Influence of nearsightedness and farsightedness on perception

• TYPE OF PSYCHOLOGY: Sensation and perception
• Nearsightedness and farsightedness result from an inability of the lens of the eye to focus the image of far or near objects on the retina. In the inherited form of the condition, the eyeball is too long or short in the anterior-posterior direction to allow correct focusing by the lens. In the form of the condition that is related to age, the lens becomes too inflexible to focus nearby objects.

Introduction

Several conditions affecting vision involve defects in the lens or cornea or the focusing mechanism of the eyes. By far the most common of these visual impairments are nearsightedness (myopia) and , the inability to focus the eyes on objects that are either far from or close to the viewer.

Light reflected or originating from distant objects enters the eye in essentially parallel rays. In a person with normal vision, the lens of the eye brings these rays to a point of focus on the . The focusing creates an image of distant objects on the retina, much like the image focused by a film or slide projector on a screen. The image focused on the retina by the lens of the eye, however, is inverted and greatly reduced in size. The image of a person’s head and shoulders at a distance of twenty feet from the viewer, for example, is focused upside down on the retina in a spot only a half millimeter wide.

Many people have visual impairments caused by an inability to focus on near or far objects. In such people, the distance between the lens and retina of the eye is too long or short to allow correct focusing. When the distance from the lens to the retina is too long, the point of focus of the lens falls in front of the retina when parallel rays from distant objects enter the eye. Such people cannot focus clearly on distant objects and are said to be nearsighted or myopic. When the distance between the lens and the retina is too short, the diverging rays from near objects are focused on a point that would fall behind the retina. Individuals with this visual impairment cannot focus on nearby objects and are said to be farsighted or hyperopic. An individual with normally shaped eyeballs, who is neither nearsighted nor farsighted, is emmetropic. Myopia and hyperopia are inherited conditions in many individuals.

Another form of farsightedness, presbyopia, occurs with advancing age and results from a gradual loss of flexibility in the lens with advancing age. The lens of the eye differs from the lens of an optical device such as a telescope or camera in being flexible and able to change in surface curvature. Instead of focusing primarily by moving the lens forward or backward with respect to the retina, the eye contracts small muscles surrounding the lens, the ciliary muscles, to change the curvature of the lens, thereby changing its focal length. (The focal length is the distance from the center of the lens to the point of focus.)

Lenses and Focus

The focusing process, first worked out by Hermann von Helmholtz, has some characteristics that at first seem unexpected. If the fully flexible lens of a young person were removed from the eye, it would assume an approximately spherical shape, with the maximum possible surface curvature. In this form, the lens would have maximum converging or focusing power. This is a result of the fact that the lens consists of a jellylike internal substance enclosed and held under pressure by a tough but elastic surface capsule. A spherical shape would allow the lens to assume a conformation of minimum surface area per volume.

In the eye, the lens is placed under constant tension by fibers that radiate from the lens and attach to the sides of the eye. These nonelastic fibers stretch the lens into a maximally flattened state. In this form, the lens has minimum converging or focusing power. When in its fully flattened form, the lens is considered relaxed. In a person with normal vision, the parallel light rays reflected from distant objects are brought to a point of perfect focus on the retina by the relaxed lens.

Light rays reflected from objects closer than about six meters diverge too widely to be focused on the retina by the relaxed lens. In response, a group of ciliary muscles surrounding the lens contracts. These muscles collectively form a sphincter, in a form similar to the pupil of the eye or to the lips compressed to form the letter O. Contraction of the ciliary muscles has the effect of opposing the zonular fibers, compressing the lens and allowing it to assume a more spherical shape. This increases the converging power of the lens, allowing the diverging rays reflected from nearby objects to be brought to a point of perfect focus on the retina.

There is a limit to the ability of the lens to round up under the action of the ciliary muscles, so that objects held too closely cannot be clearly focused. As individuals age, several interacting factors modify this ability. One factor is the consistency of the lens, which becomes less flexible with age and loses its ability to round up under the action of the ciliary muscles. Other factors, analyzed by Jane F. Koretz and George H. Handelman, include growth of the lens and changes in the tension and arrangement of the zonular fibers. These combined factors have the effect of moving the limit of nearest vision steadily farther from the eye—that is, of making the individual more farsighted—as the individual ages. In a newborn baby, the lens is so flexible that objects placed as close as 7 centimeters can be clearly focused. In young children, the nearest focusing distance lengthens to about 8.5 centimeters. By the twenties to thirties, the point of nearest focus has extended to about 10 to 15 centimeters. By age forty, the point of clearest focus has receded to about 22 centimeters for the average person. By this time, most people need glasses or contact lenses to converge light strongly enough to see nearby objects in clear focus. By age fifty, the nearest point of focus lies at about 40 centimeters, so that objects must be held at arm’s length to be seen clearly without the aid of glasses. By age seventy, the point of nearest focus has receded to hundreds of centimeters. (These figures are for people who are neither nearsighted nor farsighted in their twenties and thirties.)

Diagnosis

To test an individual’s eyes for nearsightedness or farsightedness, the ability of the lens to accommodate, or change in focus, must be eliminated. Otherwise, the condition of nearsightedness or farsightedness might be hidden by the eye’s ability to change its focus. For example, in mild farsightedness, in which the fully relaxed lens would focus light rays slightly behind the retina, the ciliary muscles can easily contract enough to bring the rays into focus on the retina. As a result, the farsightedness will pass undetected. This is an undesirable condition, because in such persons the ciliary muscles are under a constant state of contraction, which can lead to eyestrain and headaches.

