Optic pathways and visual fields

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Introduction0:00–0:24

When light enters the eye, it hits a light sensitive neural layer of tissue near the back of the eye called the retina. That’s where phototransduction occurs, which is the process by which light waves are converted into electrical signals.
Afterwards, these electrical signals are sent to the brain for visual processing. So, let’s start by taking a look at the optic pathway and understanding visual fields.

Visual fields0:24–2:51

You can think of a visual field as everything that can be seen with a single eye, so we have a left visual field for the left eye, and a right visual field for the right eye.
These visual fields overlap and produce a binocular visual field, but for now, let’s just look at the left visual field.
The left visual field can be divided into two halves. The half that’s closer to your nose is the nasal visual field, and the half that’s closer to your ear is the temporal visual field.
Similarly, the retina in each eye has a nasal and temporal region as well. When light enters the eye, the temporal field of vision is projected to the opposite side, onto the medial nasal retina, and the nasal field of vision gets projected to the opposite side, onto the lateral temporal retina.
The visual fields are further divided into the superior and inferior visual fields, so the visual fields are actually divided into quadrants.
The superior visual field projects to the inferior retina, and the inferior visual field projects to the superior retina.
So, the visual information that appears in the right upper quadrant is actually project to the left lower retina. When light hits the retina it triggers visual receptor cells in the retina, called rods and cones, to send an electrical signal The rods and cones synapse with bipolar cells which in turn synapse with ganglion cells.
These ganglion cells have long axons that travel through the retina layer to the back of the eye where they come together to form a single optic nerve, or cranial nerve II, that exits the retina from the optic disc.
This area in the retina where the optic nerve exits doesn’t have any receptor cells, so it’s also called the blind spot.
Now your other eye’s visual field covers what’s in the blindspot, but if you only have one eye, like a pirate, things can get a little weird.
Here’s a fun experiment, cover your left eye and look at the “+” on this image. Move your head back and forth slightly until the black dot disappears.
This is where the image of the black dot hits the optic disc of your right eye, and that’s why you don’t see it. Now keep your eye focused on the “+” and follow the “X” with your peripheral vision as it travels towards the dot and disappears…and then reappears!
This is the diameter of your blind spot and it’s pretty big right? Now the blind spot doesn't show up as a black, or empty spot because that would be pretty jarring!
Instead, your visual cortex fills in the gap with what it thinks should be there, in this case, a lot of green. In other words, your brain is actually showing you something that’s not there!

Optic pathway2:51–5:02

Okay, back to the anatomy. The optic nerve exits the eye through the optic disc and travels back towards the brain.
Next, it reaches the optic chiasm where both optic nerves meet and form an X-shaped structure. At the optic chiasm the axons of the optic nerves from the nasal portion of the retina, crossover to the opposite side.
Whereas the axons from the temporal retina, do not crossover. The purpose for this crossover is to organize the visual information coming from both eyes.
In other words, information from the left visual field, which is seen by the nasal retina of the left eye and the temporal retina of the right eye, goes to the right cerebral hemisphere.
And information from the right visual field, seen by the nasal retina of the right eye and the temporal retina of the left eye, goes to the left cerebral hemisphere.
Now if we look at the two visual fields together, the nasal portion of the visual fields seen by the temporal retinas overlap.
This area is called the binocular visual field. The most peripheral part of the temporal visual fields however, are only seen by the nasal retina of each eye.
This means that if you lose your left eye, the nasal portion of the left visual field can still be seen by the right eye, but you lose the peripheral vision of the left side which is only seen by the left nasal retina.
From the optic chiasm, the axons of crossed and uncrossed fibers travel together and form an optic tract which eventually synapse with cells in the lateral geniculate nucleus of each thalamus on both sides of the brain.
The lateral geniculate nuclei is a processing center for visual information that sharpens contrast and enhances depth perception.
Neurons in the lateral geniculate nucleus send out nerve fibers that form optic radiations that travel to the primary visual cortex in the brain which is located in the occipital lobe.
The optic radiations include Meyer’s loop, which carries information from the inferior retina, and Baum’s loop, which carries information from the superior retina.
The visual cortex is where the electrical signal is processed in terms of form, color, and motion which allows the brain to finally interpreted the signal as a recognizable image.
Alright, as a quick recap of the optic pathway: light entering the eye from the nasal half of the visual field falls on the temporal retina and light from the temporal half of the visual field falls on the nasal retina.

Review5:02–5:40

The retina sends a signal via the optic nerve to the optic chiasm where axons from the nasal retina crossover and the axons from the temporal retina continue on the same side - forming the optic tract.
The optic tract then synapses at the lateral geniculate nucleus of the thalamus, which send out optic radiations to the primary visual cortex of the brain where the electrical signal is interpreted as an image.