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OV2023 visual perception > Visual representations > Flashcards

Visual representations Flashcards

1
Q

what does the primary visual pathway comprise ?

A

. primary visual pathways comprises the retinal ganglion cell
. retinal ganglion cell axons that travel down the optic nerve chaism and tract to make connections with LGN
. axons that come from LGN and end up in the primary (V1) cortex

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2
Q

what is the vertical meridian ?

A

when you look into the visual world, there is a vertical line which splits the left and right visual field

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3
Q

what is the left hemi-field represented by?

A

. left hemi-field is represented on the right side of the brain and vice versa, so right V1 cortex damage causes left hemi-anopia

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4
Q

what is the left part of visual field seen by?

A

. left part of visual field is seen by the right half of each of your eye

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5
Q

what is the right part of visual field seen by?

A

. right part of visual field is seen by the left half of each of your eyes

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6
Q

how are all primary visual pathways organised ?

A

. organised as visual field maps
. large territories over-representing the fovea and macula
. central , high acuity vision

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7
Q

how are the retinal axons in the optic pathway split?

A

. the line of decussation divides nasal from temporal retina
. crossed nasal versus uncrossed temporal axons at the chiasm
. maps of opposite hemi-field beyond the chiasm (e.g., in LGN and V1 cortex)

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8
Q

how is the organisation in the lateral geniculate axons in the optic radiations?

A

. inferior verus superior visual quadrants
. take ‘direct’ versus ‘loop’ pathways in the optic radiations
. details of the hemi-field map in V1 cortex

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9
Q

what diameter is the macula and where is it located?

A

. 5mm in diameter=the central 15 deg of vision

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10
Q

what diameter is the fovea centralis and where is it located?

A

. 1.5mm in diameter = just central 3 deg

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11
Q

what do the macula and fovea contain?

A

. they contain 66-75% of all the ganglion cells of the retina
. they provide 66-75% of all the axons in the optic pathways

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12
Q

how is the over-representation of the macula and fovea maintained >

A

. the over-representation of central vision is simply maintained throughout the primary visual system
. with 66-75% of the territory in the LGN and V1 cortex devoted to processing information from theses central retinal regions

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13
Q

how many ganglion cell axons run in the NFL?

A

. 1-1.5 million ganglion cell axons run in the NFL on the inner retinal surface

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14
Q

what is the direction of axons arising from ganglion cells in the nasal, superior and inferior retinal quadrants ?

A

. axons arising from ganglion cells in the nasal, superior and inferior retinal quadrants take a direct course to the nerve head

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15
Q

what direction do axons from ganglion cells temporal to macula follow?

A

. axons from ganglion cells temporal to the macula follow an arcuate course around the fovea centralis

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16
Q

what is the direction of axons from ganglion cells in the fovea and macula?

A

. axons from ganglion cells in the fovea and macula nearer the nerve head run more directly to it as the papillomacular bundle

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17
Q

how do peripheral axons from the superior, nasal, inferior and temporal quadrants enter the ONH?

A

. peripheral axons from the superior, nasal, inferior and temporal quadrants enter the ONH peripherally and more central axons enter centrally

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18
Q

what is the gradient of retinal ganglion cells across nasal retina ?

A

. there is a centre to peripheral gradient of retinal ganglion cells across nasal retina
. one looking centrally
. one looking intermediate
. one looking in the periphery
in field of view
- that topography is maintained in the way the axons leave the eye in ONH

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19
Q

what is the precise retinotopic order of ganglion cell axons ?

A

. fovea and macula axons start temporal at ONH, but move increasingly more central/medial in the nerve and chiasm

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20
Q

where are the nasal inferior axons?

A

.nasal inferior axons are at the base of the chiasm just above the pituitary gland

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21
Q

what does partial decussation/crossing of axons in the chiasm mean

A

. the partial crossing means that

  • nasal axons are crossing the midline , where as temporal axons don’t
  • as a result the nasal ganglion cells of your left eye , their axons go to the right side of the brain
  • where as axons coming from temporal retina of right eye also looking at left half of visual field don’t cross
22
Q

what happens if damage to the optic nerve before the optic chiasm ?

