Fixation

The Fixation allows people to look at an object or subject in the outer space and is made possible by the retinal point of the highest resolution. This so-called fovea centralis stands for the main direction of vision. Fixation disorders occur, for example, when squinting.

What is the fixation?

With the expression of fixation, ophthalmology refers to the human ability to look at an object or subject in an external space. The fixation is possible with the highest resolution over the retinal point. This part of the retina is known as the central fovea. The fovea centralis is the motor zero point of the eye and the prerequisite for central fixation.

The fixation is referred to as either central or foveal fixation. The highest resolution point of the retina conveys straight ahead as a sense of direction and is representative of the main direction of vision of the eyes. This main direction lies in the physical space between the foveola and the object to be fixed. The straight line between the two points is called the line of sight. Other retinal points in the visual field correspond to the secondary directions and are only preserved as long as the person is capable of foveal fixation.

The egocentric localization with reference point to one's own body must be distinguished from these terms. In contrast to the secondary directions, the egocentric localization can also be maintained without foveal fixation.

Function & task

The fixation is one of several patterns of eye movement and, together with the other two movement patterns, characterizes the control of arbitrary and involuntary information intake by the visual system.

In the narrower sense, fixation is not a real movement, but is characterized by keeping the eyes still. During fixation, the eyes are focused on an object in the field of vision. However, even with the fixation, the eye movement does not come to a complete standstill. While the viewer fixes an object, miniature movements and micro-saccades can still be registered in the sense of the autokinetic effect in his eyes.

The saccadic movements or saccades, for example, must be distinguished from fixation as a movement pattern of the eyes, which correspond to a rapid, jerky scanning movement pattern and usually lead over from one object to another. In the broadest sense, this movement pattern is also shaped by fixations. The saccades are basically quick jumps between a large number of individual fixations.

The subsequent movements of the eye in turn correspond to slow continuous movements that maintain fixation when the visual stimulus moves as the target of fixation. The object of fixation has a static effect during these subsequent eye movements.

If a shift of the fixation point is to take place, one speaks of convergence and divergence. These slow movements of the eyes take place in relation to each other and shift the point observed by means of fixation in terms of depth. Divergence and convergence are also required to maintain a fixation of an object moving in depth.

Another eye movement is the nystagmus, which corresponds to an alternation of individual saccades and individual subsequent movements. This alternation allows the viewer to see new points for fixation, for example when looking out of the car window.

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Illnesses & ailments

Fixation can reach pathological proportions in various ways. For example, if the foveola loses its property as a place of fixation, it can give rise to different states. There is either an eccentric setting or an eccentric fixation.

There is an eccentric setting if, for example, fixation is no longer possible due to macular degeneration. The main direction of vision is retained with such degeneration, but those affected have the feeling of looking past the fixed object. You feel compelled to look past, since with direct fixation a central scotoma overlays the object. Nevertheless, the foveola is still the center of their field of vision.

Eccentric fixation is different from this phenomenon. In this case the main direction of vision is no longer the foveola, but has shifted to a different retinal point. The target point of this shift is used by those affected for fixation from now on. This phenomenon occurs, for example, in the context of strabismus and can cause amblyopia. In the course of the eccentric fixation, the main direction of vision changes to the eccentric point of the retina. Subjectively, the person concerned has the feeling that they are directly fixing the objects. Its relative localization is accordingly oriented towards a new main direction of vision. Eccentric fixation is referred to as parafoveolar fixation when the shift occurs within a wall reflex of up to around two degrees. Parafoveal fixation is referred to as an angle outside the wall reflex of up to five degrees. If the angle is more than five degrees, then the ophthalmologist speaks of peripheral fixation. The absolute lack of fixation is also referred to as afixation.

Other complaints during 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 impairment.

Pathological fixation behavior can be actively influenced in the pleoptic procedure. If these influences do not show any effects, the occlusion of the good eye is the standard therapy. Occlusion often enables a return to foveolar central fixation. The restoration of the main direction of vision achieved in this way usually improves visual acuity and the orientation of those affected.