Visual Field Representations in the Primary Visual System

Question 1

What is the vertical meridian?

Answer 1

The line that splits the left half of your visiual field from the right half of your visual field.

Question 2

What area of the eye is over represented in visual maps?

Answer 2

The macula and fovea.

Around 66-75% of axons in the optic nerve come from the macula.

Question 3

How big is the fovea centralis and how many degrees of vision does it account for?

Answer 3

size: ~1.5mm

Degrees of vision : 3

Question 4

What is the line of decussation?

Answer 4

Decussation means to cross the midline.

The dotted line corresponds with the vertical meridian- which splis your visual field into the right and lft half - it is this represented on your retina.

Question 5

Describe the routes ganglion cell axons take in the nerve fibre layer depending on where they arise from.

Answer 5

• Axons arising from ganglion cells in the nasal, superior & inferior retinal quadrants take a direct course to the nerve head
• Axons from ganglion cells temporal to the macula (high acuity region) follow an arcuate (curved) course around the fovea centralis
• Axons from temporal ganglion cells nearer the nerve head run more directly to it as the papillomacular bundle

[Basically nothing wants to run over the macula especially the fovea whether that is vessels or axons]

Question 6

Does the retinotopic order of ganglion cell axons stay the same along the optic pathway.

Answer 6

NO —Basically the order of ganglion cell axons in the optic nerve is based on location so periphery axons shall be at the periphery of the optic nerve and central axons will be at the centre of the optic nerve but then things change slightly.

Fovea & Macula axons start temporal at Optic Nerve Head, but move increasingly more central/medial in the Nerve & at the Chiasm. They then move somewhat dorsally into the deeper part of the Optic Tract , where axons representing the same visual field locations in each eye are now intermingled.

(NB: nasal inferior axons are at the base of the chiasm just above the pituitary gland)

Question 7

How are the right and left visual fields represented in the brain?

Answer 7

Left side of the brain visualises right hemi-field of vision

Right side of the brain visualises left hemi-field of vision

This happens because of the crossing over of axons in the chiasm.

Question 8

In anatomy what are the structures directly above and below the optic chiasm?

Answer 8

Pituitary gland is just below the optic chiasm

Hypothalamus and third ventricle are just above.

Question 9

Define lateral

Answer 9

In anatomy refers to things being at the sides

Question 10

What is lateral to the optic chiasm?

Answer 10

Bood vessels

Internal Carotid Artery (ICA) - big artery that supplies blood to most of the front part of your brain- it passes up through CS and branches into its first intercranial branch - the ophthalmic artery.

Cavernous sinus (CS) -is a network of small veins and arteries of which some will supply the pituitary gland.

[The sinus itself is surounded by a bag of connective tissue]

Question 11

Other than the ICA and the CS and the pituitary gland and the hypothalamus, what else is around the optic chiasm?

Answer 11

Question 12

What blood supply do the different parts of the optic nerve have:

Intra-Orbital Nerve?

Intra-Canalicular Nerve?– where it leaves the orbit through the canal

Intra-Cranial Nerve & Chiasm?

Optic Tract (& LGN)?

Answer 12

Intra-Orbital Nerve :

• perforating arteries from branches of Ophthalmic Artery

Intra-Canalicular Nerve – where it leaves the orbit through the canal:

• short branches from Anterior Cerebral & Anterior Communicating Arteries

Intra-Cranial Nerve & Chiasm

• short branches from Posterior Cerebral & Posterior Communicating Arteries

Optic Tract (& LGN):

• perforating thalamic branches of Posterior Cerebral Artery

Question 13

What can cause damage/compression of the chiasm and what are the effects of this?

[When axons are compressed they don’t convey action potentials anymore- they effectively stop working]

Answer 13

Compression at midline damages nasal axons from both eyes- causes of this are normally:

• Pituitary tumour- can only grow upwards (adenoma- term to describe tumours in glands)
• Raised pressure in 3rd Ventricle (e.g. in an adult suffering from hydrocephalus) – this would push the hypothalamus downwards therefore damaging the chias

If either of those two conditions occurred the Px would have Bi-Temporal Hemi-Anopsia.

You can also get damage of the chiasm from the side – this would damage temporal axons only from one eye – causes of this could be:

• ICA outpocketing- i.e. basically like a ballooning of the wall (due to an aneurysm)
• Cavernous Sinus expansion ( which could be due to thrombosis of veins or arteries)

If either of those two conditions occurred the Px would have (Uni- because one eye is affected) Nasal Hemi-Anopia

Damage anywhere across the chiasm or tract could be due to Meningiomas (benign tumours that affect the arachnoid layer of the meninges) or from loss of blood supply (blockage or haemorrhage).

