Download layer 4

Activity-dependent
Development
Nature vs. Nurture
Development of ocular dominance
in mammalian visual cortex
Critical period
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Nobel Prize in Physiology or Medicine 1981
Roger Sperry
David Hubel
Torsten Wiesel
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Roger Sperry
Regenerating retinal ganglion neurons
project to their appropriate position
normal frog
frog with rotated eye
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”)
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Ocular dominance (OD) in mammalian
visual cortex
left eye
right eye
left eye
right eye
ocular dominance column
layer 4
R
L
R
L
Rl
Lr
Rl
Lr
R
L
R
L
Rl
Lr
Rl
Lr
~ 0.5mm
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Afferent pathways from the two eyes
right eye
nasal
temporal
left V1
left eye
LGN
6
5
4
3
2
1
C
I
C
I
I
C
Layer 4
R
L
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Categories of cells in terms of ocular
dominance
Definition of ocular dominance index:
Response ipsi
od =
Response ipsi + Response contra
od = 1, ipsilateral only
od = 0, contralateral only
od = 0~1, binocular
Eyes
Cortical
cells
1
contra-
2
3
4
equal
5
6
7
groups
ipsi-
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OD distribution in normal adult V1
(monkey)
Number of cells
Normal V1
Equal
contralateral
ipsilateral
OD groups
Normal adult V1 -(above & below layer 4) binocular cells are common, with
each eye well represented roughly equally
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OD distribution in V1 after monocular deprivation
monocular deprivation (MD) -- suture one eye of the
newborn animal (monkey) for several months, reopen.
Number of cells
V1 after monocularly depriving the contralateral eye
Equal
contralateral
ipsilateral
OD groups
MD V1 -Ocular dominance shifts to the non-deprived eye.
Animal blind in the sutured eye.
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Effect of monocular deprivation on OD was observed
in multiple mammalian species
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Transneuronal dye to study the structure
of OD columns
left
radioactive
right
amino acid
eye
LGN
V1
6
5
4
3
2
1
C
I
C
I
I
C
L
R
2
L
R
L
layer 4
Areas which get inputs from the injected eye are labeled
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Compare OD columns in newborns,
adults and MD animals
normal adults - labeled and unlabeled alternate
layer 4
new borns - no OD column, all areas are labeled
layer 4
MD animals - deprived eye columns shrink, non-deprived eye
columns expand
layer 4
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deprived eye
non-deprived eye
Segregation of LGN afferents
- new borns
1. single LGN afferent has lots of
branches, covers a big area
layer 4
2. axon terminals from the two
eyes overlap extensively
L
R
- normal adults
1. selective elimination of axon
branches
layer 4
2. local outgrowth of new axon
branches
L
R
- MD animals
1. axon terminals from the closed
eye retract more
layer 4
2. axon terminals from the open
eye take over more areas
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open eye
deprived eye
OD column formation is an activitydependent, 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:
no social interaction
babies
foundling home
nursing home
with social interaction
withdrawn
normal
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