Brain Development

We’re not born with our brain fully-formed. During our early lives we undergo a process of brain development whereby neuronal connections are strengthened or removed. Some ethically-dubious experiments in cats showed that proper development of the visual cortex depends on visual stimulation during a ‘critical period’ in our childhood.

Visual Cortex

The visual cortex is a region at the back of our brains and forms part of the cerebral cortex. It helps us to process visual information. Neurones in the visual cortex receive information from either our right or left eye and are clustered together in structures called ocular dominance columns. Right ocular dominance columns receive information from our right eye while left ocular dominance columns receive information from our left eye. The ocular dominance columns are arranged within the visual cortex in a repeating alternating pattern (i.e. right, left, right, left, and so on).

Development of the visual cortex

When we are born, our brains contain a load of neurones which begin to form connections (synapses) or are removed, making the brain more organised and allowing our visual system to develop. This all happens very early in life and relies on both of our eyes receiving visual input. The period of early life when our brains are developing is called the critical period. During the critical period, synapses that receive visual stimulation and pass on action potentials into the visual cortex are retained and strengthened. Synapses that do not receive visual stimulation, so the neurones between them are not firing, are removed. This means that if visual stimulation does not occur during the critical period (i.e. if a baby is born with cataracts which obscure vision, or if they are born in a cave) then their visual cortex will not develop properly because many of the synapses will have been destroyed. Evidence for a ‘critical period’ comes from some ethically-dubious experiments on kittens (see below).

Hubel and Wiesel’s experiment

David Hubel and Torsten Wiesel were two scientists who studied the electrical activity of neurones in the visual cortex of different animals. They were the first to discover the presence of ocular dominance columns and they determined that both right and left ocular dominance columns exist which are stimulated by visual input from the right and left eyes respectively.

Hubel and Wiesel carried out an experiment on kittens and adults cats in 1963 which proved that the visual cortex develops during a critical period early in life. First, they prevented visual stimulation in one eye by sewing up one eye of each kitten. Several months later, they unstitched the eye and found that the kittens had gone blind in one eye. Looking at the brains of the kittens under the microscope, they found that the ocular dominance columns for the stitched up eye had shrunk and that the ocular dominance columns for the open eye had expanded, suggesting that the columns for the open eye had taken over the columns that were not being stimulated (i.e. the neurones in the visual cortex had switched dominance). When they repeated the experiments in adult cats, they found that the cats retained their normal vision after having their eye stitched closed for several months and their ocular dominance columns remained unchanged. They repeated the experiment on young and adult monkeys and achieved the same results. Their results showed that the visual cortex only develops normally if both eyes receive visual stimulation in early life.

Since the visual cortex in cats and humans is similar (they both contain ocular dominance columns), Hubel and Wiesel’s results can be applied to humans. This means that humans need to receive visual stimulation from both eyes during the early periods of life for their visual cortex to develop normally. For example, some babies are born with cataracts which makes the lens of the eye go cloudy and obscures vision. Unless the cataracts are removed, the baby’s visual cortex will not develop properly because they are not receiving sufficient visual stimulation during early life. However, if adults develop cataracts it will not affect their visual system because it has already developed.

Ethical issues with animal studies

Animals are commonly used in medical research as they enable us to test theories and ensure that drugs are safe to use, meaning that many human lives are saved as a result. However, their use is controversial and there are a number of arguments against their use. Some of the arguments for and against the use of animals in medical research are summarised below:

Arguments for the use of animal research:

• It allows us to test the effects on a whole organism. An alternative to using animals is cell culture but this has the disadvantage that the cells are not part of a whole system - other body tissues may play a role in physiological responses.

• It allows us to carry out research which would be unethical on humans and helps us to develop effective drugs which save countless human lives.

• Animal experiments are only carried out when necessary and scientists have to follow strict guidelines and procedures e.g. animals need to be well looked after and euthanized humanely.

• The similarities between animals and humans means that we can apply the results to humans, making them an invaluable research tool.

Arguments against the use of animal research:

• All organisms have the right to not suffer and it is unethical to cause pain or stress to any living creature.

• There are alternatives to using animals - i.e. cell culture or computer models.

• Animals and humans are not the same and there is the possibility (indeed, this quite often happens) that drugs which are safe in animals have a different response and side effects in humans.

Studying brain development

The way that our brains develop is a result of both our genes (nature) and our environment (nurture). Some scientists disagree about the extent that nature or nurture influence the development of the brain - this is known as the nature-nurture debate. There are five different types of study which scientists can use to work out the role that nature and nurture play in brain development:

1. Twin studies - identical twins that have been raised in different environments (i.e. adopted by separate families) can show the influence of nature vs nurture. Because identical twins have the same genes, any similarities will be due to nature and any differences due to nurture. For example, identical twins who have been separated have very similar IQ scores, suggesting that nature has a large influence in intelligence. Scientists can also compare identical twins (who haven’t been separated - so same genes, same environment) with non-identical twins (different genes, same environment). Any differences between the identical and non-identical twins will be due to nature. For example, stuttering in both twins is more identical among identical compared to non-identical twins, suggesting that speech/stuttering is largely influenced by nature.

2. Cross-cultural studies - scientists can look at data from large groups of children who have been brought up in different cultures. Any differences between them will largely be due to nurture rather than nature. For example, mapping and spatial abilities of young children is similar across cultures, suggesting that this ability is largely due to nature.

3. Animal experiments - animals of the same species have similar genes, so brain development can be compared between animals of the same species, with any differences being due to nurture than nature. For example, rats raised in a stimulating environment have larger brains and are better at problem-solving than rats raised in a dull environment, suggesting that nurture plans a role in brain size and ability to solve problems. Scientists can also genetically engineer mice that are missing certain genes to determine the effect of those genes on brain development. If scientists knock out a gene called Lgl1, the brain expanded and is filled with fluid. This suggests that nature plays a large role in normal brain development.

4. Newborn studies - apart from the womb, newborn babies have had little exposure to the environment so any abilities that they are born with (i.e. the ability to cry) is likely to be innate and due to nature rather than nurture. Any abilities which take more time to develop (such as language) is more likely to be due to nurture.

5. Brain damage - scientists can compare children with and without brain damage to determine which abilities are due to nurture. If a child with brain damage still develops a certain ability, then that skill will be a result of nurture. For example, children with brain damage to the region of the brain associated with language initially show impairments in speaking but by the age of five there is no difference between these children and healthy children. This suggests that speaking and processing language has a mostly environmental component (nurture).

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