Question 14

Are anopia and anopsia the same thing?

Answer 14

Yep the ‘s’ is optional - both mean loss of sight

Question 15

At first how would a px that is experiencing Bi-temporal hemianopsia present?

Answer 15

[Tumour won’t grow exactly straight that’s why one eye may also be more affected than the other]

Question 16

What is a macro tumour?

Answer 16

Tumour larger than 2 cm.

Question 17

How big does the pituitary adenoma need to get for px to experience complete bi-temporal hemianopsia?

Answer 17

Bigger than 2 cm so it can successfully crush all nasal axons

Question 18

Which layers of the LGN receive input temporally and which layers receive input from nasal axons?

Answer 18

Nasal axons from the opposite eye terminate in LGN layers 1, 4 & 6

Temporal axons from the ipsilateral eye (same side eye) terminate in layers 2, 3 & 5

So that each of the 6 layers of the LGN contains its own map of the opposite hemi-field originating from one eye!

Question 19

Name an organisational function of the LGN.

Answer 19

Is to organize retinal inputs from the 2 eyes:

• Into alternating maps of the opposite hemi-field
• One in each of its 6 layers

Question 20

How do axons that arise from LGN neurones in each layer travel to the V1 cortex?

Answer 20

They travel first in the posterior limb of the Internal Capsule and then in the Optic Radiations to area V1

This is the geniculo-cortical pathway.

Question 21

What is the geniculo-cortical pathway?

Answer 21

Axons that arise from LGN neurons in each layer travel first in the posterior limb of the Internal Capsule and then in the Optic Radiations to area V1

Question 22

What is the Lateral ventricle filled with?

Answer 22

CSF - cerebral spinal fluid

Question 23

How does the inferior horn of the Lateral Ventricle cause a slight problem to the Geniculo-cortical pathway?

Answer 23

[Axons can’t navigate through fluid]

Question 24

In the primary visual cortex the map of the visual field is upside down - why is this?

Answer 24

Because of the inferior horn of the lateral ventricle the axons of the lateral half of the LGN, which represent the Upper Visual Quadrant have to loop round (this loop is called Meyer’s loop), diagrammatically because of this looping, they end up at the lower bank of the calcarine sulcus.

Refer to slide

Question 25

Why is the Hemi-field map in area V1 cortex distorted and inverted?

Answer 25

Question 26

What are the blood supplies to the optic radiations and to the area V1 cortex?

Answer 26

Axons in 1st part of Radiations after the internal capsule

= are supplied by the Middle Cerebral Artery (MCA)

Axons in last part of Radiations, under Occipital lobe

= are supplied by the Posterior Cerebral Artery (PCA)

Calcarine Sulcus, upper & lower banks

= are supplied by the Calcarine Artery (a terminal branch of PCA)

Occipital pole (fovea/macula vision)

The Calcarine artery and possibly a terminal branch of the MCA called the Posterior Temporal Artery

Question 27

What visual field losses would a lesion in each of the areas lettered cause?

Answer 27

A)

B) Bi-temporal Hemi-anopisa

C)Left -Hemianopsia

D) Left-upper quadrant-opsia

E) Meant to represent damage to the whole primary cortex stemming from occlusion of the Posterior cerebral artery.

You shall get a left hemianopia - but you get sparing of the fovea and macula. This is called left hemianopia with macula sparing.

Question 28

How does macula sparing occur?

Answer 28

A posterior cerebral artery would lead to an infarct of the occipital pole thus a loss of vision. In 50% of people, it is estimated that a branch of the posterior temporal artery supplies the macula and fovea (as well as the PCA) thus damage to the PCA would mean although the px experiences hemianopsia the macula is spared.

[This is a theory]

Question 29

Which of the following is true regarding the macula?

a) Representation in area V1 is spared following posterior cerebral artery occlusion because it has another blood supply from the middle cerebral artery
b) Is located in nasal retina
c) Covers ~66-75% of the retinal surface
d) Is specifically associated with axons coursing through Meyer’s loop

Answer 29

a)Representation in area V1 is spared following posterior cerebral artery occlusion because it has another blood supply from the middle cerebral artery - (there is a theory about it being from the MCA as well as the Posterior Temporal Artery)

[D - not the whole fovea/macula just the upper visual bit of the macula/fovea)