Download Visual development.

Visual development.
How do we interpret information
from the eyes to enable us to see
what we see?
Specification
11 Discuss whether there exists a critical
‘window’ within which humans must be
exposed to particular stimuli if they are
to develop their visual capacities to the
full.
12 Describe the role animal models have
played in developing explanations of
human brain development and function,
including Hubel and Wiesel’s experiments
with monkeys and kittens.
What do you see?
What do you see?
Brain growth facts
• 21 days after conception the neural tube is formed - develops
into the brain and spinal cord.
• Baby is born with > 100 000 million neurons
• By 6 months old the brain is half the adult size
• By 2 years the brain is 80% of the adult size
• There is no huge increase in the number of neurones after
birth. So what happens as a child develops and the brain
grows?
– most brain growth and development is the result of
• elongation of axons,
• myelination,
• development of the right synaptic connections,
• We will look at this in the case of visual development.
What is the function of the thalamus is vision?
It is a relay station that carries visual information from the retina to
the visual cortex in the occipital lobe .
The visual pathway
Synapses in the Thalamus and
Visual cortex
• How are axons from the thalamus ordered as
they synapse with cells in the visual cortex?
– The cells of the visual cortex are arranged in
columns. Axons from the thalamus synapse within
these columns of cells.
• How do scientists know this? What is the
evidence?
– Staining sections of the visual cortex
– Studies using electrical stimulation
How are the cells ordered in the visual cortex? Describe in your own words
Adjacent columns of cells receive
input from the same area of the
retina of both eyes.
One column from the left and the
next column from the right eye
This is repeated across the whole
visual cortex to build up a ‘map’ of
the retina.
Is this ordering of the cells in the
visual cortex due to genetic
encoding? The environment?
Both?
- genetic
Genes or the Environment?
• The formation of the columns of cells is
genetically determined and not influenced by the
environment.
• However, there are periods of time (critical
window: before a certain age) during brain
development when the brain must obtain
sufficient stimulation from specific external
experiences (i.e. environment) to develop
properly.
The “critical window” – Hubel & Wiesel’s proof
Permanently
blind
monkeys?
Hubel & Wiesel investigated the
critical window.
They used monkeys and kittens
in their studies
Their work permanently blinded
some animals and can be argued
to be unethical.
Hubel & Wiesel’s Method:
•Raise monkeys from birth in three groups for 6 months
•Group 1 are the control (no blindfold), Group 2 are blindfolded in both
eyes, Group 3 are blindfolded in one eye (monocular deprivation)
After 6 months:
•Test the monkeys to see whether they can see using each eye
•Test the sensitivity of retinal cells
•Test the activity of nerves in the visual cortex in response to stimuli
The results:
• Monkeys in Group 2 (both eyes blindfolded) had
impaired vision
• Monkeys in Group 3 (monocular deprivation)
were blind in the deprived eye
• Retinal cells were responsive in all groups
• Cortical activity was reduced in parts of the brain
that process information from the deprived eye
• Adults (past the critical window) undergoing the
same tests showed no difference between
groups. All could see.
The Conclusion:
There is a critical window for visual neural
development, which requires stimulus from the
eye.
If this window is missed the monkey is blind,
because of events happening in the brain, not
the eye.
In humans there is a burst of new synapse
formation in the visual cortex at 3-4 months and
the maximum density of synapses is reached
between 4-12 months
Q 8.39 p236.
Fact: Kittens are born blind, with their eyes shut.
• Hubel and Wiesel tested kittens for the effects of
monocular deprivation at different stages of development
and for different lengths of time. They found:
• Deprivation at under 3 weeks had no effect
• Deprivation after 3 months had no effect
• Deprivation at four weeks had a catastrophic effect –
even if the eye was closed for merely a few hours.
• How can you explain these results?
Explanation
• Because kittens are born blind, early deprivation
(under 3 weeks) would have no effect.
• By 3 months connections to the brain have been
made, and deprivation has no effect since the
critical period has ended.
• The critical period is at about 4 weeks so lack of
stimulation from the kitten’s environment at this
time severely affects visual development.
What happens during the critical period?
Neurones in the visual cortex
Refinement of columns in the visual
cortex produces the distinct pattern
of column driven by the left and right
eyes together.
Compare the columns in the light
deprived eye with the light-stimulated
one.
Columns that receive input from lightdeprived left eye are much narrower
and there are fewer synapses.
Columns from the light-stimulated eye
are wider, more synapses and the
neurons appear to have grown more.
Impulses passing along
neurone P cause synapses
1 and 2 to release
neurotransmitter.
Fewer impulses passing
along neurone Q causes
synapse 3 to release less
neurotransmitter to cell Y
than synapse 2.
What happens to synapse
3 eventually?
Neurone Q no longer
synapses with cell Y so
column narrower
It is eventually lost if
there are many more
impulses passing
along P to cell Y than
along Q
Evidence for a critical window
• Medical observation
– Cataracts in children
– Treatment of lazy eyes
• Animal models - Hubel and Wiesel
– Deprivation of light in kittens and monkeys
Animal models
• Advantages –
– Uses animals that are easy to obtain
– Easy to breed
– Short life cycle
– Small adult size
• Therefore get reliable results quickly
• BUT is it ethical?
Mechanism of visual development in the cortex
See Q8.41 p 238. Rewrite the sentences in the correct order to
explain the mechanism for visual development.
What follows happens in the area of the visual cortex where neurones
from both eyes overlap.
1. There is a lack of visual stimulation in one eye.
5. Axons from the visually deprived eye do not pass impulses to
cells in the visual cortex (so no neurotransmitter is released).
3. Axons from the non-deprived eye pass impulses to cells in the
visual cortex (so neurotransmitter is released).
4.
Synapses made by active axons are strengthened (so release
more neurotransmitter).
2. Inactive synapses are eliminated.
So for full development of the visual cortex nerve impulses
from both eyes, and neurotransmitter release from all
neurones involved must occur.
Summary
The initial formation of the columns in the visual
cortex is genetically determined. There is a lot of
overlap between the axons from the thalamus at
birth.
Visual stimulation is required for refinement of the
column. So full development of the visual cortex
depends on environment
So full visual development is the result of genotype
+ environment (i.e. ‘nature + nurture’)
Activity 8.13 Critical Window for visual development
Background reading; see Weblinks
for Activity 8.13
• http://www.wellcome.ac.uk/assets/WTD00
3611.pdf
• http://web.sfn.org/index.cfm?pagename=br
ainBriefings_visualDevelopment
• http://nobelprize.org/nobel_prizes/medicin
e/laureates/1981/index.html