Eye movements can be divided into active and passive movements. While active eye movements are used to record visual information, passive eye movements are used to diagnose motility disorders.
What are eye movements?
The human eye in cross-section showing its anatomical components. Click image to enlarge. The totality of all eye movements is also called oculomotor activity or ocular motility. The eyeballs (bulbi oculi) are free to move in various ways. The rotational movements of the eye are called ductions. Torsions are rolling movements and versions refer to gaze turns or eye movements in the same direction. Versions can again be divided into fast versions or slow versions. The opposite of versions are vergences. They are opposite movements of the eyes. Eye movements happen arbitrarily, involuntarily, consciously and unconsciously. The eye movement is controlled by numerous control circuits. These control circuits involve not only the eye muscles, but also the central nervous system (CNS) or the retina.
Function and task
A total of six muscles on the eye are responsible for movements. The rectus lateralis muscle rotates the eyeball to the side during a contraction. It is the only eye muscle innervated by the abducens nerve (6th cranial nerve). The rectus medialis muscle causes inward rotation of the eye. The superior rectus muscle is responsible for the upward rotation of the eyeball. On the other hand, the inferior rectus muscle causes a lowering of the eye. These three eye muscles are innervated by the oculomotor nerve. The oculomotor nerve is the 3rd cranial nerve. It also supplies the obliquus inferior muscle. This rotates the eyeball upward and can also rotate the upper half of the eyeball outward. The superior obliquus muscle rotates the eyeball downward. It is innervated by the 4th cranial nerve, the trochlear nerve. The eye muscles serve to move the visual axis when tracking a visual object. Through a complicated interplay of nerves and muscles, the visual axes of both eyes are aligned and directed toward a specific object. Due to the same eye movements, both eyeballs form a functional unit. The combinations of abduction and adduction, depression and elevation, and internal and external rotation enable humans to see in three dimensions. Depending on the requirements, different eye movements are possible. A characteristic feature of conjugate eye movements is the concurrence. Conjugate eye movements include saccades, eye sequence movements, and nystagmus. Saccades are very rapid eye movements. The fixation point changes constantly. However, only the images at the time of fixation are perceived. The image shifts caused by the rapid eye movements are masked out. Eye sequence movements are rather slow in contrast to saccades. They are used to fixate an object that is moving. Nystagmus is a combination of saccades and eye sequence movements. Vergence movements change the angle of the visual axes. These eye movements are used to focus on objects. Convergence movements are necessary when viewing a near object. If an object is far away, a divergent movement occurs. The control of all eye movements can be voluntary or reflexive. However, eye movements do not only fulfill tasks during the visual process. The eyes also move during sleep. Rapid and short successive eye movements are the hallmark of so-called REM sleep. REM stands for Rapid Eye Movement. REM phases are often dream phases. Tests in sleep laboratories indicate that the eye movements in dreams are carried out by the eye muscles in real life. Normally, the muscles are not very active during sleep. Why the eyes move so violently during REM phases is not yet fully understood. Eye movements are also used therapeutically. EMDR therapy (Eye Movement Desensitization and Reprocessing) is a psychotherapeutic method used to treat trauma. The basic assumption of this form of therapy is that certain eye movements are linked to memories in the brain. Eye movements are said to activate memory centers in the brain. EMDR therapy is also said to provoke a connection between the right and left hemispheres of the brain.
Diseases and ailments
There are numerous disorders of eye movement. One very common disorder is strabismus. In medical terminology, strabismus is also known as strabismus. It is a balance disorder of the eye muscles. The extent and severity of strabismus can vary greatly. However, common to all forms is that the lines of vision deviate either permanently or when an object is fixed. Some forms are not pathological, but only deviate slightly from the norm. Problems with vision do not occur in these cases. However, a large proportion of strabismus forms are associated with serious visual impairment. Strabismus can be congenital or acquired, for example, due to a stroke or an accident. Nystagmus (eye tremor) can occur physiologically as well as pathologically. Physiologically, for example, nystagmus is seen when looking out the window of a moving car or train. Pathologically, eye tremor occurs, for example, with vertigo, cataracts, or [[scar]s on the retina. Failure of the eye muscles occurs when the nerves supplying them are paralyzed. The oculomotor nerve is most frequently affected by paralysis. This paralysis is also called oculomotor nerve palsy. Oculomotor palsy usually occurs in the context of cerebral hemorrhage. Vascular disorders or stroke can also result in cranial nerve paresis. In complete oculomotor palsy, all internal and external eye muscles are affected by the paralysis. The affected eyeball points downward and outward. In partial oculomotor palsy, not all muscles are affected. Here, an ocular malposition is not always visible. Rather, there are visual disturbances and dilatation of the pupil.
