Visual System Flashcards
Pathway of visual system
Retina - lateral geniculate nuclei - primary visual cortex (PVC)
Images on the retina are
- Inverted
2. Reversed
Normal visual fields
Extends from ~80-90 temporally
~50-60 nasally and superiorly
~60-75 inferiorly
Fovea
Central fixation point.
Highest visual acuity
Represented by half of the fibers in the optic nerve and half of the fibers in the visual cortex
Surrounded by the macula
What is represented on the fovea is projected on the occipital pole
Optic disk
Axons leaving the retina as they enter the optic nerve
No photoreceptors here. Small blind spot
No functional deficit when both eyes are used - when one one used visual system “fills in” the blind spot
Photoreceptors
Outermost layer of retina- respond to light
- Rods - low level lighting, low resolution, outnumber cones
- High resolution, highly represented in fovea, detect color
Bipolar cells
Middle layer of retina
Receive synapses from photoreceptors
Ganglion cell layer
Innermost layer of retina
Receives synapses from bipolar cells
Sends axons into the optic nerve
1. Parasol cells - gross stimulus features and movement
2. Midget cells - fine visual detail and colors
Optic chiasm
Partial crossings of the optic nerve fibers where they meet.
Nasal fibers crossover in chiasm and are responsible for temporal hemifields of vision
Visual info from both eyes in contain in optic chiasm
Optic tracts
Ends at LGN
Contains visual info from ipsilateral hemiretinas of each eye.
Right hemiretinas of both eyes
End up in the right optic tract
Left hemiretinas of both eyes
End up in L optic tract
Ipsilateral hemiretinas represent
Contralateral visual fields
Lateral geniculate nucleus
Axons in optic tracts wrap about the midbrain (laterally) to synapse at the LGN - axons leaving LGN project back to primary visual cortex
Layers 1-2 motion and spatial analysis
3-6 detailed form and color
-info from L and R eyes remain segregated while passing through the LGN - synapse on different layers
Extrageniculate pathways
Small number of fibers in the optic tract bypass the LGN to enter the superior colliculus and pretectal areas
Pretectal area
Front portion of rectus
Involved in the pupillary light reflex
Pretectal area and superior colliculus
Involved in directing visual attention and eye movements toward visual stim
Optic radiations
Axons projecting from LGN to PVC - sweep over and around the lateral ventricle
Pass through temporal and parietal lobes
Ipsilateral optic radiations carry info from contralateral visuals fields
Inferior optic radiations
Arc slightly forward and into temporal lobe forming Meyers loop.
Carry visual info from inferior retina (superior visual field)
Project to lower bank of calcarine fissue
Superior optic radiations
Pass through lower parietal lobe
Carry visual info from superior retina (inf. Visual field)
Project to upper bank of calcarine fissue
Primary visual cortex
Upper bank/gurus and lower bank/gyrus of the calcarine tissue in the medial portion of the occipital lobe
Reinotopically organized
Fovea occupies about half of it - very disproportion cortical representation despite the small retinal areas
Calcarine fissure
Cuneus - upper
Lingual - lower
PVC peripheral regions of visual fields are represented more atneriorly along the calcarine fissure
Analysis of motions and spatial relations
Dorsal/upper pathways project to parietal-occipital association cortex
“Where” analyzing motion and spatial rln b/n objects as well as between the body and visual stim
Analysis of form and color
Ventral/lower pathways project to occipitotemporal associated cortex
“What?” Form w/ specific regions IDying colors, faces, letters, and other visual stim
Assessment of visual disturbances
- Nature - time course, positive phenomena, negative phenomena
- Visual field
Positive phenomena
Simple - lights, colors, geometric shapes (arising from disturbances located anywhere from eye to cortex)
Formed - people, animals, or complex scenes (arising from
inferior temporal-occipital visual association cortex)
Assessment of visual disturbances 2
- Distinction between a monocular or binocular visual disturbance is essential for localization
- Visual disturbances are often described as being “in one eye”, when in reality the left or right visual field is affected for both eyes
- Blurred vision is hard to interpret without further description and examination
- Could be attributed to a variety of things from a cornea problem to a lesion in the visual cortex
Localization of visual field deficits
- Visual Field Testing- Tests for crude deficits in the visual fields
- Test each quadrant while making sure patient’s eyes stay centrally fixated (neuroexam.com-video 27)
- Test each eye using wiggling fingers and/or have pt count the number of fingers being held up
- Fields recorded as if viewing own visual field
- Blink to threat can be used on lethargic/uncooperative patients (neuroexam.com-video 28)
The most important information for localization of problems in the visual pathways
What is the position and shape of the visual loss?
Does it affect one or both eyes?
Blood supply of the visual pathway
Optic tracts, chiasm and nerves receive blood supply from numerous small branches of ACA and MCA, clinically signficant infants of these are rarely seen
Optic radiations passing through parietal lobe (superior) MCA superior division
Optic radiations passing through temporal lobe (inferior) MCA inferior
PVC by PCA
