Localisation of Function in the Brain and Hemispheric Lateralisation
AO1: Description of Localisation of Function in the Brain and Hemispheric Lateralisation:
Localisation of functions refers to the principle that specific functions (language, memory, hearing etc…) have specific locations within the brain.
AO1: Hemispheric Lateralisation:
The brain is contralateral (opposite sides) in most people; this means that parts of the left hemisphere deal with the right side of the body and parts of the right hemisphere deal with the left side of the body. This means that if a person has a stroke in the motor areas of their right hemisphere, this will affect the functioning of the left side of their body.
In addition, what you see in your right visual field is processed in your left hemisphere and although you gather auditory information from both ears, the information gather by the left ear is predominately processed by your right hemisphere.
AO3: Evaluation of Localisation of Function in the Brain and Hemispheric Lateralisation
(1) Point: A strength of the research conducted in the localisation of function of the brain and hemispheric lateralisation is that it has been conducted in the controlled setting of the laboratory. Example/Evidence: For example, research aiming to identify which areas of the brain are responsible for specific behaviours (e.g. language) have used methods such as fMRI scanning, EEG machines and post-mortem examinations.
Elaboration: This is a strength because, such research can be praised for being scientific and objective allowing researchers and psychologists to draw firm conclusions as regards to what parts of the brain are responsible for specific behaviours/activity thus increasing the internal validity of the research.
(1) Point: However, there are a number of problems with this research.
Additional research carried out has highlighted that there are individual differences in language areas. Example/Evidence: For example, in a study on silent reading, Bavelier et al (1997) found a large variability in individual patterns of activation of the brain across different individuals. They observed activity in the right temporal lobe as well as the left frontal temporal and occipital lobes. Elaboration: This is a weakness as it shows that brain functioning can differ across individuals suggesting that research into the structure of the brain does not consider individual differences.
(2) Point: In addition, gender differences have been identified in the size of the areas of the brain associated with language. Example/Evidence: For example, Harasty et al (1997) found that women have proportionally larger Broca’s and Wernicke’s areas than men, resulting in a women’s greater use of language. Elaboration: This is a weakness because it suggests that female and male brains operate in a different manner and don’t always follow the initial theories of brain structure as suggested by early research.
AO1: Lateralisation and Split Brain Research:
The idea that different areas of the brain might have different specialisms was put forward by Marz Dax. Through investigating brain damaged patients, Dax observed that in every case damage to the left hemisphere of the brain led to deficits in language (none of the patients had suffered right hemisphere damage). This suggested to Dax that it was the left hemisphere of the brain that was responsible for language and thus, this assumption has been further investigated using split-brain research.
The term ‘brain lateralisation’ refers to the fact that two halves of the brain are not exactly alike. Each hemisphere has a functional specialism (e.g. left hemisphere responsible for language, right for visual-motor tasks). Paul Broca established that damage to certain areas of the left side of the brain led to a deficit in language, damage to the same area of the right hemisphere did not have the same consequences. This raised an important question, if language is associated with the left hemisphere, how is it that we can talk about things that we experience in the right hemisphere (e.g. face recognition)? The answer is that the two hemispheres must be connected. Information received in one hemisphere can be sent to the other through the connect bundles of nerve fibres such as the corpus callosum.
Sperry (1968) – Split Brain Research with Split Brain Patients
Split brain patients – individuals who have had their connecting corpus callosum cut (usually patients with severe epilepsy in an attempt to prevent seizures passing from one area of the brain to the other).
Aim – to test the capabilities of the separated hemispheres.
• Patients were asked to fixate on a dot in the centre of a screen whilst information was presented to wither the left or right visual field.
• Patients were asked to make responses with either their left hand (controlled by the right hemisphere), their right hand (controlled by their left hemisphere) or verbally (controlled by the left hemisphere) without being able to see what their hands were doing.
• For example, if the patient was flashed a picture of a dog to the right visual field and asked what they had seen, they would answer ‘dog.’ If the patient was flashed a picture in the left visual field they would answer that they saw nothing. The reason for this is the information in the left visual field is processed by the right hemisphere, which can see the picture but does not have a language centre – cannot respond verbally.
• The left hemisphere (which does have a language centre) does not receive information about seeing the picture, therefore cannot say that it has seen it.
Conclusion: Due to the corpus callosum being cut in split-brain patients, the information presented to one hemisphere has no way of travelling to the other hemisphere and can only be processed in the hemisphere that received it.
