Brain development, nature and nurtureEdexcel A-Level Biology A: Revision notes

The brain does not develop from genes alone. In mammals, the neurones of the visual cortex form connections with the eyes in the first weeks and months of life. During this critical period, the visual cortex needs stimulation from both eyes for connections to develop normally.

Neurones that receive regular input form stronger connections (synapses). Connections that receive no input are not formed or are lost. The result is a visual cortex that can combine the images from both eyes for good depth perception and sharp vision.

After the critical period, the connections are largely fixed and are much harder to change.

Hubel and Wiesel sutured shut one eye of kittens (and later monkeys) for the first weeks or months of life and then reopened it. They recorded the activity of neurones in the visual cortex.

• Almost no cortical neurones responded to the eye that had been closed, and the animal was functionally blind in that eye, although the retina and optic nerve were normal.
• Most neurones responded to the eye that had been kept open.
• The same closure in adult animals had no such effect.

This shows that normal visual experience during the critical period is needed for the visual cortex to develop. The same idea explains why children with a cataract or a squint need treatment early.

Animals are used as models of the human brain. Their brains share a similar structure and function, and experiments that would be unacceptable on humans, such as depriving an eye of light, can be done under controlled conditions. Results are used to understand human development and to improve treatment.

Limits: animal brains are not identical to human brains, so results may not transfer exactly, and the experiments raise ethical issues.

Nature is the effect of genes and nurture is the effect of the environment and experience. Methods used to separate them include:

• Abilities of newborn babies: abilities present at birth, such as recognising voices or preferring face-like patterns, suggest they are innate (though learning in the womb is possible).
• Animal experiments: control experience, as in Hubel and Wiesel's work.
• People with damaged brain areas: loss of a function shows which area it depends on.
• Twin studies: identical twins share all genes, non-identical about half. If identical twins are more alike than non-identical twins, genes influence the trait. Identical twins brought up apart help to separate genes from environment.
• Cross-cultural studies: an ability found in all cultures suggests innate; one that differs suggests learned.

Moral and ethical arguments for and against animal research can be discussed from two standpoints:

• Utilitarian: the right action gives the greatest good for the greatest number. Benefits to many humans may outweigh suffering of animals, but harm must be weighed and kept to a minimum.
• Animal rights: animals are sentient and have a right not to be harmed or used as a means to an end. They cannot give consent.

A good answer states each standpoint, applies it to the research and reaches a reasoned conclusion.