The role of visual experience on development of visual cortex.txt
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The role of visual experience on development of visual cortex
- The first synapses in visual cortex are found at 23 weeks of gestation
- numbers peak at 8 months postnatal and then decline, reaching adult levels at 10 years of age.
- The fovea is not fully developed until at least 4 years of age
If cataract is not removed in the first 1-2 months, there will be irreversible loss of visual abilities in the affected eye
Each lateral geniculate layer gets input from only one eye, and the geniculate cells relay this output to the cortex in alternating left-eye right-eye zones.
Most of he cells in layer 4 of the cortex are monocular
they respond only to input through the left or to the right eye
Connections from cells in layer 4 converge in other layers
- Almost all the cells in the other layers of the cortex are binocular
- binocular cells repined best when both of the eyes are looking at the object
Ocular dominance columns
- Most cells respond more stingily to one eye than to the other
- About half of the cells are left-eye dominated and half right-eye dominated
Each ocular dominance column is about 1-2mm wide
The development of ocular dominance columns
- The connections from the geniculate sort themselves out by making and breaking connections
- most before birth
- continues the first few years of life
- the basic segregation of geniculocortical proections is complete at 4-6 months postnatal
Result of a cataract that is not corrected in time
fast majority of cells in layer 4 will respond only to the unaffected eye
Axons that fire together, wire together!
Model of activity-mediated segregation
- Initially, inputs from both eyes via the LGN connect to each cell in layer 4
- By the time of birth, about half the cells have more left-eye input, while others have more right-eye input
permanent deficiencies in acuity for lines of particular orientations
uncorrected astigmatism can lead to
- permanent deficiencies in acuity for lines of particular orientations
- this defect need not be corrected until early grade school age
- but don't really know
Strabismus (cross-eyedness or wall-eyedness)
- can lead to loss of stereopsis and suppression of the vision in one eye (amblyopia)
- Strabismus is often treated by surgery followed by patching the eye with the stronger vision
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