Hemispheric Lateralisation And Split-brain Research Flashcards
Define hemispheric lateralisation
The idea that the two halves (hemispheres) of the brain are functionally different and that certain mental processes and behaviours are mainly controlled by one hemisphere rather than the other, as in the example of language (which is localised as well as lateralised)
How is language lateralised?
It is performed by one hemisphere rather than the other
•the two main language centres are only in the LH
- Broca’s area in the left frontal lobe
- Wernicke’s area in the left temporal lobe
What is the right hemisphere’s role in language
• can only produce rudimentary words and phrases but contributes emotional context to what is being said
• this has led to the suggestion that the LH is the analyser whilst the RH is the synthesiser
Give examples if functions which aren’t lateralised
• 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 whilst the LH controls movement on the right
Explain how 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)
• the LVF of both eyes is connected to the RH
• 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 depth 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
What is split-brain research
A series of studies which began in the 1960s (and are still ongoing) involving people with epilepsy who had experienced a surgical separation of the hemispheres of their brain to reduce the severity of their epilepsy. This enabled researchers to test lateral functions of the brain in isolation
Outline Sperry’s research
Roger Sperry (1968)
• devised a system to study how two separated hemispheres deal with speech and vision
• 11 split-brain individuals were studied using a special set up in which an image or word could be projected to a ppt’s RVF (processed by the LH) and the same, or different, image could be projected to the 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
• however, presenting the image to on hemispheres of a split-brain ppt meant that the information cannot be conveyed from that hemisphere to the other
Outline the findings of Sperry’s research
• when a picture of an object was shown to a ppt’s RVF (linked to LH) l, the ppt could describe what was seen
• but they couldn’t do this if the object was shown to the LVF (RH) - they said there was ‘nothing there’
• this is because in the normal brain, messages from the RH are relayed to the language centres in the LH, but this is not possible in the split-brain
• although ppts couldn’t 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)
• the left hand was also able to select an object that was most closely associated with an object presented to the LVF (for instance, an ash tray was selected in response to a pic of a cigarette)
• if a pinup picture was shown to the PVF there was an emotional reaction (e.g. a giggle) but the ppts usually reported seeing nothing or just a flash of light
Outline the conclusion of Sperry’s reseach
These observations show how certain functions are lateralised in the brain and support the view that the LH is verbal and the RH is “silent” but emotional
Evaluate lateralisation in the normal brain
+ strength: research showing that even in normal brains the two hemispheres process information differently
• e.g. Gereon Fink et al. (1996) used PET scans to identify which brain areas were active during a visual processing task
• when ‘normal’ ppts were asked to attend to global elements of an image (such as looking at a picture of a whole forest) regions of the RH were much more active
• when required to focus in on the finer detail (such as individual trees) the specific areas if the LH tended to dominate
• this suggests that, at least as far as visual processing is concerned, hemispheric lateralisation is a feature of the normal brain as well as the split-brain
Evaluate the idea that the LH is the analyser and the RH is the synthesiser
- limitation: idea may be wrong
• there may be different functions in the RH and LH, but research suggests people don’t have a dominant side of their brain which creates a different personality
• Jared Nielsen et al. (2013) analysed brain scans from over 1000 people aged 7 to 29 years and did find that people used certain hemispheres for certain tasks (evidence for lateralisation)
• but there was no evidence of a dominant side, i.e. not artist’s brain or mathematician’s brain
• this suggests that the notion of right- or left-brained people is wrong
Evaluate lateralisation vs plasticity
• lateralisation is adaptive as it enables two tasks to be performed simultaneously with greater efficiency
• Lesley Rogers et al. (2004) showed that lateralised chickens could find food while watching for predators but normal chicken’s couldn’t
• on the other hand, 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
• for instance, language function can literally switch sides (Holland et al. 1996)
Evaluate blain-split research
+ strength: support from more recent split-brain research
• Michael Gazzaniga (Luck et al. 1989) showed that split-brain ppts actually perform better than normal controls on certain tasks
• for example, they were faster at identifying the odd one out in an array of similar objects that normal controls
• in the normal brain the LH’s better cognitive strategies are ‘watered down’ by the inferior RH (Kingstone et al. 1955)
• this supports Sperry’s earlier findings that the ‘left brain’ and ‘right brain’ are distinct
Evaluate split brain research (limitation)
- limitation: causal relationships are hard to establish
• the behaviour pf Sperry’s split-brain ppts was compared to a neurotypical control group
• an issue though is that none of the ppts in the control group had epliepsy
• this is a major confounding variable
• any differences that were observed between the two groups may be the result of the epilepsy rather than the split-brain
• this means that some of the unique features of the split-brain ppts’ cognitive abilities might have been due to their epilepsy
