Vision - MCAT Psychological and Social Foundations

The symptoms describe a “cataract.” Cataracts form when a protein coat builds up and cause a white barrier to form that limits vision. It is normally surgically removed. “Astigmatism” is a misshapen cornea that can also cause vision problems. “Conjunctivitis” is a disorder that is made evident by the inflammation of the sclera, or white part of the eye. Last, “glaucoma” describes damage caused to the optic nerve, usually by a buildup of excess fluid in the eye.

The lens of the eye must be able to change its shape in order to re-focus the vision on near or far objects as the eye moves. This process is called “accommodation.” On the other hand, “convergence” is the ability of the eyes to move toward one another in order to focus on objects that are very close up. “Mydriasis” is the term for dilation of the pupil, and "contrast" is a visible quality of difference in color or brightness of objects, and not a function of the eye itself.

Myopia, often called nearsightedness, is an inability to being distant objects into clear focus. This may be due either to excess curvature of the cornea or to length of the eyeball itself causing the focal point of light rays to fall in front of the retina, rather than against it.

“Presbyopia” is the term for the inability to focus the vision on close-up objects. It is most commonly caused by loss of elasticity of the lens of the eye as individuals age.

On the other hand, the other choices are incorrect. “Amblyopia” is a general term used to denote cases in which one eye is communicating less visual information to the brain, resulting in a loss of vision in that eye In other words, the brain learns to ignore visual stimulation from the non-dominant eye. A common cause of amblyopia is “strabismus”—commonly known as “lazy eye.” It is defined as a lack of coordination between extraocular muscles that prevents the eyes from orienting in the same direction. In this case, there will almost always be a dominant eye, and if the condition is not corrected in childhood, the non-dominant eye generally suffers permanent vision loss. “Nystagmus” is a condition of involuntary eye movement (i.e. the eye seems to shudder, generally in a horizontal plane) that may or may not be associated with visual impairment. It can have numerous etiologies, including nervous system disorders, alcohol or drug reaction, congenital defect, or inner-ear disorder.

Light is refracted and focused by the cornea, the anterior chamber, and the lens. The retina is a thin layer of nervous system tissue, which receives focused light patterns and transmits those patterns to the brain via photoreceptor cells.

The extrinsic muscles of the eye are primarily responsible for the mechanical movement of the eyeball. The main movements of the eye are convergence during accommodation, saccadic movements, tracking, and maintenance of a horizontal position. Dilation of the pupils is dependent on intrinsic muscles of the eye.

The structure responsible for changing the shape of the lens during accommodation is the ciliary muscle. This is a ring of smooth muscle in the middle layer of the eye that is attached to the lens via the suspensory ligament. Contraction and relaxation of the ciliary muscle will cause the suspensory ligament to tighten (in the case of relaxation of the ciliary muscle) or relax (in the case of contraction of the ciliary muscle). This affects the thickness of the lens, allowing for accommodation.

The sphincter pupillae and dilator pupillae are both responsible for changing the shape of the iris, rather than the lens, and the medial rectus muscle is an extrinsic eye muscle that moves the whole eye medially.

Control of the muscles responsible for dilation of the pupil is mediated by the sympathetic division of the autonomic nervous system. During states of fear, sexual arousal, or heightened attention, the pupil will dilate due to the stimulation of the dilator pupillae, or iris dilator muscle.

The levator palpebrae superioris is the muscle responsible for elevating the upper eyelid. It is innervated by CN III, the oculomotor nerve. CN IV (the trochlear nerve) and CN VI (the abducens nerve) are both responsible for innervation of various other muscles of eye movement, while the facial nerve (CN VII) is primarily responsible for control of the muscles of expression, for taste, and for motor innervation to the muscles of the inner ear.

When looking at objects that are close, the lens of the eye must “thicken” to accommodate changes in depth of field. This is accomplished by contraction of the ciliary muscle, which allows the suspensory ligament to stretch. The lack of tension then allows the lens to draw up into its thicker resting state. If the vision is shifted to a far object, then ciliary muscles will relax, which increases tension on the lens via the suspensory ligament and causes it to "flatten." The lens does not “tilt” or “rotate.”

Ciliary and pupillary muscles are innervated by CN III, the oculomotor nerve. This nerve is also responsible for elevation of the upper eyelid, and innervation of the superior rectus, medial rectus, inferior rectus, and inferior oblique muscles of the eye. The abducens nerve (CN VI) innervates a single extraocular muscle: the lateral rectus. The optic nerve is primarily involved in transmission of visual signals from the retina to the brain, and the trigeminal nerve is not directly involved in ocular movement or function.

The fovea centralis is a zone of densely packed cone cells in the center of the macula lutea of the retina. Because of this it is the region capable of the highest degree of visual acuity. The optic disc is the point of exit for ganglion cell axons of the optic nerve. This is an area absent of rods or cones, and so is considered a "blind spot" of the eye. Last, the cornea and lens are not part of the retina.

Myopia is nearsightedness, meaning the focal length is too short. This can be caused by too much refraction at the eye's lens or if the eyeball is too long. Myopia can be corrected using a concave (i.e. diverging) lens, which will increase the focal length as shown in the figure.

Weber's law states that the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus. Because the original stimulus changes in each condition (i.e. low, medium, high), the ratio for the just-noticeable difference also changes; therefore, the magnitude necessary to detect a change will vary by condition.

The eyes contain approximately seventy percent of the total sensory receptors of the body. Most of these are located in the retina, which has up to 200,000 photoreceptor cells per square millimeter in its densest location (i.e. the fovea centralis).

The light sensitive cells of the retina are called "photoreceptor cells." This is a group of cells with various functions regarding the transmission of information about brightness, color, and contrast. None of the other structures listed are cell types of the eye.

There are three types of photoreceptor cells in the retina: “rods,” “cones,” and “photosensitive retinal ganglion cells.” While rods and cones directly communicate information about sight (e.g. contrast, brightness, and color), photosensitive retinal ganglion cells do not directly communicate visual information to the brain. Rather, they are involved in the pupillary reflex.

“Rods” are incredibly sensitive to light, and can be triggered by a single photon. These cells are the primary photoreceptor cells active at very low light. The activation of cone cells requires significantly larger amounts of photostimulation; therefore, they function primarily in interpreting visual information in bright light. Photosensitive retinal ganglion cells, a group of cells that make up a very small percentage of overall photoreceptor cells, are less sensitive to photons and have a slower response time than either rods or cones.

Cones are the primary photoreceptor cells responsible for transmitting information about color. They are the only photoreceptor cell type that is capable of differentiating between colors. Rod cells are not capable of color vision and primarily transmit information in regards to contrast. They are most active in very low light, while cone cells are most active in levels of high light. This is why it is difficult to perceive color at very low light levels. Photosensitive retinal ganglion cells do not transmit visual information to the brain - rather, they are involved in physiological processes such as pupillary reflex and circadian rhythms.