Eye movement serves all aspects of vision and is also partially controlled by reflexes triggered by self-motion, such as detecting and tracking an object. In this process, the image is placed and held in the center of the yellow spot, which is the fovea. As soon as an object moves, subsequent movements of the eye are triggered, which at some point reach their limits and are interrupted by fast and jerky movements called saccades. Eye sequence movements are important conditions for optimal vision.
What are ocular sequence movements?
Ocular sequence movements are important conditions for optimal vision. When a movement occurs, the need arises to move the eyes along with the intended target so that the actual image can continue to be held in the fovea. The speed of eye movement must therefore adapt to the speed of the sighted and moving object. The eye movement pattern is composed of three different functions when a target or object is aimed at. First, and with information transmission to the central nervous system, fixation takes place, a holding function of the current object or target, whereby the eyes remain focused on it until the signals and information have been acquired and transmitted. Second, an object is noticed by gaze jumps, where the eye movements jump from one point to the next, by muscle movement, and thus serve, for example, orientation in space or the detection of individual letters when reading a text. As a third component there are the eye tracking movements, an ability to track moving targets and objects by a gliding eye movement. If this is interrupted or does not occur at all, there is, among other things, a disturbance in the coordination of both eyes and asthenopic complaints may result, which are caused by overexertion of the visual apparatus.
Function and task
The primary purpose of eye tracking is to stabilize the gaze on a moving object. Initially, the eyes are still stationary during this process, and the image shift across the retina is not compensated. Only after about 100 milliseconds the movement of the eyes starts and triggers imperceptible delays due to visual processing. Sequential eye movements minimize the image shift and capture the input signal. While saccades briefly perceive objects within milliseconds without directly capturing them, eye-tracking motion is more smooth and tied to the perceived object. In complete darkness or without an aimed target, eye tracking movements cannot take place. Rather, here the gaze would again jump through saccades from one point to another. An eye-tracking movement, on the other hand, compensates for a moving object as a fixed point. This also includes the sensory modalities and the imaginative ability of the observer. In reading, the eye sequence movement also plays a crucial role in grasping the letters as a series and finally as words and sentences. The image or object is repeatedly repositioned in the fovea. This process works similarly to a simple control loop, with an image shift taking place on the retina that the person himself is not aware of. During object tracking, retinal afferents are corrected by a signal that determines how many eye movements are needed to shift the image. This leads to seeing an illusory movement during eye tracking movements until the image is stabilized. Nevertheless, movement and change of the object is perceived, as well as the background movement itself. Accordingly, humans are able to detect moving targets without delay and keep the object in view. No actual retinal image motion is perceived; the visual background is shifted across the retina, with speed adapted to eye movement.
Diseases and disorders
In earlier studies, medical science focused its attention mainly on the mechanical aspect of ocular pursuit movement. Now, however, it is also tested with regard to perception and, consequently, the processing of visual stimuli and information. This provides information especially on whether the cerebellum is functioning and how the visual short-term memory is working. A test of the eye tracking movement gives e. g.B. some evidence of central vestibular damage when eye movement is completely or even partially disturbed. Disturbances in eye movement also occur when there is damage to the cerebellum. Then the motor ability of the eye movement is usually completely lost. If this is impaired, it can be an indication of various diseases, including schizophrenia. Especially in this clinical picture, the expression and course is extremely versatile, so that in medicine, certain basic character symptoms are searched for, in order to be able to make a better diagnosis. This includes the eye-following behavior, because it is easy to register and analyze. The eye-tracking system takes place via association fields, which in turn are controlled via the control center of the brain stem and via the cerebrum and cerebellum. This makes it possible to narrow down functional or structural damage, which allows conclusions to be drawn about the entire topography on the basis of a specific pattern of disturbance, thus also enabling brainstem diagnostics, which in turn can reveal a schizophrenic disorder. In addition to disturbed motor functions, in schizophrenia, ocular sequential movements are usually also disturbed. Staring without content or frequent blinking occurs. In addition, saccadic reaction time is greatly prolonged, which in turn entails not hitting the gaze target, underestimating or overestimating it, and frequent corrective saccades.