Neural mechanisms for eating and satiation

- Homeostasis is the body's tendency to maintain a relatively constant internal state. Mechanisms detect internal states and correct them, restoring the body's internal environment to it's optimum state. Part of this process ensures that we eat enough so that glucose levels are at a favourable level and so our body can then cope with everyday activities. The body has evolved two separate systems, one for turning eating 'on' (lateral hypothalamus/LH) and one for turning it off (ventromedial hypothalamus/VMH).

- Among humans glucose levels probably play the most important role in producing feelings of hunger, these feelings increase as glucose levels decrease. A decline in glucose levels in the blood activates a part of the brain (LH), affected by a neurotransmitter neuropeptide Y, resulting in feelings of hunger. This causes the individual to search for and consume food causing glucose levels to rise again.

- This rise in glucose levels activates the VMH which leads to a feeling of satiation, which in turn inhibits further feeding. This constant cycle of stopping and starting eating helps the body to maintain internal equilibrium. The neurotransmitter neuropeptide Y has been found to be an important factor in turning on eating, which is therefore linked to the LH.

+ There is evidence to support this explanation from Hetherington and Ranson who made lesions to the VMH of rats and looked at the effect it would have on their eating behaviour. It caused the rats to eat excessively making them obese. It can therefore be concluded that the VH is responsible for turning off eating and is our body's satiety centre.

+ Anand and Brobeck conducted a similar study but made lesions to the LH of rats, they found that the rats stopped eating and starved to death. As a result we can conclude that the LH does play a significant role in the process of eating/satiation.

+ Due to the fact there is consistent evidence which supports the assumption that neural mechanisms are the key to eating and satiation it can be argued that this theory is high in reliability. With this theory having high reliability it makes it easier for effective and appropriate treatments to be made for those with eating conditions as a cause and effect relationship has been established between the two variables.

- However there are methodological issues with the supporting evidence which in turn weakens the neural mechanism explanation to eating and satiation. Many of the studies in this area of psychology are based upon studies conducted on animals, particularly rats. This presents itself as a flaw as it means we cannot generalise the findings of the research accurately to humans. This is because animals such as rats have a different biological makeup to humans and also are less developed in terms of cognitive functioning.

- Therefore if the studies conducted on the rats were replicated on humans, which cannot be done due to ethical guidelines, then the same findings may not be found due to humans having a greater ability to think about the actions they are taking.

 - This means the findings cannot be generalised and the population validity of the study is reduced, therefore meaning the external validity of the theory is also reduced. Because of this reduced validity it may not be accurate to conclude that neural mechanisms are the sole cause of our eating and satiation.

- It has been suggested that neural mechanisms may be involved in influencing the cognitive process an individual goes through when processing food. Firstly, a part of the brain called the amygdala has been found to affect our eating as it results in us only selecting foods on the basis of previous experience. It is responsible for storing experiences about food so that the individual is selective about foods they eat, tending to only eat foods they have already experienced.

- Secondly, the inferior frontal cortex (IFC) receives messages from the olfactory bulb which is responsible for smells, and it is thought the damage to the IFC may decrease eating due to diminished sensory responses to our food odour and also probably taste. Therefore neural mechanisms can also have an effect on the cognitive process that affects our eating.

+ A strength of the explanation of the amygdala is that it has supporting evidence. Rolls and Rolls found that by surgically removing the amygdala in rats caused them to consume both familiar and unfamiliar foods indiscriminately, whereas amygdala intact rats would initially avoid foods. This shows how neural mechanisms affect our cognitive thoughts about food and influence our eating behaviour.

+ Furthermore, Zald and Parto found that the exposure to unpleasant odour produced significant blood flow increase to the amygdala; where as non-aversive odours did not cause the increased blood flow. This therefore suggests the amygdala is linked with the olfactory bulb, as it participates in the emotional processing of the olfactory stimuli. 

+ Both of these studies show evidence for neural control of cognitive factors which therefore increases the validity of this explanation due to the consistent support. Therefore we can conclude that the role of neural mechanisms on our thought processes does play a significant role in eating and satiation.

- However in order to accept the biological view to eating and satiation we must accept the biological approach. This can be problematic because the biological approach is deterministic claiming our eating behaviour is pre-determined by our biology. It ignores the role of cognitive factors such as some people eat due to being bored rather than 'biologically hungry'.

- Due to there not always being cognitive factors influencing a person's actions we can never scientifically test a cause and effect relationship between neural mechanisms and eating. As a result it can never be proven that neural mechanisms solely determine our eating patterns and satiation, so research into this area has no factual basis and therefore weakens the validity of this explanation towards eating and satiation.

- The neural mechanisms explanation to eating and satiation sit on the reductionist side of the reductionism-holism debate. This is because only biological factors have been considered and no thought has been given to other factors such as psychological or cognitive. This means that only one side of the argument is considered which then leads to only half an explanation being found.

- Being reductionist means we cannot establish a paradigm when considering reasons for eating and satiation as other approaches have been excluded.

- However being reductionist does have its strengths as it allows us to analyse the biological explanations for eating and satiation in greater detail and so from this we can generate a more valid understanding of that one factor alone.

- Overall a holistic approach would be more appropriate as it would allow consideration for how different contributing factors work with one another when causing a person to eat or feel satiated. This would then result in more effective and appropriate conditions being established for those with eating disorders and thus improve their chances of making a more successful recovery.