T/F: albino cats don’t have STEREO OR BINOCULAR FUSION
-WHY?!
TRUE! So all siamese cats are strabismic albinos! Albinism in warm parts of body
-Normally, R LGN processes info from L VF. In albino cats, R LGN processes ALL INFO from L EYE - because ALL FIBERS CROSS IN ALBINOS; not only the nasal ones!
Because almost all retinal fibers CROSS in albinos, there is effectively NO (ipsi/contra) LGN
-implications?
IPSI LGN - non-existent
-LGN receives all wiring from CONTRA eye –> entire visual field from contra eye is projected - NO binocularity possible!
How are albinos still able to function relatively well, even though they don’t have binocular fusion or stereopsis?
-when do all of these changes occur?
Fibers crossing corpus callosum (were supposed to die if normal) actually send out massive INHIBITORY signals –> acts as a SUPPRESSION mechanism.
Result: each side only responds to one half of the visual field - just like in nml cats!
- half the cortex is thus always inhibited, but it’s better than being unable to localize objects in VF
- PRENATALLY
Four findings that you’d expect to find in an albino patient:
- reduced vision (20/80-20/200)
- 2’ to reduced foveal pit, reduced rod-free zone/cone density
- reduced stereo
- strabismus
- nystagmus (opposing signals)
T/F: vision can be 20/20 in an albino.
-how is this possible?
True
-most albinos have cone densities close to normal!
What will the nerve do in FISH/AMPHIBIA if:
- it gets cut:
- it gets crushed:
- it gets cut and the eye is inverted:
- the optic tectum is partially removed:
- cut: quick atrophy, then regrowth to exact position
- crushed: edema/inflamm, then regrowth to same connections
- cut/inverted eye: ON has no idea - regrows to identical old position
- tectum: entering axons reorganize to occupy what’s left
What happens if you implant a third eye in a frog?
It will send its extra ON to one of the optic tectums (which were receiving individual L and R fibers to each respective tectum)
-whichever tectum gets the fibers develops ocular dominance columns as seen in mammals
What happens if you cut the optic nerve of a mammal (cat)?
it does NOT regrow.
it MAY sprout axons to the abandoned optic nerve - note the flexibility
What’s the key factor if sprouting is to happen 2’ to a cut optic nerve (or enucleated eye) in a cat?
-How long after the nerve has been cut/eye enucleated IN HUMANS would it be before ganglion cell sprouting would cease? (ie, when is the START of the critical period in humans)?
TIMING!!! sprouting to abandoned LGN only likely to happen w/i 30 days of life (beginning of critical period in cats)
7 months - start critical period, so NO more sprouting can occur - must be some other mechanism to describe amblyopia!
What kind of drugs exist that may lead to retention of the dendritic tree after the ON is cut?
BDNF - brain-derived neural growth factor
