Lateralisation and Split-Brain Research Flashcards
Lateralisation:
The fact that two halves of the human brain are not entirely alike. Each hemisphere has functional specialisations.
Hemispheric lateralisation:
- left hemisphere is dominant in language and speech
- right hemisphere excels at visual motor tasks
The two hemispheres communicate via the corpus callosum (a bundle of nerve fibres that connects the two halves)
What is the left hemisphere dominant in?
language and speech
What does the right hemisphere excel at?
visual motor tasks
What is the corpus callosum?
a bundle of nerve fibres that connects the two halves of the brain
Who researched split brain?
Sperry and Gazzaniga
Split brain research aim:
Roger Sperry and Michael Gazzaniga were the first to study the capabilities of the separated hemispheres; they were able to send visual information to just one hemisphere at a time in order to study what is known as hemisphere lateralisation.
Split brain research method:
Ppts were asked to cover one of their eyes and fixate on a point for a 1/10th of a second. An image would be projected to a patient’s right visual field (which is processed by the left hemisphere) and an image is projected to the left visual field (processed by the right hemisphere). In normal brains, corpus callosum would share information between both hemispheres giving a complete picture of the world. But in a split brain patient, that information cannot travel from one hemisphere to the other.
Split brain research results: RIGHT VISUAL FIELD - LEFT HEMISPHERE:
If the patient was flashed a picture of a dog to the right visual field and asked what they’d seen, they would answer ‘dog’. This is because the visual information is processed by the left hemisphere, and the language centres of the brain are also found in the left hemisphere. This allows them to vocally point out the dog.
Split brain research results: LEFT VISUAL FIELD - RIGHT HEMISPHERE:
However, if a pic of a cat was flashed in LEFT visual field- the patient processes the visual information in the right hemisphere, but would not be able to CALL it a cat. This is bc the language centre is in the LEFT hemisphere, but without a corpus callosum, the two hemispheres cannot communicate, so patient cannot name the object.
Split brain research conclusion:
- The left hemisphere is broadly responsible for speech and language
- Right hemisphere responsible for visual-spatial processing and recognising faces
- Split brain research does not show functions being localisaed to descreet, specific regions. Instead there is specialisation in hemispheres and connectivity between regions matters.
A strength of Sperry and Gazzaniga’s research is that it involved an innovative and ingeneous method which enabled the scientific study of split brain patients.
Due to the individually unique nature of split brain patients, researchers had difficulty inventing a standardised method to use on all split brain participants. Sperry’s original method of blindfolding one eye and making ppts fixate on a dot and then showing an image for 1/10th of a second is objective and operationalised. Therefore the method is standardised and thus replicable. Thus, reliability of findings can be tested, therefore there is an increase in scientific rigour of findings and understanding of lateralisation. Increases in validity
A strength of Sperry’s conclusion (that the left hemisphere is responsible for verbal tasks and right hemisphere is dominant in performing spatial tasks) has led to applications in the field.
This is bc has helped create rehabilitation programmes that focus on verbal / visuo-spatial therapy for brain damage. The type of therapy used depends on which area is damaged.
This is done by using mild, electrical stimulation of the damaged area which will lead to neuronal growth.
Thus understanding of lateralisation (that each hemisphere has different function) helps those gone through damage perform specific tasks to accelerate the rate of recovery of that specific area.
Increases the validity of our understanding of lateralisation.
A weakness of lateralisation is that it changes with age. Lateralisation of function appears not to stay exactly the same throughout an individual’s lifetime, but changes with normal ageing.
E.g. Szaflarski et al found language became more lateralized to the left hemisphere throughout childhood and adolescence but after age of 25, lateralisation decreased and the function of language became bilateral with each decade of life.
WB: shows that brain does not work in lateralized way throughout life - makes us question S+G’s conclusions regarding lateralisation. If lateralisation is the brain’s way of functioning, it should be demonstrated in individuals of all ages.
Bc of this - lateralisation can be considered an incomplete method - decreasing in validity.
Weakness of research that supports lateralisation: issue with its generalisability and internal validity.
E.g, in Sperry’s experiment, all 11 ppts had a history of epileptic seizures. Furthermore, while all ppts had their corpus callosum damaged, some had other connections between left and right hemisphere (e.g anterior commissure) still intact.
WB: due to ppts being epileptic and suffering seizures - brains are fundamentally different to neurotypicals- cannot generalise findings of brain functioning to neurotypicals.
In terms of internal validity - although they believed they were measuring fully split brain ppts, some were not. Degree of disconnection between ppts varied greatly.
Due to lack of generalisability and lack of internal validity, overall validity of lateralisation decreases.
