Visual acuity is the sharpness with which a visual impression from the environment is imaged on the retina of a living being and processed in its brain. Factors such as receptor density, receptive field size, and the anatomy of the dioptric apparatus affect visual acuity in individual cases. Macular degeneration is one of the most common causes of visual acuity loss.
What is visual acuity?
A cross-section of the human eye showing its anatomical components. Click image to enlarge. Visual acuity is known by the medical term visus. By the term, medicine refers to the potential with which a living being can perceive and identify the structures of its environment through its visual organ. Visual acuity can be measured and is sometimes used for diagnostic purposes. Various other medical terms are associated with visual acuity. The minimum visibile refers to the limit of everything visible. Minimum discriminibile is the threshold of detectability of differences between an object and its surroundings. The minimum separabile refers to the separation of adjacent contours of adjacent objects. Minimum legibile refers to reading visual acuity. It is to be distinguished from visual acuity proper. In addition to physiological vision, reading acuity requires a memory that forms logical relationships from the set of letters. Visual acuity depends primarily on the size of the receptive field, the density of retinal receptors, and the dioptric apparatus. Object texture and shape also have an effect on visual acuity.
Function and task
A person’s visual acuity depends on a variety of factors. For example, one factor influencing visual acuity is the receptive field and its size. The receptive fields of the central retina consist of small retinal cells. Those of the peripheral retina consist of larger retinal cells. A receptive field is correspondingly larger in the periphery of the retina. Within the fovea centralis there is an interconnection of cones on bipolar cells and ganglion cells, which corresponds to a 1:1 interconnection. Each cone is thus connected to only one target cell. Visual acuity in the central visual field is ideal due to the limited size of the receptive fields. In the extrafoveal region of the retina, multiple rods project to one cell and visual acuity is correspondingly poorer. Not only the interconnection of visual receptors but also their density plays a role in visual acuity. In the fovea centralis and thus the central part of the retina the density is highest. In the extrafoveal retinal regions, in turn, the density of the rods is greatest. Since there are no photoreceptors at all in the optic papilla, visual acuity in this area is zero. Hence the name ‘blind spot‘. Like the factors of receptor density and field size, the quality and anatomy of the dioptric apparatus plays a central role in visual acuity. Rays at the edge of the cornea, for example, are refracted much more strongly than those in the axial region. In this context, there is talk of spherical aberration, which can cause blurred images on the retina. The eye corresponds to an inhomogeneous medium that scatters light. This is another reason why objects can sometimes appear blurred. In addition to the aqueous humor and the vitreous humor, the lens and the cornea influence the sharpness with which an ambient image is imaged on the retina of the eyes. The cornea is more curved on its surface in the vertical direction than horizontally. If the difference in curvature is too high, this is called astigmatism (a curvature of the cornea), which causes blurred images. To some extent, the optical properties of objects and the environment also influence visual acuity. Besides contrasts, brightness and colors can be relevant in this context. The shape of an object has just as much influence on visual acuity. For example, right angles are resolved more strongly by the central nervous system than in the dioptric apparatus.
Diseases and disorders
Visual acuity has clinical relevance primarily for vision testing and the eye diseases that can be diagnosed by it. For example, writing boards can be used to determine visual acuity. Landolt rings are also used. When using the rings, the doctor shows the patient rings of different sizes, all of which have a gap. The patient must indicate the location of the gap in each case.Emmetropic patients with a visual acuity of 1 recognize a gap with a width of one angular minute. If a patient can only recognize the gap from twice the width, the visual acuity is 0.5. The writing table method is somewhat different. In this variant of visual acuity determination, the patient reads numbers or letters from a blackboard. Each row of numbers or letters is marked by a certain distance. If the patient can decipher them from this specified distance, the visual acuity is 1. Interestingly, a visual acuity of 0.1 is usually sufficient for a person to orient himself outdoors and in bright lighting. Reading, on the other hand, requires a visual acuity of at least about 0.5. Visual impairments with a reduction in visual acuity occur physiologically predominantly in old age and often correspond, for example, to a degeneration of the macula. The causes for a radical reduction of visual acuity are different. For example, in addition to macular degeneration, diabetic retinopathy is one of the most common causes of reduced visual acuity. Impaired visual acuity can also be associated with retinal detachments, a cataract or glaucoma. In addition, in the context of some congenital syndromes, a genetically pre-programmed degeneration of the relevant structures occurs, which causes a loss of visual acuity. In some conditions, visual aids can restore visual acuity.
