Split-brain research into Hemispheric lateralisation Flashcards
Hemispheric lateralisation
The idea that the two hemispheres of the brain are functionally different and that certain mental processes and behaviours are mainly controlled by one hemisphere rather than the other.
Split-brain research
A series of studies that began in the 1960s involving epileptic patients who had experienced a surgical separation of the hemispheres of the brain; allowed researchers to investigate the extent to which brain function is localised.
Who conducted split-brain research?
Roger Sperry et al. (1968)
Sperry’s (1968) sample
A unique group of individuals, all of whom had undergone the same surgical procedure - an operation called a commissurotomy - in which the corpus callosum and other tissues which connect the 2 hemispheres were cut down the middle to separate the 2 hemispheres and control frequent and severe epileptic seizures.
What did mean when the corpus callosum was cut?
The main communication line between the 2 hemispheres was removed.
What did the operation allow for Sperry to see?
Allowed Sperry and colleagues to see the extent to which the 2 hemispheres were specialised for certain functions, and whether the hemispheres performed tasks independently of one another.
What procedure did Sperry devise?
In which an image or word could be projected to a patient’s right visual field (processed by left hemisphere) and the same or different image could be projected to the left visual field (by the right hemisphere).
The comparison of the normal brain and the participants
Normal - the corpus callosum would immediately share the information between both hemispheres giving a complete pic of the visual world.
However, presenting an image to 1 hemisphere of a split-brain patient meant that the info couldn’t be conveyed from that hemisphere to the other.
Sperry’s results - describing what you see.
When a pic was shown to a patient’s RVF, patient could see clearly.
If same object shown to a patient’s LVF, patient couldn’t describe what was seen, and typically was reported that there was nothing there.
Patient’s inability to describe objects in LVF was because of the lack of language centres in the RH.
Normal brain - messages from RH would be relayed to language centres in LH.
Sperry’s results - recognition by touch.
Although pps couldn’t attach verbal labels to objects in LVF, they were able to select a matching object from a grab-bag of diff objects using left hand.
Objects were placed behind a screen so as not to be seen.
Left hand able to select an object that was most closely linked with an object presented to LVF.
In each case, patient wasn’t able to verbally identify what they’d seen but could nevertheless understand what the object was using RH and select the corresponding object accordingly.
Sperry’s results - composite words.
2 words were presented simultaneously, one on either side of VF, patient would select a key with their left hand and say the word ‘ring’.
Sperry’s results - matching faces.
RH appeared dominant.
Asked to match a face from a series of other faces, pic processed by RH was consistently selected, whilst pic presented to LH was ignored.
A composite pic made up of 2 different halves of a face were presented - 1 half of each hemisphere - LH dominated in verbal description whereas RH dominated in selecting a matching picture.
Strength - Demonstrated lateralised brain functions
Sperry’s (and later Micheal Gazzaniga’s) pioneering work into the split-brain phenomenon has produced an impressive and sizeable body of research findings, the main conclusion of which appears to be that the LH a more geared towards analytic and verbal tasks while the right is more adept at performing spatial tasks and music. The RH can only produce rudimentary words and phrases but contributes emotional and holistic content to language.
Research suggests that LH is the analyser whilst the RH is the synthesiser - a key contribution to our understanding of brain processes.
Strength - Methodology
Experiments involving split-brain patients made use of highly specialised and standardised procedures. Sperry’s method of presenting visual info to 1 hemispheric field at a time was quite ingenious. Pps would be asked to stare at a given point - the fixation point.
Image projected would be flashed up for one-tenth of a second, meaning the split-brain patient wouldn’t have time to move their eye across the image and so spread the information across both sides of the visual field. and subsequently, both sides of the brain.
This allowed Sperry to vary aspects of the basic procedure and ensured that only one hemisphere was receiving information at a time. Thus he developed a very useful and well-controlled procedure.
Strength - Theoretical basis
Sperry’s work prompted a theoretical and philosophical debate about the degree of communication between the 2 hemispheres in everyday functioning and the nature of consciousness.
Some theorists, for example, Roland Pucetti (1977), have suggested that the 2 hemispheres are so functionally different that they represent a form of duality in the brain - that in effect we are all two minds.
In contrast, other researchers have argued that, far from working in isolation, the two hemispheres form a highly integrated system and are both involved in most everyday tasks.
