• Age-Related Macular Degeneration (AMD)
• Cataract
• Glaucoma
• Diabetic Retinopathy
• Strabismus
• Amblyopia
• Refractive Errors

In the United States, more than 4.2 million individuals are considered by the Social Security Administration (SSA) to be legally blind or have a low vision [1].

1. How Is the Structure of the Eye Related to Its Function?

The eye is made of structures that allow light to enter the eye, refract it, and transform it into two-dimensional images by the retina [2]. The eye contains the following structures:

The eye (eyeball) occupies a space or cavity in the skull known as the orbit where it is connected to the optic nerve, muscles, and blood vessels.

The optic nerve is a sensory nerve located at the back of the eye and responsible for the transmission of visual information from the eye to the brain.

The eye muscles are responsible for eye movements.

The blood vessels provide oxygen and nutrients for the tissues of the eye.

Around the eye, there are structures involved in the production of tear (lacrimal gland), and its draining, including the tear sac (lacrimal sac), the tear ducts (canaliculi), and the lacrimal duct (nasolacrimal duct).

The eyeball is made of the following structures and tissues:

1.1. The Episclera

The episclera is a loose conjunctive tissue that covers the back of the eye (the part inside the socket) and connects with the conjunctiva.

1.2. The Sclera

The sclera or tunica albuginea oculi is the white part of the eye made of collagen and elastic fibers and which role is to protect the eye.

1.3. The Choroid

The choroid is the middle part tissue layer of the eye found between the sclera and the retina. It mostly contains blood vessels that supply oxygen and nutrients to the outer part of the retina.

1.3. The Retina

The retina is the innermost layer of the eye responsible for creating two-dimensional images of the visual world.

The retina is made of blood capillaries and photoreceptors, including cones and rods.

Cones are photoreceptors that detect color and are also responsible for the production of sharp and detailed central vision.

Cones are mostly concentrated in a region of the retina known as the macula

Rods are photoreceptors responsible for night vision and side vision. They are not capable of detecting color but are more sensitive to light.

Rods are mostly found at the periphery (side) of the retina.

The visual information from the cones or rods is then transferred through synapses to neurons known as retinal ganglion cells that are just below their ends.

1.4. The Optic Disc

The optic disc is the beginning of the optic nerve and contains the axons of retinal ganglion cells.

1.5. The Conjunctiva

The conjunctiva is an epithelial membrane that covers the front of the eye.

1.6 The Cornea

The cornea is the transparent part in front of the iris and the pupil and which role is to refract light entering the eye.

1.7. The Pupil

The pupil is the black hole in the middle of the iris which lets the light through the light coming from the cornea and towards the retina.

1.8. The Iris

The iris is a structure made of pigmented epithelial cells that control the diameter and size of the pupil.

1.9. The Lens

The lens is the structure found behind the iris and is involved in refracting light coming to the cornea and pupil.

It is suspended in place by the suspensory ligament of the lens.

1.10. The Vitreous Body

The vitreous body or vitreous humor is a clear gel that fills the inside of the eye between the retina and the lens.

It is made of a fibrillous mesh of extracellular matrix and which role is to keep the eye clear to let the light coming through the lens toward the retina.

2. Age-Related Macular Degeneration (AMD)

Age-related macular degeneration is a chronic degenerative disease mostly affecting individuals over the age of 60 years.

It is characterized by atrophy of the epithelium of the macula and the degeneration of the photoreceptors.

AMD manifests with the following symptoms:

• Difficulties seeing faces, reading smaller prints, and driving.
• Blind spot in the center of the vision.
• Blurred or distorted vision
• Seeing straight lines as wavy
• Objects looking smaller than normal
• Hallucinations

There are several factors that increase the risk of AMD such as being over 50 of age, smoker, being hypertensive (hypertension), and having a high-fat saturated diet.

Unfortunately, there is no available treatment for AMD.

3. Cataract

A cataract is characterized by the clouding of the eye lens that can affect one or both eyes. There are several causes of cataracts:

• Age
• Genetics (a family history of cataract)
• Trauma (eye injury)
• Medications (long-term treatment with steroids)
• Alcohol
• Smoking

A cataract is characterized by a clouded or blurred vision that can lead to blindness.

Treatment of cataracts involves surgical removal of the cataract.

4. Glaucoma

A glaucoma is a group of diseases that are mainly characterized by an increased pressure in the eye which damages the optic nerve; however, other types of glaucoma do not involve high intraocular pressure.

They are divided into open-angle and close-angle glaucoma based on the timing of appearance. Open-angle glaucoma develops slowly and without evident signs until the glaucoma is at an advanced stage, while a close angle appears suddenly.

