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Course: Health and medicine > Unit 8
Lesson 6: Sight (vision)Visual field processing
In this video, I review how we process our visual field, and how information from the right and left visual field is broken down and sent to the brain. By Ronald Sahyouni. . Created by Ronald Sahyouni.
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So why is the first ray of light at around1:10bent "AWAY from the normal" (as if the eye's lens' refractive index was LESS than the refractive index of the air), and therefore angles AWAY from the back of the eye, but then the second ray of light bends "TOWARD the normal" (as if the eye's lens' refractive index was GREATER than the refractive index of the air), and therefore angles TOWARD back of the eye?(11 votes)
Maybe it's a mistake in the drawing? Either way, the end result is the same. The right visual field will hit the nasal side of the right eye and the temporal side of the left eye, and same for the left visual field.(8 votes)
what if the light rays from the right visual field hit the temporal side of the right eye? would that mean that information will travel to the right hemisphere ?(4 votes)
Yes, information processed on the temporal side travels ipsilateral in the brain. Meaning information will travel to the same side. However, information processed on the nasal side will travel contralateral in the brain. Meaning that information will be processed on the opposite side of the brain.(4 votes)
- I assume the "eyeballs" represent visual fields. From a neuro/medical standpoint, doesn't the pathway in the optic tract and optic nerve depend on retinal fields as well? In fact, visual fields are inverted and reversed in relation to retinal fields. Optic nerves project posteriorly to form the optic chiasm and continue as the optic tract. Axons from the temporal hemiretinas (nasal visual hemifields) project posteriorly in the IPSILATERAL optic tract (because it's contingent on the retina). The video shows axons (or information) going to the contralateral side (passing through the optic tract). Am I confusing axons with light information? Or is there a mistake in this case?(4 votes)
What about the rest of the pathway ? through the Thalamus then to the occipital lobe I believe ...? It would have been nice of you to finish the "visual processing" story ;) I have know for sure then ;)(2 votes)
The optic nerve axons have four targets: the hypothalamus, the pretectum, the superior colliculus, and the lateral geniculate nucleus of the thalamus. From the thalamus, the signals travel up the internal capsule to the occipital lobe of the cerebral cortex.(4 votes)
- How much visual information from each eye crosses to the opposite hemisphere and why(1 vote)
Fibres from the inside (nasal) half of each retina cross to the opposite side at the optic chiasm. The result is that information from the right side of the visual field goes to the left side of the brain, and vice versa. This happens because that's how the nerve cells grow.(2 votes)
Why do defects in the Optic nerve cause full blindness in the same eye (say, Left Optic Nerve defect--> Left eye blindness)? Shouldn't it cause Binasal Hemianopsia instead, since the optic nerve(found on temporal side of eye) defect should prevent transmission of signals from the opposite Nasal visual field? How is the full blindness in one eye explained from a defect in the nerve when it is only supposed to be carrying light from one side of each visuaa field? And what, then, is the cause of Binasal Hemianopsia?(1 vote)- For this, we need to go over the anatomy of the second cranial nerve (CNII), because it can be a bit confusing at first. It's also important to remember that the visual fields are named for the stimulus is relative to the eye, not for where the light hits the retina: so a stimulus in a temporal field is seen by nasal retina, and vice versa.
The optic nerve runs from the retina to the optic chiasm, and carries all of the visual information from the ipsilateral (same side) eye. At the optic chiasm, the nerve fibres from the nasal half of the retina (corresponding to the temporal fields) cross to the opposite sides, while the nasal field (i.e., the temporal half of the retina) stays on the same side. After the chiasm, the optic tracts carry visual information from the contralateral fields - so the left optic tract has all of the information from the right visual field.
From this, we can start predicting field defects based on nerve lesions. If the optic nerve is damaged, then that causes monocular blindness because no information from that eye is reaching the optic chiasm. If the optic tract is damaged, that causes a homonymous hemianopsia - an entire half of the visual field is missing from both eyes. If the optic chiasm is damaged, then the fibres crossing over are most at risk, resulting in a bitemporal hemianopsia - as no information from the nasal retina is getting to the visual cortex.
