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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 in 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