Glaucoma is caused an increased pressure in the eye associated with the build-up of fluid (aqueous humor). Another cause is genetics which involves a family history of glaucoma.

Open-angle glaucoma manifests with symptoms such as Blindspot in the center of the vision, and tunnel vision.

Closed-angle glaucoma presents symptoms including blurred vision, ocular pain, severe headaches, nausea, and vomiting, and decreased vision.

The treatment of glaucoma is preventive and involves early detection to prevent or slow down the progression of glaucoma.

5. Diabetic Retinopathy

Diabetic retinopathy is associated with Diabetes Miletus that causes the blockage of blood vessels that supply oxygen and nutrients to the retina causing its damage.

However, diabetic retinopathy develops through stages including:

• Stage 1: Microaneurysms
• Stage 2: Blockage of some blood vessels of the retina
• Stage 3: Blockage of more blood vessels leading to ischemia
• Stage 4: Retinal damage

Diabetic neuropathy manifests with the following symptoms:

• Blurred vision
• Gradual worsening of vision and its sudden loss
• Redness of the eye with pain
• Floating shapes in the field of vision

The diagnosis of diabetic retinopathy is performed using diabetic eye screening to see if there are anomalies in the eye.

The treatment involves laser treatment, injection of steroids (Triamcinolone) in the eye, and surgery.

6. Strabismus

Strabismus is a lack of alignment between the two eyes when looking at an object. It is caused by anomalies with the extraocular muscles or nerves that control them.

The extraocular muscles are responsible for controlling the eyes’ movements.

Strabismus manifests with symptoms such as double vision, eye strain, fatigue when reading, and headaches.

Treatment of strabismus involves the use of eyeglasses or surgery to align the extraocular muscles.

7. Amblyopia

Amblyopia or lazy eye is a visual impairment due to the brain failure in processing visual stimulation in the early first few years of life [3].

It can be caused by strabismus or a difference in refractive errors (anisometropia) between the two eyes.

Amblyopia manifests with symptoms that affect one eye such as poor visual acuity, low sensitivity to contrast and motion, and poor pattern recognition.

The treatment involves correcting the refractive errors between the two eyes by putting a patch on the normal eye.

8. Refractive Errors

Refractive errors are anomalies that affect the capacity of the eyes to focus light on the retina.

Refractive errors include myopia (near-sightedness), farsightedness (hyperopia), astigmatism, and presbyopia [4].

• Myopia (Near-Sightedness)

Myopia is a disorder of the eye where distant objects appear blurry. Other symptoms of myopia may involve headache and eye strain.

Although the causes are unknown, genetic, and environmental factors may be involved.

• Farsightedness (hyperopia)

Farsightedness is a disorder of the eye where close objects appear blurry. Other symptoms of farsightedness may involve headache and eye strain.

It is caused by variations during the development of the eye.

• Astigmatism

Astigmatism is due to an anomaly of the shape of the eye which affects its refraction of light causing objects that are close and far away to look blurry.

Other symptoms of astigmatism may involve headache, migraine, eye strain, and double vision.

Although the causes of astigmatism are unclear, genetics appears to play a role.

• Presbyopia

Presbyopia is a progressive decrease in the capacity to clearly focus on objects that are close and that affects individuals over the age of 40.

Other symptoms of presbyopia may involve headache and eye strain.

It is caused by the hardening of the lens due to age.

Refractive errors are treated using eyeglasses or corrective lenses and surgery.

Conclusion

The eye is a very sensitive organ and any anomalies, even smaller, that affect the eye tissues results in changes in the vision.

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There are two parts of the visual cortex according to the two brain hemispheres: the right visual cortex and the left visual cortex.

The right visual cortex controls the visual information coming from the left visual field of the left eye, while the left visual cortex is responsible for the visual information coming from the right visual field of the right eye.

The left visual field means what the right eye can see in all possible directions and the same applies to the right eye.

If an eye is closed and the other eye is left open while also moving left and right, the visual field is what can be seen with the open eye.

1. How Does Visual Perception Work in the Brain?

Before reaching the visual cortex for processing, the visual information that is captured by the retina is transmitted through the optic nerves (left and right) and the lateral geniculate nucleus [1].

• What Is the Function of the Retina in the Eye?

The retina is an internal layer of the eye responsible for translating light from the eye into a focused two-dimensional image of the things being seen (visual information).

Within the retina, there are photoreceptor cells known as cones and rods. Cones require brighter light and are responsible for the perception of colors, while rods detect black-and-white visual information.