AO3: Lateralisation and Split Brain Research Evaluation
(1) Point: A strength of split brain research is that it has led to the understanding the brain lateralisation can increase neural processing capacity. Example/Evidence: For example, by using only one hemisphere to engage in a particular task (e.g. language, mathematical ability), this would leave the other hemisphere free to engage in another function. Elaboration: This is a strength because, such research does provide evidence that brain lateralisation enhances brain efficiency in cognitive tasks that demand the simultaneous but different use of both hemispheres. This therefore highlights that split brain research has been useful in helping individuals to understand more clearly the role of the brain/different hemispheres of the brain.
(1) Point: However, there are a number of problems with this research:
Additional research has suggested that language may not be restricted to the left hemisphere. Evidence/Example: For example, Gazzinga (1998) suggests that some of the early discoveries from split-brain research have been disconfirmed by more recent discoveries. The case study of JW demonstrated that the right hemisphere may play some part in language like the left hemisphere as he developed the capacity to speak out of the right hemisphere (Turk et al 2002). Elaboration: This is a weakness because, it suggests that there are inconsistent findings across split brain research making it difficult to from firm conclusions.
(2) Point: In addition, there are limitations in carrying our split-brain research due to the fact that in the current day and age split-brain patients are very rare.
Example/Evidence: For example, Andrews (2001) argues that many studies are presented with as few as three participants, some studies have only a single participant making up the sample. Elaboration: This is a weakness because, Andrews argues that those patients seemingly making split-brain processing possible are based on small samples that are not representative of the wider population, therefore such research lacks population validity. In addition, Andrews states that such findings are very rarely replicated/consistency isn’t always obtained.
AO1: Plasticity of the Brain
Brain plasticity refers to the brain’s ability to change and adapt as a result of experience. This ability to change plays an important role in brain development and behaviour. Researchers used to believe that changes to the brain took place only in infancy and childhood, but more recent research has demonstrated that the brain continues to create new neural pathways and alter existing ones to adapt to new experiences as a result of learning. The brain also appears to show evidence of functional recovery, moving functions from a damaged area of the brain (after trauma) to an undamaged area.
Increased Brain Stimulation
As neurons are damaged there is an effect on the neighbouring neurons as they no longer have input. This happens with the hemispheres too. Although damage may only be on one side, the other hemisphere functions at a lower level too, as it has reduced input. Takatsura et al (2009) demonstrated that if the undamaged hemisphere is stimulated, recovery from a stroke can be improved.
• As people gain new experiences, nerve pathways that are used frequently develop stronger connections, whereas those used rarely eventually die.
• By developing new connections and pruning away old weaker ones the brain is able to adapt to the changing environment.
• There is a natural decline in cognitive functioning with age which can be attributed to the changing brain – researchers are therefore looking for ways in which new connections can be made in order to reverse this effect.
This occurs when axons that do a similar job become aroused to a higher level to compensate for the ones that are lost. However, it can have the unfortunate consequence of over-sensitivity to messages such as pain. This increases the pain levels in individuals.
When an axon is damaged its connection with neighbouring neurons is lost. In some cases, other axons that already connect with that neuron will sprout extra connections to the neuron, replacing the ones that have been destroyed. It is compensating for the loss of a neighbour. This occurs for the most part two weeks after the damage has happened. It helps replace function, but only if the damaged axon and the compensatory axons do a similar job. If not, problems can occur with function.
AO3: Plasticity of the Brain Evaluation
(1) Point: There is research from animal studies to support the idea that the brain has the ability to change as a result of experience. Example/Evidence: For example, Kempermann et al (1998) investigated whether an enriched environment could alter the number of neurons in the brain. They found evidence of an increased number of new neurons in the brains of rats housed in complex environments in comparison to rats housed in lab cages. Elaboration: This is a strength because, in particular, the rats housed in complex environments showed an increase in neurons in the hippocampus (a part of the brain associated with the formation of new memories) which supports the idea that as individuals gain new experiences, nerve pathways that are used frequently develop stronger connections, whereas those used rarely eventually die.
(1) Point: However, this research can be criticised for extrapolation.
Example/Evidence: For example, Kempermann’s research used a sample of rats. Rats and humans are physiologically different and it can be assumed that the brain functioning of rats is possibly very different to the way in which a human brain functions. Elaboration: This is a weakness because, findings from research conducted on non-human animals cannot be generalised to humans, thus making it difficult to draw firm conclusions about human brain functioning.