A

. if the damage to the optic nerve before the optic chiasm , visual field loss will only occur in the affected eye

23
Q

what happens if damage to the optic chiasm or beyond?

A
  • if damage within the optic chiasm or beyond (e.g. in the optic tract) both eyes will be affected
24
Q

how is the area around the optic chiasm ?

A
  • optic chiasm is a crowded region, with things above it and below it - which can get pathologies and damage axons as they are travelling through optic chiasm
25
Q

what is the optic chiasm surrounded by?

A
  • above the optic chiasm is the hypothalamus
  • on the side of the optic chiasm is the 3rd ventricle - which contains CSF
  • below the optic chiasm is the pituitary gland
  • laterally to the optic chiasm : internal carotid artery (ICA) and cavernous sinus (CS)
26
Q

what is the cavernous sinus (CS) ?

A

. network of smaller arteries and veins which sit behind superior orbital fissure
. encased in a bag of connective tissue
. situated lateral to optic chiasm

27
Q

what else surrounds the optic chiasm ?

A

. arteries around the optic chiasm forming the circle of willis

28
Q

what is the blood supply to the optic pathway?

A

. blood supplies to the optic pathways have an anterior-posterior sequence

  • intra-orbital nerve: axons supplied from branches of ophthalmic artery
  • intra-canalicular nerve: axons supplied by short branches from anterior cerebral and anterior communicating arteries
  • intra-cranial nerve and chiasm : supplied by short branches from posterior cerebral and posterior communicating arteries
  • optic tract ( and LGN): supplied by perforating thalamic branches of posterior cerebral artery
29
Q

what happens when the compression of chiasm occurs?

A

. axons become compressed , they can’t conduct action potential
. end up with visual field loss as the retina is no longer reporting information to the brain

30
Q

what are the two ways of getting compression at the midline of optic chiasm ?

A
  • at midline, damages nasal axons from both eyes:
    1. pituitary tumour (adenoma) - benign - the only direction tumour can grow is upwards , and can hit optic chiasm
    2. raised pressure in 3rd ventricle (hydrocephalus )
31
Q

what do compression at the midline of optic chiasm cause ?

A
  • both types of compression will affect axons crossing the midline in the optic chiasm - this causes bi-temporal hemi-anopsia
32
Q

whay are the ways of getting compression from the side?

A
  • from the side, damages temporal axons , same eye
  1. ICA outpocketing (aneurysm)- wall of artery becomes weak
  2. cavernous sinus expansion (thrombosis )
33
Q

what do compression from the side of optic chiasm cause ?

A
  • cause (uni)nasal hemi-anopia

. temporal axons affected

34
Q

what else can you get compression ?

A
  • anywhere : from meningiomas (arachnoid tumours ) or from loss of blood supply ( blockage or haemorrhage )
35
Q

what is the reality of Bi-temporal ‘hemi-anopia’ ?

A
  • pituitary ademonas initially compress axons from the nasal inferior retina in both eyes at the chiasm , leading to superior temporal visual field losses at first
  • in order to get the nasal superior , the ademona has to be macro
36
Q

what does the px present with when they got a pituitary ademona ?

A
  • visual field loss in the superior temporal field of left and right eye
37
Q

why are visual field loss incongruent in left and right eye ?

A
  • the loss in one eye can be bigger, this is because pituitary ademonas don’t grow with respect to the midline
38
Q

how many layers does the LGN have?

A

. 6 layers

- 4 parvo cellular layers and 2 magno cellular layers

39
Q

what does each layer of LGN contain ?

A
  • each of the 6 layers of the LGN contains its own map of the opposite hemi-filed orginating from one eye
40
Q

where do nasal axons from the opposite eye terminate in LGN layers?

