The eyes constantly move to keep objects being viewed on the fovea, which is a small area of the retina that contains only cones and is responsible for the best peripheral visual acuity. The six extraocular muscles that move each eye are innervated by the oculomotor (III), trochlear (IV), and abducens (VI) cranial nerves. Alterations in ocular movement include strabismus, diplopia, and nystagmus.

Visual acuity refers to the ability to see clearly. A lack of visual acuity is commonly associated with refractive errors. In nearsightedness, or myopia, the eye focuses the visual image in front of the retina, causing objects in close view to be seen clearly and those at a distance to be blurry. In farsightedness, or hyperopia, the eye focuses the visual image behind the retina, causing objects in close view to be blurry and those at a distance to be clear. Both these problems are caused by an alteration in the shape of the eyeball. Other causes of reduced visual acuity include aging, amblyopia, cataracts, glaucoma, papilledema, dark adaptation, and scotoma (an area of diminished visual acuity surrounded by an area of normal vision within the visual field).

Amblyopia is severely decreased visual acuity or virtual blindness in a structurally intact eye. It occurs when there is interference in the stimulation of the vision-receptive cells in the brain. When normally stimulated, these cells allow for sensory development of visual acuity. Amblyopia is a form of treatable vision loss if recognized early. Causes may include strabismus, congenital cataract, abnormal refractive error, tumor, trauma, or infection.

If amblyopia is detected and treated before age 5, cure is almost assured. Treatment begun after age 10 is rarely successful.

With age, the pupil becomes smaller, which decreases the amount of light that reaches the retina. Older adults need about three times as much light as a younger person to see objects clearly.

Accommodation occurs as the thickness of the eye’s lens changes to maintain visual acuity. For near vision, the ciliary body contracts and relaxes the zonules, the lens becomes spherical, the pupil constricts, and the eyes converge. For far vision, the ciliary body relaxes, the zonules tighten, the lens becomes flatter, the eyes straighten, and the pupils dilate. The oculomotor nerve and coordinated brain stem pathways account for accommodation. Alterations in accommodation may be caused by pressure, inflammation, aging, or disorders affecting the
oculomotor nerve. The result of impaired accommodation may be diplopia, blurred vision, or headache.

Refraction is the process of bending light rays so that they fall on the retina. As rays of light reach the surface of the cornea from all directions, the cornea directs them toward the lens. The lens further bends the light and directs the light rays to one spot on the retina. The greater the refractive power, the more the light rays are bent. Emmetropia is a condition in which light rays fall exactly on the retina. Alterations in refraction occur when light isn’t properly focused. Causes include abnormalities in curvature of the cornea, focusing of the lens, and eye length. Results include myopia, hyperopia, and astigmatism. (See Refractive errors.)

The retina’s cones are responsible for color vision. Each cone contains one of three different pigments (red, green, or blue) that absorb light waves of different wavelengths.

Color blindness is inherited on the X chromosome and therefore usually affects males. Acquired color blindness may also be due to
diabetes, bilateral strokes affecting the ventral portion of the occipital lobe, or disease of the macula or optic nerve.

Older adults typically experience impaired color vision, especially in the blue and green ranges, because cones in the retina deteriorate. Yellowing of the aging lens also impairs color vision.

The sensory receptors for taste are the taste buds, concentrated over the surface of the tongue and scattered over the palate, pharynx, and larynx. These buds can differentiate among sweet, salty, sour, and bitter stimuli. Taste and olfactory receptors together perceive more complex flavors. Much of what’s considered taste is actually smell; food odors typically stimulate the olfactory system more strongly than related food tastes stimulate the taste buds.

A factor interrupting the transmission of taste stimuli to the brain may cause taste abnormalities. Taste abnormalities may result from trauma, infection, vitamin or mineral deficiencies, neurologic or oral disorders, and the effects of drugs. Moreover, because tastes are most accurately perceived in a fluid medium, dryness of the mouth may interfere with taste. Two major pathologic causes of impaired taste are aging, which normally reduces the number of taste buds, and heavy smoking (especially pipe smoking), which dries the tongue.

Alterations in taste may include:

♦ ageusia—a complete loss of taste

♦ hypogeusia—a partial loss of taste

♦ dysgeusia—a distorted sense of taste

♦ cacogeusia—an unpleasant or revolting taste of food.

Young children typically can’t differentiate between an abnormal taste sensation and a simple taste dislike.

As air travels between the septum and the turbinates of the nose, it touches sensory hairs (cilia) and olfactory nerve endings in the mucosal surface. The resultant stimulation of cranial nerve I sends impulses to the olfactoryreceiving area, primarily in the frontal cortex. Temporary impairment in the sense of smell can result from a condition that irritates and causes swelling of the nasal mucosa and obstructs the olfactory area in the nose, such as heavy smoking, rhinitis, or sinusitis. Permanent alterations in the sense of smell usually result when the olfactory neuroepithelium or a part of the olfactory nerve is destroyed. Permanent or temporary loss can also result from inhaling irritants, such as cocaine or acid fumes, that paralyze nasal cilia. Conditions such as aging, Parkinson’s disease, Alzheimer’s disease, or Kallmann’s syndrome (a congenital disorder) also may alter the sense of smell. Because combined stimulation of taste buds and olfactory cells produces the sense of taste, the loss of the sense of smell is usually accompanied by the loss of the sense of taste.

Alterations in smell include:

♦ anosmia—a total loss of the sense of smell

♦ hyposmia—an impaired sense of smell

♦ parosmia—an abnormal sense of smell.

The causes of macular degeneration are unknown but may include:

♦ aging

♦ infection

♦ inflammation

♦ injury

♦ nutrition.

Age-related macular degeneration results from hardening and obstruction of retinal arteries, which probably reflect normal degenerative changes. The formation of new blood vessels in the macular area obscures central vision. Underlying pathologic changes occur primarily in the retinal pigment epithelium, Bruch’s membrane, and choriocapillaries in the macular region.