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Hemispheric lateralisation and split-brain research - Coggle Diagram
Hemispheric lateralisation and split-brain research
Hemispheric lateralisation
localisation & Lateralisation
localisation refers to fact that some functions, such as vision & language, are governed by very specific areas in brain.
the brain is lateralised, i.e. there are two sides which are called hemispheres
for some functions, localised areas appear in both hemispheres, e.g. vision - visual area is in left & right occipital lobe - located in left & right hemispheres.
hemispheric lateralisation
: idea that two hemispheres of the brain are
functionally different
& that certain mental processes & behaviours are mainly controlled by one hemisphere rather than the other, as in the example of language (which is localised as well as lateralised).
Left and Right Hemispheres
in cases of language, two main centres are only in LH (for most people) - Broca's area in Left frontal lobe & Wernicke's area in left temporal lobe.
as result we say language is
lateralised
- i.e. performed by one hemisphere rather than the other.
RH can only produce rudimentary words & phrase but contributes emotional context to what's being said.
has led to suggestion that LH is the
analyser
whilst RH
synthesiser
many functions aren't lateralised e.g. vision, motor & somatosensory areas appear in both hemispheres.
but there's a further twist, in case of motor area the brain is
Cross-wired (contralateral wiring)
- the RH controls movement on left side of body whilst LH controls movement on right side.
in case of vision, situation is even more complex - it's both contralateral and
ipsilateral (opposite and same sided)
.
each eye receives light from Left Visual Field (LVF) and Right visual field (RVF). LVF is connected to RH and RVF of both eyes is connected to LH - enables visual areas to compare the slightly different perspective from each eye & aids depth perception.
there-s similar arrangement for auditory input to the auditory area and disparity from the two inputs helps us locate the source of sounds.
Evaluation of Lateralisation Theory
P: there is research showing that the 2 hemispheres process info differently
E: Gereon Fink et al. (1996) used PET scans to identify what areas of brain were active during visual processing task. when ppts w/ connected brains were asked to pay attention to global elements of image (e.g. picture of whole forest) regions of RH were much more active. when asked focus on finer details, specific areas of LH tended to dominate.
E: strength -suggests, as far as visual processing's concerned, hemispheric lateralisation is feature of connected brain as well as split-brain.
P: there may be different functions in each hemisphere but research suggests that one hemisphere is not dominant over the other to create different personalities.
E: Jared Nielsen et al. (2013) analysed brain scans from over 1,000 people aged 7- 29 years & did find that people used certain hemispheres for certain tasks (evidence of lateralisation). but there was no evidence of a dominant side, i.e. not artist's brain/mathematician's brain.
E: limitation - suggests notion of right - or left-brained people is wrong.
Split-brain research
split-brain operation involves severing connections between RH & LH, mainly the
corpus callosum
.
this is a surgical procedure to reduce epilepsy. during epileptic seizure the brain experiences excessive electrical activity which travels from one hemisphere to the other. to reduce fits, these connections are cut 'splitting' brain into 2 halves.
split-brain research
studies how hemispheres function when they can't communicate with each other.
Sperry's research
roger sperry 1968 devised system to study how 2 separate hemispheres deal with, e.g., speech & vision.
procedure
11 people who had split-brain operation were studied using special set up where image could be projected to ppts RVF (processed by LH) and the same/different image could be projected to LVF (processed by RH).
in 'normal' brain, corpus callosum would immediately share info between both hemispheres giving a complete picture of visual world. but presenting image to one hemisphere of split brain ppt meant info couldn't be conveyed from that hemisphere to the other.
findings
when picture of object shown to ppt RVF ppts could describe what was seen but couldn't do this if object was shown to LVF - they said nothing was there.
this is because, in connected brain, messages from RH are relayed to language centres in LH but this isn't possible in split-brain.
although ppts couldn't give verbal labels to objects projected to LVF, they could select matching object out of sight using their left hand (linked to RH). Left hand was also able to select an object that was most closely associated with object presented to the LVF.
if pinup picture was shown to the LVF was an emotional reaction (e.g. giggle) but the ppts usually reported seeing nothing or just flash of light.
conclusions
these observations show how certain functions are lateralised in the brain & support the view that LH is verbal and RH is 'silent' but emotional.
Evaluation of Split-Brain Research
P: there is support from more recent split-brain research
E: Michael Gazzaniga (Luck et al.1989) showed split-brain ppts actually perform better than connected controls on certain tasks. e.g. they were faster at identifying the odd one out in an array of similar objects than normal controls. in normal brain. the LH's better cognitive strategies are 'watered down' by the inferior RH (Kingston et al 1995)
E: strength - supports Sperry's earlier findings that the 'left brain' and 'right brain' are distinct.
P: causal relationships are hard to establish in Sperry's research
E: behaviour of Sperry's ppts were compared to a
neurotypical control group
. an issue though is that none of the ppts in the control group had epilepsy. this is major
confounding variable
. any difference which were observed between the 2 groups may be result of epilepsy rather than split brain.
E: limitation - means some of unique feature of split brain ppts cog abilities might have been due to their epilepsy.