Near fixation is the visual concentration on a stimulus in the immediate vicinity. The optic pit is the retinal point of sharpest vision and is used for fixation. In addition to the visual pit, the near accommodation of the eye is required for near fixation.
What is near fixation?
In medicine, near fixation is the focused viewing of an object in the external space of short distance. Fixation occurs on the retina of highest resolution. Retina shown in figure as yellow surrounding layer. The fact that humans ideally see things in the distance just as sharply as things in the immediate vicinity is due to the accommodative ability of the eyes. Accommodation is the near and distance adjustments that the eyes make by changing the curvature of the lens. The adjustments are made reflexively by the ciliary muscle. Its contraction state regulates the tension of the zonula fibers on the lens, changing its degree of curvature and refraction. During near accommodation, for viewing close objects, the ciliary muscle tenses and thus allows the zonular fibers to relax. In this way, the lens curves. At the same time, its refractive power increases. In medicine, near fixation is the specific viewing of an object in an external space at a short distance. Fixation occurs on the retinal site of highest resolution (visual pit in foveola). In physical space, near fixation is a straight line between the foveola (yellow spot) and a fixation object in the immediate vicinity of the observer. This straight line is also called the line of sight.
Function and task
The ciliary muscle is a paired smooth muscle. When this muscle contracts, the zonula fibers at the opposite end of the lens relax. The inherent elasticity of the lens is thus dislocated and the refractive properties change. To see nearby objects, the lens thus deforms due to the contraction of the ciliary muscle. The simultaneous occurrence of a convergence movement, near accommodation and pupil constriction is also referred to as near adjustment triad and is coupled to each other via a neurophysiological control loop. The extent of the convergence movement is directly related to the accommodative power. Like near accommodation, distance accommodation is controlled by the ciliary muscle. The zonular fibers tighten when viewing distant objects due to a relaxation of the ciliary muscle. In this way, the lens curvature and the refractive power of the lens decrease. Through these accommodation processes, humans see objects near as sharply as objects far away. Accommodation also plays a role in fixation. In fixation, the eye rests on a specific visual stimulus of the visual field. Fixation always occurs on a straight line between the visual pit and a fixation object. The visual pit is located in the center of the yellow spot, where it appears as a depression. This area of the retina is the place of sharpest vision, as it is a prerequisite for fixation. In humans, the optic pit has a diameter of 1.5 millimeters. In the visual fossa lies a receptor cell whose signal is transmitted to and from a single bipolar ganglion cell to reach a single multipolar ganglion cell. Transmission losses or signal attenuation of the optical information do not occur in this way. Signal convergence decreases to nearly 0. Fixation is the main process for conscious vision. Actual information acquisition via the visual sense is therefore mostly bound to fixation processes via the optic pit. All other retinal points or objects outside the line of sight are merely secondary directions. Fixation is often associated with the notion of visual attention, as the observer applies focused concentration to specific objects in the visual field through fixation. Reading is an example of near fixation. Because reading is about actual information acquisition, near fixations account for 90 to 95 percent of total reading time, making them the essentially visual process in reading.
Illness and discomfort
The near fixation of the eye is lost, for example, with a loss of accommodative ability. Such loss may be due to paralysis of the ciliary muscle. In addition to damage to the third cranial nerve (oculomotor nerve), lesions on the optic nerve can also make near fixation impossible.When the oculomotor nerve fails, the eyeball is turned outward and downward and the pupils are dilated. Due to the simultaneous failure of the ciliary muscle, accommodation movements are no longer possible with the damaged eye. Especially the convergence movements of near fixation are disturbed. In case of a failure of the second cranial nerve opticus, the affected eye is completely blind. If the optic nerve is not completely destroyed, but only the middle part at the optic nerve junction is damaged, the patient suffers from heteronymous hemianopsia. Contralateral hemianopsia results from destruction of the optic cord. Cranial nerve destruction can occur, for example, in the context of neurological disease patterns such as multiple sclerosis. However, fixation may also be impaired by direct diseases of the optic fossa. Such impaired fixation manifests itself in an eccentric setting or eccentric fixation. An eccentric setting prevents the use of the optic pit by macular degeneration. The main direction of vision is thus preserved. Instead of seeing fixed objects clearly, they are covered by a central scotoma (visual field loss) during fixation. Affected individuals must therefore look past objects to actually see them. In eccentric fixation, unlike in the eccentric setting, the visual pit is no longer the main direction of vision. Another point on the retina has taken over this function and is henceforth used for fixation. This phenomenon is present, for example, in strabismus and often triggers amblyopia. Subjectively, an affected person has the impression to fixate an object directly. For fixation, he or she orients to the new main direction of sight, which henceforth corresponds to the retinal location of eccentric fixation. A special form of fixation loss is nystagmiform fixation. It is characterized by unsteady or restless fixation of objects and is accompanied by eye tremor.
