activity-dependent development
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Activity-dependent Development. Nature vs. Nurture Development of ocular dominance in mammalian visual cortex Critical period. Nobel Prize in Physiology or Medicine 1981. Roger Sperry. David Hubel. Torsten Wiesel. Roger Sperry. - PowerPoint PPT PresentationTRANSCRIPT
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Activity-dependent Development
Nature vs. Nurture
Development of ocular dominance in mammalian visual cortex
Critical period
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Regenerating retinal ganglion neurons project to their appropriate position
Axons know where to go; this process is NOT experience dependent. However, the details of the connection patterns between retina and tectum can be affected by experience (and electrical activity)
There is also some difference between regeneration (more specificity) and development (more “trial and error”)
Roger Sperry
normal frog frog with rotated eye
4
Ocular dominance (OD) in mammalian visual cortex
layer 4
R L R L
R L R L
Rl Lr Rl Lr
Rl Lr Rl Lr
~ 0.5mm
left eye right eye left eye right eye
ocular dominance column
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Afferent pathways from the two eyes
right eye
left eye
LGN
left V1
nasal
temporal
6
5
43
21
CI
II
C
C Layer 4
left eyeright eye
R L
6
Categories of cells in terms of ocular dominance
od =
Response ipsi
Response ipsi + Response contra
od = 1, ipsilateral only
od = 0, contralateral only
od = 0~1, binocular
Eyes
Corticalcells
groups1 2 3 4 5 6 7
contra- equal ipsi-
Definition of ocular dominance index:
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OD distribution in normal adult V1 (monkey)
Normal adult V1 --
(above & below layer 4) binocular cells are common, with each eye well represented roughly equally
contralateral ipsilateralEqual
OD groups
Num
ber
of c
ells
Normal V1
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OD distribution in V1 after monocular deprivation
MD V1 --
Ocular dominance shifts to the non-deprived eye.
Animal blind in the sutured eye.
monocular deprivation (MD) -- suture one eye of the newborn animal (monkey) for several months, reopen.
Num
ber
of c
ells
V1 after monocularly depriving the contralateral eye
Equal
OD groups
contralateral ipsilateral
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Transneuronal dye to study the structure of OD columns
Areas which get inputs from the injected eye are labeled
left right radioactive
amino acid
2CI
II
C
C
layer 4
R L R LL
LGN
V1
eye
65
43
21
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Compare OD columns in newborns, adults and MD animals
normal adults - labeled and unlabeled alternate
new borns - no OD column, all areas are labeled
MD animals - deprived eye columns shrink, non-deprived eye columns expand
layer 4
layer 4
layer 4
deprived eye non-deprived eye
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Segregation of LGN afferents
- MD animals
1. axon terminals from the closed eye retract more
2. axon terminals from the open eye take over more areas
- normal adults
1. selective elimination of axon branches
2. local outgrowth of new axon branches
L R
layer 4
- new borns
1. single LGN afferent has lots of branches, covers a big area
2. axon terminals from the two eyes overlap extensively
L R
layer 4
layer 4
deprived eyeopen eye
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OD column formation is an activity-dependent, competitive process
Experiments:
1. Binocular injection of TTX, blocks segregation of OD columns
- segregation is activity dependent
2. If both eyes are deprived (binocular deprivation), OD columns are normal!
- segregation also depends NOT on the absolute level of activity, but on the balance between the input from the two eyes, thus seems to be competitive
Mechanism:
1. Normal development
- initially the axon terminals from the two eyes overlap
- at local region, inputs from one eye happen to be stronger
2. Monocular deprivation
- open eye more active, take over more territory
- deprived eye less active, lose most of the territory
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Critical period
1. Monocular deprivation (MD) causes a shift of OD toward the non-deprived eye. This is effective only before certain age. MD has no effect on adult animals.
critical period: a period in early life that the neural circuit is susceptible to external sensory inputs (e.g. MD). This period depends on the species and the neural circuit.
For OD in V1:
cat: 3rd week -- 3 months
monkey: first 6 months
human: 1st year most important, but extends to 5-10 years
2. MD within the critical period, the effect is permanent and irreversible.
This finding has profound implications in treatment of congenital cataracts in children
3. MD within the most sensitive part of the critical period (e.g., first 6 wk for monkey), a few day’s MD results in a complete loss of vision in the sutured eye.
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Critical periods of other neural functions
• visual system
- OD
cat: 3rd week ~ 3 months
monkey: first 6 months
human: 1st year, also extends to 5-10 year
- more complex visual functions (e.g., contour integration) often have longer critical period
• other aspects of brain function
- Bird imprinting behavior
Konrad Lorenz (1903-1989)
- Monkey social interaction
newborn monkeys reared in isolation for 6-12 months, behaviorally abnormal
- Human
- language: 2-7 years of age
- early social interaction:
withdrawn
nursing home babies
foundling homeno social interaction
normalwith social interaction