The visual information from the cones or rods is then transferred through synapses to neurons known as retinal ganglion cells that are just below their ends. This transmission of information is known as neurotransmission.

The activated neurons transmit the visual information to the optic nerves (right and left).

• What Nerve Is the Optic Nerve?

The optic nerve is the 2nd cranial nerve responsible for the transmission of visual information from the retina to the brain such as colors, brightness, and contrast (acuity).

There are two optic nerves, the right and the left optic nerves that cross each other at the optic chiasma and continue their extension to the lateral geniculate nucleus.

There are two lateral geniculate nuclei, left and right.

At the optic chiasma, the optic nerve comes from the left eye extent towards the right hemisphere of the brain, while the optic nerve comes from the right eye extent towards the left hemisphere of the brain.

• What Is the Role of Lateral Geniculate Nucleus?

The lateral geniculate nucleus is part of the thalamus and serves as a relay for the transmission of visual information coming from the optic nerve.

When each optic nerve reaches the corresponding lateral geniculate nucleus, neurons are sent to layer V1 of each visual cortex and known as the primary visual cortex.

• What is the Visual Cortex?

The visual cortex is the part of the occipital lobe located at the back of the brain and responsible for processing visual information [2].

The visual cortex is divided into 5 different layers: V1, V2, V3, V4, and V5. These layers are highly specialized, and each plays a complex role in processing visual information according to the orientation of edges and lines, orientation and directions, and object recognition.

2. What Part of the Brain Controls Eye Movement?

Eye movements are controlled by the brainstem through the oculomotor nerve. The brainstem is located in the posterior part of the brain.

2. What Neurological Diseases Cause Blurred or Double Vision?

There are several disorders affecting the optic nerve and eye movements that can cause blurred or double vision:

• Ischemic Optic Neuropathy

This disorder is characterized by a reduction in blood flow (ischemia) to the optic nerve that may be due to diabetes, autoimmune diseases, migraine, inflammation, or collagen-vascular disease [3].

• Compressive Optic Neuropathy

This disorder is due to the compression of the optic nerve, the optic chiasma, or optic tracks by a tumor (e.g., meningioma), a hematoma, an inflammation, infection, or any type of mass [4].

• Chiasm Disorders

These disorders are characterized by alteration of the optic chiasma that can be caused by tumors such as meningiomas, pituitary adenomas, craniopharyngiomas, or optic chiasmal glioma [5].

• Infiltrative Optic Neuropathy

This disorder is due to the infiltration of cancerous white blood cells that affects the function of the optic nerve [6].

• Traumatic optic Neuropathy

This neuropathy is caused by direct ocular or head trauma that affects the optic nerve [6].

• Optic Neuritis

This disorder is associated with inflammation that affects the function of the optic nerve. Optic neuritis is a common symptom of multiple sclerosis (MS) [7].

• Gian Cell Arteritis (Temporal Arteritis)

This disorder is an inflammatory autoimmune disease that affects large vessels causing them to swell which prevents blood supply to the optic nerve [8].

• Mitochondrial Optic Neuropathies

These are disorders that affect the ganglion cells of the retina, the optic nerve, the optic chiasma, or the optic tracks.

They are due to mutations in the DNA of mitochondria (source of energy for the body’s cells) or alterations in the function of the mitochondria (e.g., vitamin deprivation) [9].

• Nutritional Optic Neuropathy

This disorder is associated with an insufficient supply of nutrients such as vitamin B12 or folic acid, to the cells involved in the visual system [10].

• Toxic Optic Neuropathy

This disorder is caused by the damage of the optic nerve by different toxins, including drugs, methanol, metals, methanol, organic solvents, and carbon dioxide [11]. 

• Hereditary Optic Neuropathies

These disorders by inherited damages of the optic nerve:

Leber Hereditary Optic Neuropathy

This is a mitochondrial disease that affects young individuals causing symptoms associated with poor visual acuity or loss of vision [12].

Dominant Optic Atrophy

This disorder is also a mitochondrially inherited disease due to mutations in the mitochondrial gene OPA1 that is important for mitochondria membranes. The disease is associated with a progressive loss of vision [12].

Eye Movement Disorders

Eye movements including eyes’ rotation and eyelids are controlled by the oculomotor nerve. Alterations affecting this nerve are associated with symptoms such as double vision, nystagmus (involuntary eye movement), anisocoria (unequal size of the pupils), and oscillopsia (blurred and oscillating vision).

Conclusion

Vision is a fascinating subject as it allows us to see the beauty of the world; however, a lot of things are still not well-known, particularly the role of the different layers of the visual cortex.

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