Vision information relayed from the eyes retinas is processed by the visual cortex which is a part of the occipital lobe found at the back of the brain.
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 .
- 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 .
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
- 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 .
- Chiasm Disorders
- Infiltrative Optic Neuropathy
This disorder is due to the infiltration of cancerous white blood cells that affects the function of the optic nerve .
- Traumatic optic Neuropathy
This neuropathy is caused by direct ocular or head trauma that affects the optic nerve .
- 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) .
- 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 .
- 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) .
- Nutritional Optic Neuropathy
- 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 .
- 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 .
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 .
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).
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.