Hemisphere lateralisation

• Basic introduction of hemisphere lateralisation?

  The two halves of the brain are completely separate in terms of function. Certain mental processes and behaviours are controlled by one hemisphere.
• Split brain patients?

  • The two hemispheres are joined together by tissue called the corpus callosum.
  • This tissue is the way that the hemispheres communicate with one another, they are completely separate.
  • Performing this operation was beneficial for people of epilepsy as it would reduce epileptic fits.
  • This gave researchers an opportunity to research them. 
  • Split-brain patients provided researchers with an opportunity to study lateralisation of the brain. 
• Processing in the brain?

  • In a normal brain, both halves would communicate with each other, left with right and vice versa .
  • Information then passes across the brain via the corpus callosum which shares information in the whole brain. 
  • In cases of split-brain patients, they do not have the corpus callosum so information can only be processed by one side of the brain.
  • Despite this, split-brain patients are able to lead normal lives and undertake day to day activities. 
  • For example, the left hemisphere is for language and the right is for visual processing. 
• Sperry study procedure?

  • 11 split brain patients had their corpus callosum cut out because of advanced epilepsy not controlled by medication
  • He used a quasi experiment in a lab comparing the 11 people to people without split brains
  • Images or words were presented to the left or right visual field so tasks were set separately to either visual field. 
  • The participant had one eye covered and images flashed at 1/10th of a second on a projection screen. This was to prevent the participant moving their eye across their picture. 
  • Below the screen was a gap so that the p’s could reach the objects but could not see their hands
  • P’s were flashed images on the screen and were asked to respond verbally or non-verbally (drawing the object or selecting it from under the screen)
• Results - Describing what you see (Verbal)

  •  When a picture was shown in the right visual field, they could describe it but if shown in the left, they could not and would say that they reported nothing was there 
  • This is due to the fact that language is processed in the left hemisphere and objects in the left visual field could not be described due to a lack of language centres in the right hemisphere. 
  • In a normal brain, the messages would have been relayed to the language centres in the left hemisphere
• Recognition by touch (non-verbal)

  • Objects were placed behind a screen so they could not be seen
  • An image was presented to the left visual field
  • P’s could select the matching object by touch using their left hand. 
  • They could not verbally say what was seen due to the right hemisphere not having any language centres. However, they could understand the object by picking up the corresponding object from the image on the screen they had seen.
• Conclusion of Sperry?

  • The study supports the argument of lateralisation of function.
  • Different hemispheres specialise in different tasks such as the left hemisphere being responsible for language.
  • Further trials of Sperry’s study found that the right hemisphere of the brain is dominant in having drawing abilities and facial recognition. 
• Positive evaluation for Sperry's study being controlled

  • A strength is that the experiments made use of highly specialised and standardised procedures controlling for extraneous variables.
  • For example, the p’s had one eye covered and were instructed to look at a fixed point in the centre of a projection screen.
  • The pictures were also shown at a high speed of 1/10th of a second and this prevented moving their other eye across the picture to process the information of the other side of the visual field and therefore both hemispheres of the brain.
  • This procedure allowed Sperry to infer that one hemisphere was receiving information.
  • Therefore, he developed a well controlled procedure meaning we can infer cause and effect. 
• Problem with Sperry's study with more appropriate control group?

  • However, a problem with Sperry’s study is that the control group used by Sperry was made up of people with no history of epilepsy.
  • It would have been more appropriate to use a control group of people with epilepsy but with no split brain surgery done on them, meaning that the difference would only have been whether the group had split brain surgery or not.
  • This would have stopped the history of epilepsy from becoming a confounding variable.
  • Therefore, there could be problems with making conclusions based on Sperry’s research, it is unclear whether it is the epilepsy or the split brain causing the results. 
• Criticism of Sperry's study being an unrepresentative sample?

  • Another possible criticism is that it was done with an unrepresentative sample, it was a small sample of split brain participants used in all variations of the basic procedure.
  • All of them also had epilepsy which may have caused unique changes in the brain influencing the study; hemisphere lateralisation of epileptics may not represent that of normal people.
  • Many researchers have urged caution on the widespread acceptance of the results as there is issue with generalisation to the wider population of non-epileptics.
• Weakness of the differences in function being pop psychology in Sperry's study?

  • A weakness of the theory is that the differences in function between the hemispheres may be overstated.
  • Unfortunately, one legacy of Sperry’s study is the growing body of pop-psychology which overemphasises and oversimplifies the functional distinction.
  • Although these labels of the right hemisphere being creative and the left hemisphere being verbal, neuro-scientists argue that the distinction is less clear-cut than this. In the normal brain, the two hemispheres are in constant communication when performing everyday tasks
  • Many of the behaviours typically associated with one hemisphere can be effectively performed by the other when the situation requires it. 
• Gazzaniga's criticism study?

  • A final issue is that the language may not be restricted to the left hemisphere; Gazzaniga suggests that some of the early discoveries from split brain research have been disproved by more recent studies.
  • For example, the split brain research suggested that the right hemisphere is unable to handle even the most rudimentary language.
  • However, case studies show that this is not the case as seen in JW who developed the ability to speak out of the right hemisphere and can now speak about information presented to the left or right brain.
  • Therefore language may not be lateralised in the left hemisphere.