular eye movements are utilized when the head moves but the target does not. One interesting way to observe this eye movement is to focus on a point and move your head. You should notice that your eyes, particularly the foveas, are able to stay in this spot. Essentially, the muscles in the eye make up for the movement of your head to keep your vision fixed. These movements integrate movement from the vestibular system, in particular the semicircular canals, and sight to create these quick corrective eye movements. These eye movements are made all the time, even when you're just walking!
Overall, saccades, smooth pursuit, vergence, and vestibular eye-movements allow us to keep our focus on certain objects in a variety of situations. While they are grouped separately scientifically, in the real world there is rarely ever just one type of these eye movements in isolation. Many eye movements are normally used together to effectively focus or track an object.
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