A

. terminate in LGN layers, 1,4 and 6

41
Q

where do temporal axons from the ipsilateral eye terminate ?

A

terminate in layers 2,3 and 5

42
Q

what is a function of the LGN?

A
  • to organize retinal inputs from the 2 eyes:
    . into alternating maps of the opposite hemi-field
    . one in each of its 6 layers

before relaying theses maps on to V1 cortex

  • on the same (ipsilateral ) side of the brain
  • via axons that arise from the LGN neurons in each layer
  • and travel first in the posterior limb of the internal capsule
  • and then in optic radiation to area V1
43
Q

what is the geniculo-cortical pathway ?

A

. axons coming from neurons 6 layers of the LGN leave the tissue
. they first enter the white matter pathway ( called the internal capsule )

44
Q

what is the problem the inferior horn of the lateral ventricle ?

A

. axons coming from the medial half of the LGN, which represents lower visual quadrants of the hemifield map, are able to pass straight over it to enter the optic radiations directly

. but it forms an obstruction to axons coming from the lateral half of the LGN, which represents the upper visual quadrant of the hemifield map. They are initially forced to travel anteriorly (opposite direction) and then loop around it to join the optic radiations- which is not what is wanted

45
Q

what are the 2 components to the optic radiations ?

A
  • lower visual quadrant: over inferior horn of the lateral ventricle and below the parietal lobe straight to upper bank of calcarine sulcus
  • upper visual quadrant : ‘myers’s loop’ around inferior horn of the lateral ventricle, below the temporal lobe, then to lower bank of calcarine sulcus
  • this means the map of visual field is inverted/upside down- as axons representing the lower visual quadrant to upper bank of calcimine sulcus and upper visual quadrant to lower bank of calcimine suclus hence why map is inverted
46
Q

explain the hemi-field map in area V1 cortex?

A

. the hemi-field map in area V1 cortex is inverted and distorted

  1. inverted: the lower visual quadrant is represented in the upper bank of the calcarine sulcus and upper visual quadrant ( via meyer’s loop) in its lower bank
  2. distorted: because 66-75% of the posterior region of both banks is devoted to ‘ over’ representing the fovea and macular ( central vision), with more peripheral vision anteriorly ( towards the parieto-occipital sulcus )- the horzinontal meridian is represented right in the depths of the fundus of the calcarine sulcus
47
Q

what is the binocular portion ?

A

. region of space only seen by nasal retina of left eye

48
Q

explain the blood supplies to the optic radiations and area V1 cortex?

A

. axons in 1st part of radiation after the internal capsule = supplied by middle cerebral artery (MCA)

. axons in last part of radiations, under occipital lobe= supplied by posterior cerebral artery (PCA)

. calcarine sulcus, upper and lower banks = supplied by calcarine artery ( a terminal branch of PCA)

. occipital pole( fovea/macula vision) = dual supply : the calcarine artery + a terminal branch of the MCA

49
Q

what are the causes of visual field losses following local lesions/damage to the primary visual pathway?

A

A. damage to right to optic nerve - cause by optic neuritis - lose vision in right eye

B.midline lesion- chopped nasal axons from left and right eyes crossing the midline- caused by pituitary adenoma

C. transection of optic tract in right side - lead to left hemianopia

D. lesion that is knocked out Myers loop ( Myers loop ends up in lower bank of calcimine sulcus and that represents the upper quadrant which is where you lose vision in both left and right eyes)- left (both left sides of both eyes) upper quadrant hemianopsia

E. wipe out primary visual cortex - occlusion to posterior cerebral artery - left hemianopia with macular sparing

50
Q

what is the posterior pole of the occipital lobe?

A

. this is where the fovea and macula are represented

51
Q

what happens in most people with PCA INFARCT?

A

. just branches of the posterior cerebral affected

52
Q

What can lead to left hemianopia?

A
  • C
  • and if you wipe out the right LGN
  • wipe out the whole V1

OV2023 visual perception (20 decks)

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