6. split brain research (hemispheric lateralisation)

what is hemispheric lateralisation?

when specialised areas are only found in one hemisphere rather than both e.g. language in left hemisphere

what is corpus callosum?

large white tract that connects the two hemispheres of the brain (tissue that connects the 2 brain hemispheres)

what does commissurotomy involve and why do people have this surgery?

the corpus callosum and other tissues which connect the 2 hemispheres are cut down the middle.

this was done as a treatment for people who had frequent and severe epileptic seizures.

the aim of Sperry’s split brain research?

to see the extent to which the 2 hemispheres were specialised for certain functions and whether the hemispheres performed tasks independently of one another

what are the key features of Sperry’s sample used in split-brain research?

the sample was a unique group of individuals who had all undergone the commissurotomy operation

Sperry’s procedure

• used visual and tactile tasks to test hemispheric lateraliation

• used a piece of equipment called a T-scope- this allowed each hemisphere to be tested in isolation of the other

• ps were asked to focus on the ‘fixation point’ then an image/word was projected very quickly (1/10th of a second)to one or both visual fields.

• to test for non-verbal processing ps were asked to pick up or match objects which were out of sight

• in a normal brain the corpus callosum would immediately share info between both hemispheres giving a full picture of the visual world. However, presenting the image to only one visual field here means info can’t be conveyed across hemispheres

Sperry’s 4 key findings

1. when a picture/word was projected to right VF (processed by left hemisphere) the patient could easily describe what they had been shown. However, when projected to left VF (processed by right hemisphere) patient could not describe what was shown. SUPPORTS hemispheric lateralisation.

2. patients could use their left hand to point to a matching object or picture which had only been shown to their left VF- right hemisphere has processed the info but can’t descibe what was shown.

3. if 2 words/pictures were projected simultaneously one on either side of VF (e.g. dollar sign on left and question mark on right) patients would say they saw a question mark but when patients were asked to draw what they saw with their left hand they would draw a dollar sign. Patients would not have been aware that they drew and said separate things. SUPPORTS- 2 hemispheres working separtely from one another

4. an object placed in patient’s right hand (could not see, only feel) could be easily described or named in speech or writing, whereas could not if same object was placed in left. However, when this object was placed in a bag along with other objects the patients could search for and recieve the object with their left hand- SUPPORTS- left dominant for speech and writing, right can comprehend what it is but can’t verablly identify it

Conclusions

1. Brain is lateralised (each hemisphere responsible for different functions on body

2. Cortices are contralateral= each hemisphere process/controls the opposite side of the body

3. In most people language centres are in left hemisphere

4. Without the corpus callosum left and right hemispheres cannot communicate

AO3 methodology pros

p- experimental and standardised research

e- used specialised equipment (T-scopes) that can objectively measure the lateralisation in each hemisphere. an image/word can be projected for 1/10th of a second to one or both visual fields (too quick that they could look with other visual field by turning head)-. increases internal validity

e- standardised- ps given same task and used standardised equipment (T-scope). research can be repeated and checked for reliability

AO3 methodology cons

p- issues with methodology

e- small sample size (11 people), generalisation concerns, split brain procedure rarely carried out now so have to use small samples. -

e- Sperry’s control group had no history of epileptic seizures, therefore this is not an appropriate comparison, perhaps the differences are due to epilepsy rather than commissurotomy. A better comparison would have been to use patients with a medical history of epilepsy but who did not have a commissutotomy.

AO3- contradictory research

P- Danelli provides evidence that brain can adapt

E- investigates 17yr old boy who had entire left hemisphere removed at 2yrs old. By age 17 language abilities almost normal- right hemisphere has learnt language abilities

E- language centres Broca’s and Wernicke’s suggested to be lateralised in left hemisphere BUT this suggests that lateralisation may not be fixed and brain can adapt after damage

L- perhaps lateralisation is not as strong as the theory and this research suggests