Visual acuity

The Visual acuity is the sharpness with which a visual impression from the environment is reproduced on the retina of a living being and processed in its brain. Factors such as the receptor density, the size of the receptive field and the anatomy of the dioptric apparatus influence visual acuity in individual cases. Macular degeneration is one of the most common causes of loss of visual acuity.

What is visual acuity?

Visual acuity is under the medical term VA known. With 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 diagnostics.

Various other medical expressions are associated with visual acuity. The minimum visibile denotes the limit of everything visible. The minimum discriminibile is the recognizability threshold for differences between an object and its surroundings. The minimum separabile relates to the separation of neighboring contours of adjacent objects. With the minimum legibile reading acuity is meant. It is to be distinguished from the actual visual acuity. In addition to physiological vision, reading visual acuity requires a memory that forms logical relationships from the stock of letters.

Visual acuity depends primarily on the size of the receptive field, the density of the retinal receptors and the dioptric apparatus. The nature of the object and its shape also have an effect on visual acuity.

Function & task

A person's visual acuity depends on various factors. One factor influencing vision is, for example, 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 the cones with bipolar cells and ganglion cells, which corresponds to a 1: 1 interconnection. Each cone is only connected to one target cell. Visual acuity in the central field of vision is ideal due to the limited size of the receptive fields. In the extrafoveal region of the retina, several rods project onto a cell and visual acuity is correspondingly poor.

Not only the interconnection of the visual receptors, but also their density plays a role in visual acuity. The density is highest in the fovea centralis and thus the central part of the retina. The density of the rods is greatest in the extrafoveal retinal regions. Since there are no photoreceptors at all in the papilla nervi optici, visual acuity in this area is zero. Hence the name 'blind spot'.

Just like the factors receptor density and field size, the quality and anatomy of the dioptric apparatus play a central role in visual acuity. Rays at the edge of the cornea are refracted much more strongly than those in the axial region. In this context, we are talking about spherical aberration, which can give rise to blurred images on the retina.

The eye corresponds to an inhomogeneous medium that scatters light influences. For this reason too, 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 image of the surroundings is displayed 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 great, it is referred to as astigmatism (a curvature of the cornea), which creates blurred images.

The visual quality of objects and the environment also influences visual acuity to some extent. In addition to contrasts, brightness and colors, for example, can be relevant in this context. The shape of an object has just as much influence on vision. For example, right angles are resolved more strongly by the central nervous system than in the dioptric apparatus.

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Illnesses & ailments

Visual acuity is of clinical relevance above all for the eye test and the eye diseases that can be diagnosed with it. For example, writing tables 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 position 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 see the gap from twice the width, the visual acuity is 0.5. The tablet procedure is a little different. In this variant of the 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 orientate himself outdoors and in bright light. For reading, however, a visual acuity of at least about 0.5 is required.

Physiologically, visual impairments with impaired visual acuity occur predominantly in old age and often correspond to degeneration of the macula, for example. The causes of a radical decrease in visual acuity are different. In addition to macular degeneration, diabetic retinopathy is one of the most common causes of reduced visual acuity.

Retinal detachments, cataracts or glaucoma can also be associated with impaired visual acuity. In addition, a genetically pre-programmed degeneration of the relevant structures occurs in the context of some congenital syndromes, which causes a loss of vision. In some diseases, visual aids can restore visual acuity.