Fixation lets a person look specifically at an object or subject in external space and is made possible by the retinal site of highest resolution. This so-called fovea centralis represents the main direction of vision. Disorders of fixation are present, for example, in strabismus.
What is fixation?
By the term fixation, ophthalmology refers to the human ability to look specifically at an object or subject in external space. By the term fixation, ophthalmology refers to the human ability to selectively view an object or subject in external space. Fixation becomes possible through the retinal site with the highest resolution. This site of the retina is known as the fovea centralis. The fovea centralis is the motor zero point of the eye and the prerequisite for central fixation. Fixation is referred to as either central or foveal fixation. The highest resolving point of the retina mediates straight ahead as a sense of direction and is thus representative of the eyes’ main direction of sight. This main direction of fixation lies in the physical space between the foveola and the object of fixation. The straight line between the two points is called the visual line. Other retinal points in the visual field correspond to the secondary directions and remain so only as long as the person is capable of foveal fixation. Egocentric localization with reference point to one’s own body is to be distinguished from these terms. Unlike the secondary directions, egocentric localization can be preserved even without foveal fixation.
Function and task
Fixation is one of several patterns of eye movement and, together with the other two movement patterns, characterizes the control of voluntary and involuntary information acquisition by the visual system. In a narrower sense, fixation is not a true movement, but is characterized by holding the eyes still. In fixation, the eyes are purposefully fixed on an object in the visual field. However, a complete standstill of the eye movement does not occur even with fixation. While the observer fixates an object, miniature movements and micro-saccades in the sense of the autokinetic effect can still be registered at his eyes. From fixation as a movement pattern of the eyes are to be distinguished, for example, the saccadic movements or saccades, which correspond to a fast, jerkily scanning movement pattern and usually transition from one object to another. In the broadest sense, this movement pattern is also characterized by fixations. Thus, saccades are basically rapid jumps between a large number of individual fixations. In turn, the subsequent movements of the eye correspond to slowly continuous movements that maintain fixation as the visual stimulus moves as the target of fixation. The object of fixation appears static during these subsequent eye movements. If a shift of the fixation point is to take place, we are talking about convergence and divergence. These slow movements of the eyes take place in relation to each other and shift the point viewed by means of fixation in terms of depth. Divergence and convergence are also necessary to maintain a fixation of an object moving in depth. Another eye movement is the nystagmus, which corresponds to an alternation of single saccades and single following movements. This alternation allows the observer, for example, to repeatedly locate new points for fixation when looking out of a car window.
Diseases and ailments
Fixation can reach pathologic proportions in several ways. For example, when the foveola loses its property as a site of fixation, it can cause different conditions. Either eccentric setting or eccentric fixation is present thereafter. Eccentric setting prevails when, for example, fixation is no longer possible due to macular degeneration. The main direction of vision is preserved in such a degeneration, but the affected persons have the feeling to look past the fixed object. They feel forced to this looking past, because with direct fixation a central scotoma overlays the object. Despite this, the foveola is still the center of their visual field. Eccentric fixation differs from this phenomenon. In this case, the main direction of rotation is no longer the foveola, but has shifted to another retinal point.The target point of this displacement is henceforth used by the affected person for fixation. This phenomenon is present, for example, in the context of strabismus and can cause amblyopia. In the course of the eccentric fixation, the main direction of vision is transferred to the eccentric point of the retina. The affected person subjectively has the feeling to fixate the objects directly. Accordingly, his relative localization aligns with a new main direction of fixation. The eccentric fixation is called parafoveolar fixation if the shift occurs within a Wall reflex up to about two degrees. Parafoveal fixation is referred to when the angle outside the Wall reflex is up to five degrees. If the angle is more than five degrees, the ophthalmologist speaks of peripheral fixation. Absolute lack of fixation is also called afixation. Other complaints of fixation can manifest themselves, for example, as an unsteady or restless fixation variant and are then called nystagmiform fixation. The more eccentric the fixation, the more likely it is to be associated with severe visual deterioration. Pathological fixation behaviors can be actively influenced in pleoptic procedures. If these influencing procedures do not show any effects, occlusion of the good eye is considered the standard therapy. Occlusion often enables a return to foveolar central fixation. The resulting restoration of the main direction of vision usually improves the visual acuity and orientation of the affected person.
