NB8-4 - Visual Pathways and DLA Flashcards
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Explain the technical term for this defecit
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Right superior homonymous quadrantanopia
Homonymous - present in both eyes
Quadrantanopia - only present in one quadrant
What are the fibers called that are leaving the retina and carrying visual information?
Retinofugal projections
Describe where the main retinofugal projection and its collaterals synapse and their functions.
- The main retinofugal projection will synapse at the lateral geniculate nucleus in the thalamus. It’s postganglionics will then go to the primary visual cortex to provide conscious vision.
- One collateral will synapse at the suprachiasmatic nucleus in the hypothalamus to provide info for biological rhythms and sleep
- One collateral will synapse at the pretectal nucleus in the midbrain for pupillary light reflexes
- One collateral will synapse at the superior colliculus in the midbrain for automatic saccades (quick simultaneous movement of both eyes)
Describe the visual pathway for conscious vision
- The retinofugal projections from the temoporal and nasal hemiretinas bundle into the optic nerve which travels to the optic chiasm
- At the OC, the nasal hemiretinal fibers decussate while the temporal fibers DO NOT. the fibers bundle together again into the optic tract
- The optic tract synapses at the lateral geniculate nucleus of the thalamus
- Postganglionic fibers, on both sides, bundle into two separate optic radiations. Meyer’s loop which projects through the temporal lobe on its way to the lower visual cortex carrying information from the upper visual fields. Baum’s loop which projects through the parietal lobe on its way to the upper visual cortex carrying information from the lower visual field.
- The optic radiations synapse on layer 4 of V1
List the parts of the conscious visual pathway that carry ipsilateral and contralateral visual information.
- The retinofugal projections and optic nerve carry both. The temporal hemiretinal fibers from the right eye and nasal hemiretinal fibers from the left eye carry contralateral information and vice versa.
- The optic tracts, radiations, and primary visual cortices (V1) will all carry or process contralateral visual information
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Where in the occipital lobe is V1 located?
Just above and below the calcarine sulcus in the cuneal and lingual gyri
Explain the concept of ocular dominance columns.
Fibers from one eye do not terminate in exactly the same area of the primary visual cortex than fibers from the other eye. Instead there are areas (columns) dedicated to input from one eye. These columns alternate so that one is from one eye and the next in the row is from the other eye. It should be noted that the organization in columns refers to the fact that most (if not all) layers of the cortex are involved, from layer 1, close to the pial surface, down to layer 6, close to the white matter. Refer to image.
Describe what orientation columns are and the experiment that discovered them.
Hubel and Wiesel performed an experiment where they measured the AP frequency of a V1 cell while showing an animal a light bar in varying orientations. What they discovered is that different columns of cells had higher AP frequencies when different bar orientations were shown (refer to image). They discovered that the ocular dominance columns were also orientation columns.
Describe the parallel visual pathways and what they do.
Within the conscious visual pathway and beyond there are parallel pathways that carry depth/motion or form/color information. Refer to image
- The Depth/Motion pathway starts with the magnocellular neurons (M ganglion cells) and synapse in the top two layers of the LGN and then synapses in layer IV of V1 before forming a dorsal (parietal) pathway. AKA the “where” pathway
- The for/color pathway starts with the parvocellular neurons (P ganglion cells) and synapses in the bottom four layers of the LGN and then synapcses in layer 4 of V1 before formina a ventral (inferior temporal) pathway. AKA the “what” pathway
What is achromatopsia and what usually causes it?
Achromaopsia is TOTAL color blindness which usually is not caused by conal defects (since all three types of cones would have to be affected). This is usually caused by a lesion to the inferior temporal lobe affecting the form/color pathway (Brodmann areas 18 or 37)
List the visual field defecits.
Hemianopia - visual loss in one half of the visual field
Quadrantic Anopia - visual loss in one quadrant of the visual field
Homonymous Anopia - same visual field defecit in both eyes
Heteronymous Anopia - different visual field defecits for both eyes
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