Counter acting strength:
(2) Point: However, there has been research conducted on humans which has supported the idea that the brain has the ability to change as a result of experience/exposure to enriched environments. Evidence/Example: For example, Maguire et al (2000) studied London taxi drivers to discover whether changes in the brain could be detected as a result of their extensive experience of spatial navigation. Using MRI scans, the researchers calculated the amount of grey matter in the brains of the taxi drivers and a set of control participants. The hippocampus of the taxi drivers was significantly larger in comparison to the control group. Furthermore, there was a positive correlation between the length of time they had been a taxi driver and the volume of their brain.
Elaboration: This is a strength because, this research supports the idea that the more an area of the brain is used, the stronger that area of the brain becomes (i.e. nerve pathways that are used frequently develop stronger connections, whereas those used rarely eventually die).
AO1: Functional Recovery of the Brain after Trauma
Functional Recovery – Much recovery after trauma is due to anatomical compensation brought about by intensive rehabilitation. The brain learns to compensate for the function. The brain can be taught to learn how to use the working faculties and function to compensate for the ones that are lost forever. In the 1960s, researchers studied cases in which stroke victims were able to regain functioning. They discovered that when brain cells are damaged or destroyed, as they are during a stroke, the brain re-wires itself over time so that some level of function can be regained. Although parts of the brain may be damaged or even destroyed as a result of trauma, other parts appear able to take over the functions that were lost. Neurons next to damaged brain areas can form new circuits that resume some of the lost function.
Regenerative developments in brain function arise from them brain’s plasticity, its ability to change structurally and functionally following trauma. Two ways in which the brain is able to do this is:
(1) Neuronal Unmasking:
Wall (1977) first identified what he called ‘dormant synapses’ in the brain. These are synaptic connections that exist anatomically but their function is blocked.
Under normal circumstances these synapse may be ineffective because the rate of neural input to them is too low for them to be activated. However, increasing the rate of input to these synapses, as would happened when a surrounding brain area becomes damaged, can then open (or ‘unmask’) these dormant synapses. The unmasking of dormant synapses can open connections to regions of the brain that are not normally activated, creating a lateral spread of activation which, in time, gives way to the development of new structures.
(2) Stem Cells
Stem cells are unspecified cells that have the potential to give rise to different cell types that carry out different functions, including taking on the characteristics of nerve cells.
There are a number of views on how stem cells might work to provide treatments for brain damage caused by injury or neurodegenerative disorders.
The first view is that stem cells implanted into the brain would directly replace dead or dying cells.
A second possibility is that the transplanted stem cells secrete growth factors that somehow ‘rescue’ the injured cells.
A third possibility is that the transplanted cells form a neural network, which links an uninjured brain site, where new stem cells are made, with the damaged region of the brain.
AO3: Functional Recovery of the Brain after Trauma Evaluation
(1) Point: There is research from animal studies to support the idea that the brain has the ability to recover after trauma. Example/Evidence: For example, Tajiri et al (2013) provided evidence for the role of stem cells in recovery from brain injury. They randomly assigned rats with traumatic brain injury to one of two groups. One group received transplants of stem cells into the region of the brain effected by the traumatic injury. The control group received a solution infused into the brain containing no stem cells. Three months later the brains of the stem cell rats showed clear development of neuron-like cells migrating into the brain’s site of injury. This was not the case with the control group. Elaboration: This is a strength because, in particular, this shows that with the help of stem cells, the brain is able to undergo functional recovery after injury.
(2) Point: There is research to suggested that functional recovery of the brain is affected by age. Example/Evidence: For example, Huttenlocher (2002) suggested that the only option following traumatic brain injury after childhood is to develop compensatory behavioural strategies to work around the deficit that older ages poses (such as seeking social support or developing strategies to deal with cognitive deficits). Elaboration: This is a strength because, Huttenlocher’s research confirms scientific and objective findings from research suggesting that with age, the brain’s ability the functionally recovery declines.
(1) Point: However, this research can be criticised for extrapolation. Evidence/Example: For example, Tajiri et al (2013) research used a sample of rats. Rats and humans are physiologically different and it can be assumed that the effects of stem cells on the brains of rats may not be the same as the effects of stem cells on the brains of humans. Elaboration: This is a weakness because, it means that research based on non-human animals cannot be generalised to humans and therefore it is difficult to draw firm conclusions.