Eye movement refers to the voluntary or involuntary movement of the eyes, helping in acquiring, fixating and tracking visual stimuli. Specific systems are used in maintaining fixation, when reading and in music reading. A special type of eye movement, rapid eye movement, occurs during REM sleep.Rapid eye movement sleep (REM) sleep is the portion of sleep when there are rapid eye movements (REMs).EM sleep is characterized by a number of other features including rapid, low-voltage brain waves detectable on the electroencephalographic (EEG) recording, irregular breathing and heart rate and involuntary muscle jerks. During movements, certain muscles increase their activity while others decrease it. The movements of the eye include: adduction (the pupil directing toward the nose); abduction (the pupil directed laterally); elevation (the pupil directed up); depression (the pupil directed down); intorsion (the top of the eye moving toward the nose); and extorsion (the superior aspect of the eye moving away from the nose). Horizontal eye movements are rather simple. Increased activity of the lateral rectus will direct the pupil laterally, while increased activity of the medial rectus will direct it medially. The central control of eye movement can be distilled into the principle types of functions. These include voluntary, conjugate horizontal gaze (looking side-to-side); voluntary, conjugate vertical gaze (looking up and down); smoothly tracking objects; convergence; and eye movements resulting from head movements. All movements of the eyes that are produce by the central nervous system are conjugate (i.e., both eyes moving in the same direction in order to keep the eyes focused on a target) except for convergence, which adducts the eyes to focus on near objects.