Visual Pathways (Special Senses) Flashcards
Describe the path of information starting at the retina, to the LGN.
Information goes down optic nerve to optic chiasm, and then optic tract, and then to LGN (main synapse before entering brain proper).
At the chasm, information on the temporal (outside) half of each eyeball remains on the outside half of the chiasm (does not cross over) but info on nasal (inside) side does cross over at the chiasm. As a result, everything on R side of visual world is dealt with by the L half of the brain and vice versa.
Describe the structure of LGN.
Six layers:
- Layers 1 and 2 receive signals that travel by the Magnocellular pathway (through magnocellular cells)
- Layers 3 through 6 receive signals that travel by the Parvocellular pathway (through parvocellular cells)
Info from contralateral eye goes to LGN layers 1, 4, 6
Info from ipsilateral eye goes to LGN layers 2, 3, 5
From the right retinal hemifield of each eye (the nasal hemifield of the L eye, and the temporal hemifield of the right eye), the visual information is routed to the right LGN.
VICE VERSA
Distinguish between magnocellular and parvocellular ganglion cells.
MAGNOCELLULAR Ganglion Cells
• Large axons – lots of myelin – respond quickly
• Receive information from large number of photoreceptors
• Movement, brightness, depth perception
• Project to parietal lobes – the ‘where’ stream
PARVOCELLULAR Ganglion Cells
• Thin axons – less myelin – respond slowly
• Receive information from small number of photoreceptors
• Detail of objects assisting in recognition
• Project to temporal lobes – the ‘what’ stream
Describe the path of visual information after the LGN.
Information form the LGN go to the occipital lobe (specifically the striate cortex, first area to receive input from LGN)
Describe the connection between LGN and striate cortex.
Line of Gennari – Myelinated fibres running from LGN to synapse in layer 4 of the ‘striate’ cortex
Describe the relationship between spots on the retina and location on the occipital lobe.
• Retinotopic map – direct physical relationship between spots on the retina and spots in the occipital lobe (topographic relation)
Define neural tuning, in the context of vision.
Specific populations of cells in the occipital lobe will respond to;
- Colour
- Spatial frequency
- Orientation
Describe the retinotopic map in the occipital lobe.
Center part of the retina (fovea) corresponds to most posterior aspect of the occipital lobe
Superior part of retina projects to part of occipital lobe inferior to part of occipital lobe to which inferior part of retina projects
Refer to image on slide 10
Identify the different part of the vision pathway (from retina to occipital lobe) which can be damaged, and state the effect on vision. Draw these.
1) Lesion in L/R superior temporal retina: corresponding field defect in ipsilateral inferior nasal visual field
2) Complete lesion of L/R optic nerve: total blindness in corresponding eye
3) Chiasmal lesion: bitemporal hemianopia (only damages information that crosses over, i.e. info that comes from nasal retina, which sees the
temporal hemifields, so get bitemporal hemianopia)
4) Lesion of L/R optic tract: contralateral incongruous hemianopia (e.g. if lesion of left optic tract then right incongruous hemianopia)
5) Lesion of inferior optic radiations in temporal lobe: homonymous superior quadrantanopia
6) Involvement of optic radiations: homonymous inferior quadrantanopia (upper L optic radiation would affect bottom R quadrant)
7) Congruous incomplete homonymous heminopia
8) Lesion in R/L hemisphere (e.g. occipital lobe infarct): Homonymous hemianopia in contralateral side
Refer to slide 13 for images
Which is the least common site of hemianopia ?
Optic tract
Which kinds of damage along the vision pathway do not affect pupillary light reflex ?
Pupillary light reflex not impaired if beyond tract
Identify possible causes of chiasmal lesion, and the associated visual defect. Draw these.
1) Pituitary lesion (pituitary is in the midline, and close to the chiasm, so can damage the latter if grows)
2) Craniopharyngioma
EFFECT: BITEMPORAL HEMIANOPIA (Bottom part of retina remains in bottom part of chiasm, and sees top of visual field, and vice versa. Hence, pituitary lesion associated with denser hemifield loss at top of visual field because tend to come from bottom, and vice versa in craniopharygnioma)
Refer to slide 17
Identify a possible cause of retinal lesion. Draw the pattern of visual loss associated with this.
Age related macular degeneration
Refer to slide 15
Explain how quadrantanopias occur. Draw these.
As information moves from eyeball, to chiasm, to the occipital cortex, it actually splits anatomically. Info which came from the top part of the retina goes to the parietal lobes, and the info from the bottom part
of retina goes through the temporal lobes before gets to occipital lobe.
Damage exclusively to right temporal lobe (e.g. due to CVA), will result in L visual field problem, specifically affecting the superior hemifield (because that info has come from lower part of retina), so
L superior quandratnanopia (R inferior temporal lobe
problem)
Parietal lesions (e.g. CVA) lead to inferior quadrantanopias affecting the side of the visual field contralateral to the lobe involved.
Refer to slide 20
Identify techniques to test visual field.
SIT OPPOSITE PATIENT, AT SAME HEIGHT
-Ask patient, ‘while looking at me, can you see all of my face, and the surrounding room ? ‘
-Present your open palsms in the temporal quadrants ‘point to the hand that is waving’
-Present targets to the patient:
With the patient’s R eye covered/your L eye closed (and vice versa)
In the plane midway between two
Moving from periphery to the center along a line of 45o
Without crossing the vertical or horizontal meridian
-Start with a white cotton bud, moving all the way from periphery to fovea
-Map out macular function with a red cotton bud ‘tell me when you see this as red’
-Map out the blind spot, moving from non-seeing towards seeing areas
