VIsual System Flashcards
What forms the optic nerve?
The ganglion cells from the retina.
Fovea
Avascular zone in the back of the eye in which all the ganglion cells and interneurons are moved out of the way, so that light can hit directly on the photoreceptor. CONES ONLY.
Optic disc
Where the optic nerve is formed by the ganglia and where the blood vessels enter the eye.
Where does optic information ultimately end up?
Calcarine sulcus (occipital cortex- Broadmann’s 17)
Difference in the amount of information being sent to the cortex, based on the part of the retina it comes from.
For the photoreceptors in the fovea (almost all cones), there is a 1:1 or even a 1:2 ratio. Each has its own path to the cortex. As you move to the periphery of the retina, this ratio becomes more diluted, so that as many as 50-60 receptors will share 1 path.
70% of information comes from fovea.
The sides of the retina
Temporal and Nasal. This is important because the visual fields cross so things in the middle land on the temporal part and vice versa.
Which retinal axons cross and where/why do they cross?
The Nasal axons cross at the Optic chiasm so that they can be with the corresponding temporal information coming from the other eye.
Pathway from the Retina
(1) Optic nerve
(2) Optic chiasm (nasal cross here)
(3) Optic tract
(4) Lateral geniculate (mostly cones)
(5) Optic radiations
(6) Visual cortex
2 types of lateral geniculate cells
(1) M-type: respond to movement. Associated with Rods. Fewer of these. These are larger.
(2) P-type: Color and fine detail. Associated with Cones. More of these. These are smaller.
Some light information does not go to the lateral geniculus. Where does it go?
2 locations.
(1) Pretectal area- for pupillary reflexes
(2) Superior colliculus- for head and eye movements
How does lower/upper visual field info differ in its movement?
LOWER: Info is taken in by upper parts of retina. It travels along upper fibers and streams via the PARIETAL LOBE to synapse on upper calcarine sulcus.
UPPER: Info is taken in by lower parts of retina. It travels along lower fibers and streams via the TEMPERAL LOBE to enter lower calcarine sulcus (meyer’s loop).
THESE ARE ALL KNOWN AS OPTIC RADIATIONS.
What happens if there is a lesion on…
(1) Optic nerve
(2) Optic Chiasm
(3) Optic tract
(4) Lateral geniculate
(5) Meyer’s loop
(6) Occipital lobe
(1) Full vision loss in that eye
(2) Full loss of nasal fibers which means loss of lateral vision (Bitemporal hemianopsia)
(3) Opposite visual fields for both eyes ( Cut the right optic tract, lose the left optic field for both eyes).
(4) Same as cutting the optic tract^^^
(5) “Pie-in-the-sky”. Quadrantanopia. Cuts out right or left upper quadrant of vision field.
(6) Loss of half of visual field (hemianopia), but leaves small piece in the middle (macular sparing- fovea fibers survive)
Direct vs consensual response
Direct-constriction of the eye in which the light was shown
Consensual- constriction of the other eye
Happens due to BILATERAL CONNECTIONS TO THE EDINGER-WESTPHAL.
Presentation of optic nerve damage vs damage to CN III
OPTIC NERVE: produces equal pupils, neither of which respond to the light shone via direct reflex, but still have functioning consensual reflex (assuming it is only one eye with damage).
OCULOMOTOR: For damaged eye, no response to light shown in either eye. For healthy eye, response is present both direct AND consensual, because the optic nerve of the damaged eye still functions properly.
What is the benefit of the expanded layer IV of the visual cortex?
It is the input layer. Expansion allows you to process via 3 columns at once:
(1) Orientation (special representation and edges)
(2) Blobs (color specificity)
(3) Occular dominance (which retina did this come from)
