Hemispheric lateralisation and split brain research Flashcards
Localisation and Lateralisation
Localisation: that some functions (such as vision and language) are governed by very specific areas of the brain
Brain is lateralized: there are 2 sides - the hemispheres ( hemispheric lateralisation)
For some functions - localised areas appear in both hemispheres
E.g. vision - the visual area is in the left and right occipital lobe (located in the left hemisphere LH and right hemisphere.
lateralized language
For language - there are 2 main centres only in the LH (most people)
Broca’s area - left frontal lobe
Wernicke’s area - left temporal lobe
So language is lateralized - performed by 1 hemisphere rather than the other
RH only produce rudimentary words + phrases but contributes emotional context to what is being said
Suggests LH is the analyser and the RH is the synthesiser
non lateralized
Vision, motor and somatosensory areas in both hemispheres are not lateralized
Motor Area: is cross-wired (contralateral wiring) - RH controls the movement of the left side. The LH controls the movement of the right
non lateralised vision
vision is contralateral and ipsilateral (opposite and same sided)
Each eye receives light from the left visual field LVF and the right visual field RVF
LVF of both eyes is connected to the RH and the RVF of both eyes is connected to the LH
This enables the visual areas to compare the slightly different perspective from each eye and aids perception
There is a similar arrangement for auditory input to the auditory area and the disparity from the two inputs helps us locate the source of sounds
split brain operation
‘Split brain’ operation involves severing connections between RH and LH - mainly the corpus callosum. This is a surgical procedure used to reduce epilepsy.
During an epileptic seizure, the brain experiences excessive electrical activity which travels from one hemisphere to the other. To reduce the fits the connections are cut - ‘splitting’ the brain into 2 halves.
Split brain research studies how hemispheres function when they can’t communicate with each other
Sperry’s procedure
11 people who had split brain operation studies using special set up
An image projected to a participants RVF (processed by the LH) and the same - image projected into LVF (processed by the RH)
In the ‘normal’ brain the corpus callosum would immediately share the information between both hemispheres giving a complete picture of the visual world
Presenting the image to one hemisphere of a split hemisphere of a split brain participants - the information can’t be conveyed from that hemisphere to the other
sperry’s findings
When a picture of an object was shown to a participant’s RVF (linked to LH) - the participant could describe what was seen
They could not do this if the object was shown to the LVF (RH) - they said there was nothing there
Because - in the connected brain - messages from the RH are relayed to the language centres in the LH - this is not possible in the split brain
Although participants could not give verbal labels to objects projected to the LVF - they could select a 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 an object presented to the LVF -
If a pinup picture was shown to the LVF there was an emotional reaction (giggle) but the participants usually reported seeing nothing or just a flash of light
conclusions
Observations show how certain functions are lateralized in the brain and support the view that the LH is verbal and the RH is silent but emotional
Evaluation (Hemispheric Lateralisation) Lateralisation in the connected brain - Strength
Strength : research show even in connected brains, the 2 hemispheres process information differently
e.g. Fink used PET scans to identify which brain areas were active during visual processing tasks
when pp’s with connected brains were asked to attend to global elements of an image (e.g. looking at a pic of a whole forest) regions of the RH were more active
When required to focus in on the finer detail (individual trees) the specific areas of the LH tended to dominate
Suggests that at least as far as visual processing is concerned, hemispheric lateralisation is a feature of the connected brain as well as the split brain
HL - One brain LIMITATION
L- idea that LH is the analyser and RH - synthesiser may be wrong
May be different functions in the RH and the LH - but research suggests people don’t have a dominant side of their brain which creates a different personality
Nielsen analysed brain scans from over 1000 people aged 7-29 and found people used certain hemispheres for certain tasks (shows lateralisation)
But there is no evidence of a dominant side
suggests that the notion of right/ left brained people is wrong
extra - Lateralisation versus plasticity
Lateralisation is adapative - it enables 2 tasks to be performed at the same time with greater efficiency
Rogers showed that lateralised chickens could find food while watching for predators but ‘normal’ chickens could not
BUt neural plasticity could also be seen as adaptive. Following damage through illness or trauma, some functions can be taken over by non specialised areas in the opposite hemisphere
e.g. language function can literally switch sides (Holland)
SBR Research support - Strength
Strength - support from more recent split brain research
Gazzaniga showed that split brains 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 the normal brain, the LH’s better cognitive strategies are ‘watered down’ by inferior RH (Kingstone)
SBR Generalisation issues LIMITATION
Limitation of Sperry’s research is that casual relationships are hard to establish
Behaviour of sperry’s split brain pp’s was compared to neurotypical control group
issue is none of the participants in the control group had epilepsy - major confounding variable
any differences observed between the 2 groups may be the result of epilepsy rather than the split brain
means that some of the unique features of the split-brain participants cog abilities may have been due to their epilepsy
EXtra - Ethics
Split brain operation not performed for the purpose of the research. In addition, all procedures were explained to the split-brain participants and their full informed consent was obtained
BUT the trauma of the operation may mean that the participants did not later fully understand the implications of what they had agreed to
they were subject to repeated testing over a lengthy period (years in some cases) - this may have been stressful at times
