hemispheric lateralisation Flashcards
hemispheric lateralisation
-idea that certain functions in the brain are dominated by one hemisphere or the other
split brain research
Sperrys research was conducted in 1968 and they were a series of studies that involve the unique group of individuals all of which and under a surgical procedure known as a commissurotomy is a procedure in which corpus callosum and other tissues that connect the two hemispheres are severed in order to control severe epileptic seizures. Allows two hemispheres to communicate.
-that then allowed Sperry and his colleagues to see the extent to which two hemispheres were actually specialised for certain functions and whether the hemispheres performed the tasks independently of one another.
-The general procedure in which an image or a word could be projected to a patient right visual field which would be processed by the left hemisphere and then the same or different image could be projected to the left visual field which would be processed by the right hemisphere, a normal brain ,Colpus callosum would actually share the information between both hemispheres, giving complete picture of outside world, in split brain patient that information couldn’t be conveyed from one hemisphere to another.
split brain research results
• Describing what you see-word was flashed in right visual field patient would be able to say what they saw-left hemisphere has language centres in.-If image was in left visual field, connected to right hemisphere, patients wouldn’t report seeing anything or say what they saw- doesn’t have any language centres.
• Recognition by touch-If word/image was flashed in right visual field, could say what they saw, but left visual field they couldn’t but with left hand they could pick blindly an item out of the bag which matched what they saw on the screen.
-If a pinup picture was shown to the LVF there was an emotional reaction eg giggle but the ppts usually reported seeing nothing or just a flash of light
limitation-issues of generalisation
A limitation of Sperry’s research into split brain patients is that it has been argued the findings are not generalisable.
This is largely down to the sample of patients that were used. Split-brain patients are such an unusual sample of people and they are rarely encountered in sufficient numbers to be useful for research.
There were only 11 people who took part in all variations of the basic procedure, all of whom had a history of epileptic seizures. It has also been argued that this may have influenced the findings.
Furthermore, it is also the case that some participants had experienced more disconnection of the two hemispheres than others. Finally, the control group Sperry used made up of 11 people who did not have epilepsy, may have been inappropriate. It may have been more valid if the control group had been made up of epileptics who had not had a commissurotomy, as a much more valid group would be epileptic people who had not had the operation. The epilepsy or the drugs used to treat the epilepsy may have affected the patients’ brains irrespective of the split-brain procedure and could be responsible for the results.
Issues such as those outlined above seriously impede the validity of the research, as it reduces the generalisability, but also the internal validity, as variables were not being controlled appropriately.
Left and right hemispheres book
In the case of language, the two main centres are only in the LH (for most people) - Broca’s area is in the left frontal lobe and Wernicke’s area is in the left temporal lobe. So we say that language is lateralised - that is, 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. This has led to the suggestion that the LH is the analyser whilst the RH is the synthesiser.
Many functions are not lateralised. For example vision, motor and somatosensory areas appear in both hemispheres. But there is a further twist, 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.
In the case of vision the situation is even more complex - it is both contralateral and ipsilateral (opposite and same-sided). Each eye receives light from the left visual field (LVF) and the right visual field (RVH). The LVF of both eyes is connected to the RH and the RVF of both eyes is connected to the LH (see diagram on right).
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.
Strength-lateralisation in connected brain
One strength is research showing that even in connected brains the two hemispheres process information differently.
For example, Gereon Fink et al. (1996) used PET scans to identify which brain areas were active during a visual processing task. When participants with connected brains 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 of the LH tended to dominate.
This 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.
limitation- one brain
One limitation is the idea that the LH as analyser and RH as synthesiser may be wrong There may be different functions in the RH and LH, but research suggests people do not 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.
strength-Sperry-research support
One strength is support from more recent split-brain research
Michael Gazzaniga (Luck et al. 1989) showed that split-brain participants actually perform better than connected controls on certain tasks. For example, 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 the inferior RH (Kingstone et al. 1995).
This supports Sperry’s earlier findings that the ‘left brain’ and ‘right brain’ are distinct.
