Saccade: Function, Tasks, Role & Diseases

Human eyes are constantly in motion. In this process, the eyeballs turn consciously or unconsciously in different directions and perceive different objects arbitrarily or involuntarily. This happens through the reception of all visual stimuli by both eyes, which as a functional unit make three-dimensional vision possible. A distinction is made between vergence movements and conjugate eye movements. The former are changes in the angle of both instantaneous axes, the latter are changes in the direction of gaze of both eyes. Saccades, in turn, are the rapid movements of the eyes to acquire a new target before it is fixed. Since the saccade cannot be voluntarily changed, it can narrowly miss the target and must be recorrected, which occurs as a reflex. At this moment, no information enters the nervous system, so the eyes are blind during this very short period of time.

What are saccades?

Saccades are the rapid movements of the eyes to acquire a new target before it becomes fixed. Human eye movement occurs under three aspects, which are also useful for medical evaluation of diseases and disorders. A distinction is made between saccades, fixations, and regressions, and in the movement pattern itself, again between saccades, eye tracking movements, and a combination of both changes in gaze direction called “nystagmus.” The saccade is understood to be the erratic and rapid eye movement that takes place between fixations. The human eye focuses spontaneously on objects without perceiving them directly. Thus, with this jerky eye alignment to the object or event, no information acquisition takes place yet. Medicine also calls this movement a scanning jump, since perception is limited at this point. Rather, the object is simply located and eye contact is established. This occurs, for example, when reading letters or fixating on points during a train ride.

Function and task

Saccades are a brief visual process that occurs before fixation, through which information is then received and processed. The movement of the eyes as a pattern is effected via two messages that occur from the vestibular organ and take place as a visual message via the retinal image shift. In this process, the vestibular organ, with its sensors for linear and rotational acceleration, responds to rapid changes in head position. The vestibular organ triggers eye movements within 10 milliseconds via short brainstem reflexes, which always result in an image movement in the direct visual field. Such movements cause slow subsequent movements of both eyeballs, which, as soon as they reach their mechanical limits, are interrupted by jerky rapid resetting movements. The actual stimulus is not a consequence of self-motion, but results from movements in the external world, e.g., a departing car perceived from the window of a stationary car, triggering the feeling that one’s own train is moving. The false assessment is called the “self-motion illusion.” Since resolution occurs only immediately in the visual field, in the center of the yellow spot (fovea centallis), it must be directed toward the stationary object. This is called fixation. The occurring change of the fixated target and the individual fixation moments takes place by saccades. Besides these jerky fast movements, there is also the slow eye following movement, where saccades and eye following movement are two forms of the goal-directed eye movement, which complement each other in this form, nevertheless proceed on different basis. With respect to the fovea, saccades move object images from the retina and its outer sheath into the fovea, whereas slow eye-following movement rather tracks the fovea as soon as an object moves. Both saccades and eye-tracking movements are supported by head movement. Straight moving objects are first perceived by saccades, then fixated by slow or smooth eye-following movements and more accurately held in the visual field and fovea. If the external object moves too fast, catch-up saccades are started which bring the image back into focus again and again. In this process, the duration of the saccade is shorter than the saccade-based reaction time. The visual response in the central nervous system is also outside the saccade.This again shows that during a saccade no visual perception and information intake takes place. It is rather a kind of intermediate process, which can nevertheless be changed by information. The information, however, does not represent visual signals, but internally generated ones, which approach a certain eye position. While the eye detects an object and the signal is compared with the target orientation, the saccade continues until both are aligned and thus identical. If the eye misses the moving gaze target, a correction saccade takes place, pushing the image back to the foveal area.

Diseases and complaints

Saccades are medically checked by the physician holding his or her two hands in front of the patient’s eyes from a distance of one meter and asking the patient to look at and fixate both hands alternately. The speed of the eyeballs and the accuracy of fixation are checked. Also, how quickly the target is acquired. If both eyes are healthy, the target is immediately recognized and the saccade does not have to correct anything, or at most only to a very small extent. If, on the other hand, there is a pathological disorder, the saccade may be hypometric or hypermetric. In the hypometric saccade, the eye movement is slowed down. From this it can be concluded that a neurodegenerative disease is possibly present, i.e. direct damage to the nervous system, which is the case, for example, in dementia, Alzheimer’s disease or Parkinson’s disease. In these conditions, the patient’s eyes lose the ability to perform fast saccades. A hypermetric saccade is when the corrective saccades occur significantly more frequently than usual. Usually results when the cerebellum is damaged.