• Regulation of motor movements (directed movements and reflexes)
• Balance control

However, functional imaging studies indicated its potential involvement in cognition, including working memory, executive functioning (e.g., attention, reasoning, and problem-solving), time perception, language, and emotional processing [1].

1. What Is the Structure of the Cerebellum?

The cerebellum contains almost 80% of the brain neurons and consists of two lateral hemispheres involved in the control of quick and finely coordinated movements of the arms.

The lateral hemispheres are separated by a narrow midline zone, known as the vermis, involved in coordinating the movements of the trunk and legs [2].

The cerebellum is divided into 3 lobes, the anterior lobe, the posterior lobe, and the flocculonodular lobe.

The anterior lobe is separated from the posterior lobe by the primary fissure and the posterior fissure from the flocculonodular lobe by the posterior fissure.

the flocculonodular lobe is involved in maintaining equilibrium and coordinating the movements of the eye, head, and neck.

The cerebellum has an outer layer of intricately folded grey matter (accordion-like structure) known as the cerebellum cortex that surrounds a layer of white matter known as the arbor vitae.

The arbor vitae surrounds 3 pairs of nuclei (compact groups of specialized neurons) that form the center of the white matter.

These pairs of nuclei are known as fastigial (deep nuclei), interposed, and dentate nuclei.

The cerebellar cortex is made of 3 layers: the molecular layer (outer layer), the Purkinje layer (middle layer), and the granular layer (deep layer).

The molecular layer is made of dendritic arborizations from the Purkinje cells in the Purkinje layer and fibers from the granule layer.

The Purkinje layer is made of neurons known as Purkinje cells involved in movement coordination.

The granule layer is made of granule neurons which role is unknown.

2. What Causes Cerebellum Disorders?

They are several disorders that can affect the cerebellum:

• Ischemia or stroke of the cerebellar artery (cerebellar stroke) results in reduced or absent supply of blood to the cerebellum
• Tumors (e.g., medulloblastoma) or abscess in the cerebellum
• Alcoholism (alcoholic cerebellar degeneration)
• Thiamine (vitamin B1) deficiency
• Vitamin E deficiency
• Celiac disease
• Toxin exposure such as carbon dioxide, lithium, and heavy metals.
• Brain trauma
• Excessive consumption of certain drugs (e.g., sedatives and antiseizure medications).
• Cerebellar degeneration
• Congenital malformations
• Genetics
• Metabolic and endocrine disorders such as Wilson’s disease, hypothyroidism, and inherited metabolic disorders (e.g., mitochondrial disorders).

3. What Are Some of the Cerebellum Disorders?

Cerebellum disorders can be due to congenital malformations associated with inherited genetic mutations or acquired after birth [3].

3.1. Congenital Malformations

3.1.1. What Is Joubert Syndrome?

Joubert Syndrome is an inherited disorder of brain development that affects the cerebellar vermis and the brainstem.

This disorder may manifest with the following symptoms:

• Low muscle tone resulting in muscle weakness
• Intellectual disability
• Abnormal eye movements
• Ataxia (difficulty coordinating movements)
• Episodic breathing anomalies
• Distinctive facial features.

3.1.2. What Is Dandy-Walker Malformation?

Dandy-Walker malformation is a congenital malformation that affects the development of the cerebellum and the 4^th ventricle (space around the cerebellum).

This malformation during embryonic development results in the underdevelopment of the cerebellar vermis, cystic expansion of the 4th ventricle, and an enlargement of the base of the skull.

This disorder may manifest with the following symptoms:

• Ataxia
• Muscle weakness due to low muscle tone
• Mental retardation
• Seizures
• Enlarged head circumference

3.1.3. What Is Rhombencephalosynapsis?

Rhombencephalosynapsis is a congenital abnormality of the cerebellum characterized by the absence of the vermis.

This disorder may manifest with the following symptoms:

• Truncal and/or limb ataxia
• Head stereotypies (e.g., body rocking, head banging)
• Abnormal eye movements
• Delayed motor development

3.1.4. What Is Pontocerebellar Hypoplasia?

Pontocerebellar hypoplasia is a group of inherited degenerative disorders characterized by the underdevelopment (hypoplasia) of the pons and cerebellum.

This disorder may manifest with the following symptoms:

• Mental retardation
• Slow development
• Ataxia
• Nystagmus (involuntary eye movement)

3.1.5. What Is Cerebellar Hypoplasia?

Cerebellar hypoplasia is a congenital malformation characterized by the underdevelopment (hypoplasia) of the cerebellum.

This disorder may manifest with the following symptoms:

• Intellectual disability
• Seizures
• Bilateral spastic cerebral palsy (stiff and tight muscles reducing movement ability)
• Microcephaly
• Loss of hearing

3.1.6. What Are Spinocerebellar Ataxias?

Spinocerebellar Ataxias (SCA) are a group of genetic diseases (over 25 SCA) characterized by the degeneration of the spinal cord and cerebellum.

These disorders may manifest with the following symptoms:

• Poor coordination of the eyes
• Poor speech coordination
• Progressive Gait incoordination
• Poor hand coordination

3.2. Acquired Cerebellum Disorders

Acquired cerebellum disorders are characterized by cerebellar degeneration associated with ischemic or hemorrhagic stroke, which limits blood flow or oxygen to the cerebellum.

These acquired disorders can also be associated with Multisystem atrophy, cerebellar cortical atrophy, and olivopontocerebellar degeneration.

Transmissible spongiform encephalopathies, such as Creutzfeldt-Jakob disease, can cause inflammation in the brain, including the cerebellum.

Damages to the neurons’ myelin sheath during diseases, such as multiple sclerosis can also affect nerve cells of the cerebellum.

Chronic alcohol abuse can lead to temporary or permanent cerebellar damage that can cause cerebellar degeneration.

Finally, paraneoplastic disorders that are triggered by the body’s own immune system in response to cancers (e.g., lung, ovarian, lymphatic, or breast cancer) can result in the production of substances that cause immune system cells to attack neurons in the cerebellum leading to cerebellar degeneration.

Conclusion

Although the cerebellum plays an important role in the regulation of motor movements and balance control, little is known about its development.

Little is also known about the role of the cerebellum in cognition. Interestingly, using advanced imaging techniques, some studies indicated the cerebellum may have a potential role in working memory, executive functioning, time perception, language, and emotional processing.

This potential role of the cerebellum in cognition is less surprising when we consider that it is interconnected with brain networks involved in cognition such as the cerebrum, the thalamus, and the cerebral cortex.

However, more research on this topic will certainly uncover more details about the mechanisms by which the cerebellum contributes to cognition.

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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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