Comparing the contributions of ‘nature’ and ‘nurture’ in brain development Sammir Bushara

Introduction – ‘Nature vs nurture’ There is clearly at least some genetic basis for human brain function. We are inherently distinct from other species, and some vital functions must be shared across all humans (an example being the control of breathing/heart rate and sensory processing) However, there are some issues which complicate this. There are differences between individual humans, and there is development in brain function after birth. The relative contributions of nature and nurture to these have be explored in a variety of case studies.

Abilities of newborn babies At birth, the earliest ‘reflexes’ of a newborn baby exist as a result of ‘natural’ brain development ‘in utero’ (i.e. in the womb). These include ‘grasping’ when startled, gripping a finger which touches their palm, etc.

Abilities of newborn babies One ability which appears early in life is the perception of depth. This was investigated using a ‘visual cliff’ experiment on babies of six months. The arrangement pictured right was set up; the children were encouraged to cross the ‘cliff’ covered by Perspex or glass. However, the children all refused to do so. Although this complex facility (to judge depth and assess consequent risk) has been demonstrated at such an early age, it is unknown whether it is a ‘hardwired’ or learned ability in human infants. However, the experiment has been repeated on animals which walk straight after birth, for example goats (kids) and lambs. All these animals refused, suggesting it is a ‘hardwired’ response for them.

Twin Studies Studies involving twins have often been used in order to determine the relative contributions of genotype and environment to an individual phenotype (with regard to a particular characteristic. Two ‘types’ of twin are often compared, fraternal and identical twins. Fraternal twins Identical twins Same environment, Same genotype Same environment, Different genotype Alzheimer’s study One recent study has shown that there is a 40% chance that an identical twin will contract late-onset Alzheimer’s disease if their twin has done so. This can be contrasted with a likelihood of around 10% among non-identical twin. This shows that it is likely that there is a genetic basis for the development of Alzheimer’s in later life, but environmental factors also play a significant role.

Cross – cultural studies Studies involving comparison of groups of participants from different cultures are useful in gauging differences in behaviour and perception which arise from being accustomed to a different environment. In some cases, significant differences were seen between different cultural groups. Take the example of the Muller-Lyer illusion Observe the two lines below. Which is the longest?

Cross – cultural studies While the two lines are the same length, the majority of people living in urban areas, who are accustomed to living around straight edges, would perceive the higher line to be longer. However, a study on a group of Zulu people in Southern Africa showed that people living in these cultures were less likely to be ‘fooled’ by the illusion, even when compared to local natives who lived in urban areas. This is thought to be due to the fact that they are more accustomed to living in circular huts than in built-up areas, with straight edged, angular buildings. It is hypothesised that they are therefore less likely to make the assumptions that we do about perceived ‘depth’ based on angles and lines.

Effect of occupation on brain structure? volume of the posterior hippocampi A study conducted by Maguire et al (2000) showed a significant difference between the volume of the posterior hippocampi of a group of taxi drivers and that of a control group. The results demonstrated both a larger volume for the taxi drivers and an apparent progressive increase in volume with time spent as a taxi driver. Changes in hippocampi of taxi drivers spatialmemory navigation. According to past research, the function of the hippocampus is predominantly spatial memory and navigation. This appears to be evidence for the influence of an environmental factor (i.e. occupation) having an effect on the structure of a part of the brain.

Brain damage and recovery of function One rather astounding example of the brain’s capacity to adapt to change is the recovery of the brain after damage (e.g after physical trauma or a stroke). This is due to the brain forming new connections to replace the function of the damaged tissue. This is not due to the production of new neurones in the brain, but the formation of new connection between existing neurones through the growth of new dendrites This occurs as a result of training (‘rehabilitation’) of undamaged areas of the brain to assume the role of the damaged tissue.

Conclusion It depends It depends – different functions can be developed due to ‘nurture’ at different stages of development to different extents. Reflexes and abilities of newborns and infants of humans/other animals show that some functions can arise from structural changes in utero (strongly determined by genotype), and are common across all (healthy) infants within a particular species. The cross-cultural studies show that some elements of ‘perception’ and behaviour which we may consider to be ‘universal’ can be affected by the culture in which we are brought up.

Conclusion However, the twin studies show that even individual differences between humans in brain development can be influenced by genotype. The ability of brain-damaged people to recover brain function shows that parts the brain can be trained to ‘acquire’ certain functions from others, through the fromation of new connections. However, the study on taxi drivers shows that the hypothalamus a highly ‘plastic’ area of the brain, can even be structurally enlargened through demands of use.