hemispheric lateralisation/split brain research

• • hemispheric lateralisation: two halves of the brain are functionally different and certain mental processes and behaviours are controlled by one hemisphere in particular
  • the right hemisphere controls movement on the left side of the body and the left hemisphere controls movement on the right side of the body: this is called contralateral control
  • in the motor area the brain is cross wired
  • language is lateralised in the left hemisphere (Broca's and Wernicke's area)
  • the LH is the analyser and the RH is the synthesiser
• • in case of vision, it is both contralateral and ipsilateral control (opposite and same sided)
  • each eye receives information from the left visual field and the right visual field
  • the LVF of both eyes is connected to the right hemisphere
  • the RVF of both eyes is connected to the left hemisphere
  • this adds depth perception and allows us to compare different perspectives
• • during an epileptic fit the brain experiences an electrical storm which travels from one hemisphere to another
  • to prevent this the corpus collosum, which connects the hemispheres, is severed, 'splitting' the brain in two halves
  • sperry did split-brain research on 11 split-brain patients
  • a picture was flashed for 1/10th of a second to the left or right visual field
  • if it was flashed to the LVF it was processed by the right hem and drawn by the left hand
  • presenting image to one hem meant the information could not be shared across hemispheres
• • when a picture was shown to the RVF (processed by left hem) the pps could describe what was seen
  • but if the picture was shown to the LVF (right hem) they could not describe anything as there was 'nothing there'
  • in non-split brain messages from RH are relayed to language centres in LH but this is not possible in split-brain
  • pps could not give verbal labels to objects projected to the LVF but they could select/draw a matching object using their left hand
  • suggests that right hemisphere has no verbal ability
• • both hemispheres have similar capabilities other than verbal communication
  • the RH can communicate using drawings but cannot verbally describe what it saw
  • shows that the RH can give an emotional response and has its own set of schemas
  • RH is better at spatial tasks like drawing and facial recognition
  • supports that left hemisphere is where language is lateralised
• • image was shown on fixation point for 1/10th second whilst one eye was blindfolded so pps could not move eye across the screen
  • if longer than that then both RVF and LVF would process the stimulus as visual information would cross over to the other hemisphere via the optic chaism so would no longer be testing the hemispheres in isolation
  • increases the validity as it is precise, empirically scientific methodology and standardised procedure means experiment can be repeated so high in internal reliability
• • Sperry has been criticised for being low in external validity
  • Split-brain patients have said in real life there is virtually no difference because they can use both eyes and wont be seeing things for 1/10th of a second
  • Study only shows artificial stimulus which lowers the validity as they can't be applied in a real-life setting
  • Raises the question of whether there's any point in the research if split-brain patients have no difference in every day life
  • decreases validity
• • the research supports lateralisation as it shows that only the left hemisphere has the ability to verbalise words
  • supports localisation of function as we know that language is lateralised in the left hemisphere
• • underestimates the ability of the right hemisphere
  • suggests that RH doesn't have any ability to communicate as LH would always say it was unsure why the left hand drew something
  • but it shows that the RH can still communicate through drawings
  • later research has shown that RH has emotional response, individual schemas, separate memory and is better at spatial tasks such as facial recognition
  • RH is very capable and sperry's research underestimates its abilities when it has the same and is better at certain things
  • lateralisation was overemphasised as it shows that both are cognitively advanced
• • issue of generalising the findings
  • all the pps suffered with epilepsy so the validity of the sample is questionable as only neurodivergent pps were used
  • to increase validity should have compared to neurodivergent control group with intact corpus collosum
  • means that we cannot be certain that the findings of lateralisation are valid as epilepsy may have caused functional changes in the brain so pps cannot be sure differences in functionality are 100% due to severed corpus collosum
  • however unlikely still decreases the validity