hemispheric lateralisation and split-brain research Flashcards
hemispheric lateralisation - left and right hemispheres
- for language, the two main centres are only in the LH, so we can say that language is lateralised (performed by one hemisphere rather than the other)
- the RH can only produce rudimentary words and phrases but contributes emotional context to what is being said (suggestion that LH is analyser but RH is synthesiser)
- vision, motor and somatosensory areas appear in both hemispheres, but in the case of the motor area, the brain is cross-wired (contralateral wiring), the RH controls movement on the left side of the body and vice versa
- vision is both contralateral and ipsilateral (opposite and same-sided)
- each eye receives light from the left visual field (LVF) and the right visual field (RVF)
- the LVF of both eyes is connected to the RH and the RVF is connected to the LH
- enables visual areas to compare slightly different perspectives from each eye and aids depth perception
- similar arrangement for auditory inout to auditory area and the disparity from two inputs helps us locate the source of sounds
split-brain research
- split-brain operation involves severing connections between RH and LH (mainly corpus callosum) in order to reduce epilepsy
- during a seizure, the brain experiences excessive electrical activity which travels from one hemisphere to the other
- these connections are cut, splitting the brain in two halves
- split-brain research studies how the hemispheres function when they can’t communicate with each other
split-brain research - Sperry’s research
- found a system to study how two separated hemispheres deal with things like speech and vision
procedure -
- 11 people who had a split-brain operation studied using a set up where an image could be projected to their RVF (processed by LH) and the same or different image was projected to the LVF (processed by RH)
- in a ‘normal’ brain, corpus callosum would immediately share info between both hemispheres, giving a complete picture of the visual world
- however, presenting the image to one hemisphere of a split-brain participant meant that the info cannot be conveyed from that hemisphere to the other
findings -
- when the picture was shown to the RVF, the participant could describe what was seen
- they couldn’t do this if the image was shown to the LVF (they said that there was nothing there)
- this is because, usually, messages from RH are relayed to language centres in the LH, but this is not possible in the split-brain
- although participants couldn’t give verbal labels to objects projected to LVF, they could select a matching object out of sight using their left hand (linked to RH)
- left hand could also select an object that was most closely associated with an object presented to LVF
- would also be able to draw what they saw in the LVF by using their left hand
- if a pinup picture was shown to the LVF there was an emotional reaction (eg. a giggle) but participants usually reported seeing nothing or just a flash of light
conclusions -
- shows how certain functions are lateralised in the brain
- supports view that LH is verbal and RH is silent but emotional
evaluation strength of hemispheric lateralisation - lateralisation in the connected brain
- research shows that, even in connected brains, the two hemispheres process information differently
- Gereon Fink used PET scans to identify which brain areas were active during visual processing task
- when participants with connected brains were asked to attend to global elements of an image, regions of the RH were much more active, and when asked to focus in on finer detail, specific areas of LH tended to dominate
- suggests that, at least as far as visual processing is concerned, hemispheric lateralisation is a feature of both connected brains and split-brains
evaluation limitation of hemispheric lateralisation - one brain
- the idea of LH as analyser and RH as synthesiser may be wrong
- research suggests that people do not have a dominant side of their brain which creates a different personality
- Jared Nielsen analysed over 1000 brain scans of people aged 7-29
- found that people did use certain hemispheres for certain tasks (lateralisation), but there was no evidence of a dominant side
- suggests that the notion of left or right brained people is wrong
evaluation strength of split-brain research - research support
- Michael Gazzaniga showed that split-brain participants perform better than connected controls on certain
- they were faster at identifying the odd one out in an array of similar objects
- in a normal brain, the LH’s better cognitive strategies are ‘watered down’ by the inferior RH
- supports Sperry’s findings that the left brain and white brain are distinct
evaluation limitation of split-brain research - generalisation issues
- hard to establish causal relationships from Sperry’s research
- behaviour of split-brain participants was compared to a neurotypical control group, however none of these control participants had epilepsy
- any differences that were observed between the two groups may have been a result of the epilepsy rather than the split-brain
- some unique features of the split-brain participants’ cognitive abilities might have been due to their epilepsy
evaluation strength of split-brain research - nobel prize
- sperry won a nobel prize for his research in 1981
- this proves that this is a highly valid and reliable method that is still true today