A binasal hemianopsia would be uncommon, because it would mean that the nasal fibres crossing in the chiasm were spared while the temporal fibres passing along the outside were damaged - an unlikely (but not impossible!) pattern of damage. It's more likely that a nasal hemianopsia would be from one eye only.(2 votes)
- So this basically creates a mirror image which our brains then flip the right way? Or am I wrong?(1 vote)
- How would vision be affected if one of the eyes were removed/sight was lost in one eye?(1 vote)
If I close my left eye then the right side of my brain will not recieve any information of what I'm seeing?(1 vote)
He told us which side of the brain the light that hits the side of the eye goes, but where does the light that hits the back of the eye go?(1 vote)
1:10Video transcript
In this video, we're going to
talk about visual processing, so how our brain is able to make
sense of what we're looking at. So in most of our body,
we have the right side of the body being controlled
by the left side of the brain and the left side of
body is controlled by the right side of the brain. So how does this work in vision? So let's imagine that
this rectangle that I drew is our entire visual field. And if these two
eyeballs were focused in at the center
of this rectangle, so if they were both
focusing on this purple line, all they can see are
these two colors. So we have a ray of light coming
in from the left visual field. It'll hit the eyeball. It'll hit the left eyeball. It'll kind of be bent a
little bit by the lens. And it'll hit the right
side of this eyeball. And so the inner side of
the eyeball-- so this side of the left eyeball and this
side of the right eyeball is known as the nasal side
because the nose would be right in the middle the eyes. So the nasal side is
the side of the eyeball closest to the nose. And then this outside
part of the eyeball, which is over here
and over here, is known as the temporal
side of the eyeball because it's closest
to your temples. So this is the side
closest to the temples. This is the side
closest to the nose. So a ray of light coming
from the left visual field will hit the nasal
side of the left eye. And a ray of light coming
from the left visual field will hit the right eye, be
bent a little bit by the lens, and it'll hit the temporal
side of the right eye. So let's look at a
ray of light coming from the right side
of the visual field. A ray of light coming from
here would enter the right eye. And it would be bent
a little by the lens and hit the nasal
side of the right eye. Whereas, a ray of light coming
in would hit the left eye, be bent a little, and hit the
temporal side of the left eye. So let's look at
what happens next. So the eye is
basically connected to the brain via
the optic nerve. So there is an optic
nerve that kind of exits the back of the eye
and goes into the brain. So interestingly, the
optic nerve from both eyes actually converge. So they actually reach a
point where they converge. And this point right
here where they converge is known as the optic chiasm. So they kind of converge and
then break off again and then move even deeper into the brain. So this point
where they converge is known as the optic
chiasm, so optic chiasm. Let's look at how this
information is transmitted to the brain through
the optic chiasm. The retina is lining
the back of the eyeball. And we had this
yellow ray of light hit the nasal side
of the left eye and it hit the temporal
side of the right eye. So let's go ahead and trace
this information to the brain. So the information
will be sent via axons through the back of the
eye into the optic nerve. And basically, it'll come in. And what it'll do
is it'll actually cross at the optic chiasm
and then go this way. And what we also have
is this ray of light will come into the
back of the eye and it will actually go
down the optic nerve. But it's not going to cross. So all light that hits the
temporal side of either eyeball does not cross the optic chiasm. So let's go ahead and
trace this green ray. So the green ray coming from the
temporal side of the left eye is going to exit the back of the
eye, go down the optic nerve, and it's just going
to stay in place. So it's not going to
cross the optic chiasm. Whereas, this ray
of light is going to go through the
back of the eye and it's going to
cross over here. It's going to cross
the optic chiasm and go down to the brain. And so what this
effectively does is it actually takes
the right visual field and allows all the
information that's entering the eye from
the right visual field to go to the left
side of the brain. So this is the left
side of the brain. This is the right
side of the brain. So like the rest of the
body, all the information coming from the
right visual field actually goes to the
left side of the brain and all the information coming
from the left visual field goes to the right
side of the brain.