To eliminate as a source of error, drops are usually added to paralyze the ciliary muscles temporarily. The point of focus of the fully relaxed lens can then be accurately determined. The ability of the eye to accommodate and conceal inherent nearsightedness or farsightedness is one of several reasons that it is not advisable to correct faulty vision by trying on the glasses available at drugstores and department stores until a pair is found that apparently provides clear vision.

Treatment

Nearsightedness and farsightedness have been problems for the human population since long before recorded history. In ancient times, people noticed that glass spheres, or a spherical bottle filled with water, could magnify objects and make them more clearly visible. Crude lenses of this type were probably used, at least by farsighted persons, to provide a partial correction from the earliest times. More highly developed, handheld lenses correcting for nearsightedness and farsightedness were used in both Europe and China; by the fourteenth century, the first eyeglasses had been invented in Italy. Credit for this invention is generally given to Alessandro di Spina of Florence, Italy. A portrait painted in Italy in 1352 is the first known depiction of a person wearing eyeglasses.

Nearsightedness and farsightedness can be corrected by lenses placed in front of the eye or directly on the cornea. For correction of farsightedness, a converging lens is placed in front of the eye. This lens bends parallel rays reflected from distant objects into converging pathways. Because the rays are now converging rather than parallel, the lens can bring the rays to focus precisely on the retina rather than behind the lens. For the correction of nearsightedness, a diverging lens is placed in front of the eye. The diverging rays are focused precisely on the retina rather than off the retina, as they would be in the uncorrected eye. The degree of or divergence needed to correct for farsightedness or nearsightedness is usually determined simply by trial and error, by the selection of progressively stronger or weaker lenses until the correcting lens that gives maximal visual acuity is found.

In 1784, Benjamin Franklin invented bifocals by combining lenses correcting for near and far vision. The lenses were cut in half, placed one above the other, and held in a common frame. By the early twentieth century, bifocals were cut from a single piece of glass. Later, trifocals have allowed correction for near, intermediate, and far vision. The ultimate correction is now obtained by multifocal lenses, which have a complex surface curvature that increases the power of the lens continuously from the top to the bottom. The top of the lens corrects for distant vision and the bottom for near vision. By tilting the head, the wearer can find a point on the lens that gives clear vision for any distance between near and far vision.

Because eyeglasses are fixed in place on the head, they cannot move with the eyes. As a consequence, the distance from the eye to the lens changes as the eyeball rotates. This change in distance is compensated for by a difference in curvature in the front and back of the eyeglass lens, so that the power of the lens varies from the center to the edges. The correction is imperfect even in modern glasses, so that objects viewed through off-center regions of the lenses appear distorted and slightly out of focus. This has the effect of reducing the clarity of peripheral vision. To compensate for this deficiency, most persons wearing glasses learn to turn the head instead of the eyes to view objects in the periphery. Eyeglasses present another problem because they are located several centimeters from the lens of the eye. Because of their position, lenses correcting for nearsightedness or farsightedness have the effect of changing the size or magnification of the image.

Whether the correcting lens is placed a few centimeters in front of the eye, as it is in glasses, or directly on the cornea, as it is in contact lenses, makes essentially no difference to the correction of nearsightedness or farsightedness. Contact lenses, however, have the advantage of automatically correcting astigmatism caused by defects in the curvature of the cornea. Because the cornea has a curved surface, it acts as a fixed lens and contributes to the focus of the eye. In corneal astigmatism, the cornea, rather than having a spherical shape, is slightly flattened or rounded too greatly in one direction over the surface. The effect makes one region of the cornea converge light rays more strongly than other regions. As a result, not all parts of the field of view can be placed in focus. If a series of lines radiating from a point is viewed by an astigmatic individual, some of the lines are seen in focus and some out of focus. Usually, the in-focus and out-of-focus lines are ninety degrees apart. When a contact lens is placed over the cornea, the contact lens, in effect, becomes the cornea. Because a contact lens is constructed with a perfectly spherical surface curvature, any astigmatism caused by imperfections in the cornea is relieved. Because most astigmatism is corneal, rather than caused by imperfections in the lens of the eye, contact lenses are usually effective in eliminating astigmatism as well as nearsightedness.

Since contact lenses are seated directly on the cornea of the eyes, they turn with the eye. As a result, they provide a wide field of undistorted view comparable to normal vision. Because they are placed close to the lens of the eye, in a position that is essentially the same as the cornea itself, they correct for nearsightedness and farsightedness without significantly affecting the size of the image. The several advantages of contact lenses are offset for many persons, however, by their greater expense and irritation of the eyes.

Refractive Surgeries

Many options for vision correction are available to the nearsighted, the farsighted, and the astigmatic in the form of refractive surgery. Refractive surgeries include all procedures that reduce refractive errors, correcting myopia, hyperopia, and astigmatism. There are many refractive procedures available to those with impaired vision.

Photorefractive keratectomy (PRK) has been used since 1989, when clinical trials began. This procedure employs a laser to modify the curvature of the cornea, thereby correcting nearsightedness, farsightedness, or astigmatism.

Laser-assisted in situ keratomileusis (LASIK) is a type of refractive surgery, which was first studied in clinical trials in 1995. Using a microkeratome, the surgeon creates a thin layer of cornea that can be folded back. The laser correction is then applied underneath the flap. When this technique is used, there is little scarring, recovery of vision is quick, and minimal pain occurs.

Other treatments for nearsightedness and farsightedness include intraocular rings, also called intracorneal ring segments (INTACS), laser thermokeratoplasty (LTK), and SmartSurfACE. Which procedure is best for any individual depends on the subject’s age, type and degree of refractive error, and the risks and benefits for each person as well as the subject’s personal circumstances.

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