Specification: Hemispheric lateralisation: split brain research.
Hemispheric lateralisation
Lateralisation is the idea that the two halves of the brain are functionally different and that each hemisphere has functional specialisations, e.g. the left is dominant for language, and the right excels at visual motor tasks. The two hemispheres are connected through nerve fibres called the corpus callosum, which facilitate interhemispheric communication: allowing the left and right hemispheres to ‘talk to’ one another.
Split-brain research
Sperry and Gazzaniga (1967) were the first to investigate hemispheric lateralisation with the use of split-brain patients.
Background: Split-brain patients are individuals who have undergone a surgical procedure where the corpus callosum, which connects the two hemispheres, is cut. This procedure, which separates the two hemispheres, was used as a treatment for severe epilepsy.
Aim: The aim of their research was to examine the extent to which the two hemispheres are specialised for certain functions.
Method: An image/word is projected to the patient’s left visual field (which is processed by the right hemisphere) or the right visual field (which is processed by the left hemisphere). When information is presented to one hemisphere in a split-brain patient, the information is not transferred to the other hemisphere (as the corpus callosum is cut).
Sperry and Gazzaniga conducted many different experiments, including describe what you see tasks, tactile tests, and drawing tasks.
In the describe what you see task, a picture was presented to either the left or right visual field and the participant had to simply describe what they saw.
In the tactile test, an object was placed in the patient’s left or right hand and they had to either describe what they felt, or select a similar object from a series of alternate objects.
Finally, in the drawing task, participants were presented with a picture in either their left or right visual field, and they had to simply draw what they saw.
Findings:
Conclusion: The findings of Sperry and Gazzaniga’s research highlights a number of key differences between the two hemispheres. Firstly, the left hemisphere is dominant in terms of speech and language. Secondly, the right hemisphere is dominant in terms of visual-motor tasks.
Evaluation of split-brain research
It is assumed that the main advantage of brain lateralisation is that it increases neural processing capacity (the ability to perform multiple tasks simultaneously). Rogers et al. (2004) found that in a domestic chicken, brain lateralisation is associated with an enhanced ability to perform two tasks simultaneously (finding food and being vigilant for predators). Using only one hemisphere to engage in a task leaves the other hemisphere free to engage in other functions. This provides evidence for the advantages of brain lateralisation and demonstrates how it can enhance brain efficiency in cognitive tasks. However, because this research was carried out on animals, it is impossible to conclude the same of humans. Unfortunately, much of the research into lateralisation is flawed because the split-brain procedure is rarely carried out now, meaning patients are difficult to come by. Such studies often include very few participants, and often the research takes an idiographic approach. Therefore, any conclusions drawn are representative only of those individuals who had a confounding physical disorder that made the procedure necessary. This is problematic as such results cannot be generalised to the wider population.
Furthermore, research has suggested that lateralisation changes with age. Szaflarki et al. (2006) found that language became more lateralised to the left hemisphere with increasing age in children and adolescents, but after the age of 25, lateralisation decreased with each decade of life. This raises questions about lateralisation, such as whether everyone has one hemisphere that is dominant over the other and whether this dominance changes with age.
Finally, it could be argued that language may not be restricted to the left hemisphere. Turk et al. (2002) discovered a patient who suffered damage to the left hemisphere but developed the capacity to speak in the right hemisphere, eventually leading to the ability to speak about the information presented to either side of the brain. This suggests that perhaps lateralisation is not fixed and that the brain can adapt following damage to certain areas.
Possible exam questions
Briefly explain how split-brain patients could be examined in an experiment. (4 marks)
Briefly explain what split-brain research has shown. (4 marks)
Evaluate research using split-brain patients to investigate hemispheric lateralisation of function. (4 marks)
Joseph suffered a stroke when he was 45 years old. He could move his left arm and leg but was paralysed down his right side. While Joseph could understand what was said to him, he was unable to speak. Referring to Joseph, discuss hemispheric lateralisation of language centres in the brain. (16 marks)
Describe and evaluate research into lateralisation and/or the split brain. (16 marks)
