Lesson 7: Hemispheric Lateralisation Flashcards
What is hemispheric lateralisation
Hemispheric lateralisation refers to the notion that certain functions are principally governed by one side of the brain
Systematic research has demonstrated that….
in most people language centres are lateralised to the left hemisphere. The Broca’s Area was thought to be responsible for the production of speech, however, this is now thought to involve a wider network than just the Broca’s Area. Damage to the Broca’s Area leads to expressive aphasia. The Wernicke’s Area is considered to play a vital role in understanding language/interpreting speech. Damage to the Wernicke’s Area leads to receptive aphasia. The right hemisphere is dominant for visuo-spatial functions and facial recognition.
Right and left hemisphere
The right hemisphere of the brain is responsible for the left hand side of the body, and the left hemisphere is responsible for the right hand side of the body. If a patient is experiencing right sided paralysis this means there is lateralised damage to the left hemisphere.
The two hemispheres are connected by a bundle of nerve fibres known as the corpus callosum which enables information to be communicated between the two hemispheres. Many researchers suggest that the two hemispheres work together to form most tasks as part of a highly integrated system
Advantages of Hemispheric Lateralisation
+ An advantage of hemispheric lateralisation is that it makes sense from an evolutionary perspective. It increases neural processing capacity, which is adaptive. By using one hemisphere to engage in a particular task it leaves the other hemisphere free to engage in another function. Rogers et al. (2004) found that hemispheric lateralisation in chickens is associated with an ability to perform two tasks simultaneously (finding food and being vigilant for predators).
+ Patients who have extensive damage to their left hemisphere can experience global aphasia (loss of speech production and speech comprehension). This suggests that language is lateralised to the left hemisphere.
Disadvantages of Hemispheric Lateralisation
- Lateralisation patterns shift with age (Szaflarski et al 2006) with most tasks generally becoming less lateralised in healthy adulthood.
- JW (a split-brain patient) developed the capacity to speak using his right hemisphere, with the result that they could speak about information presented in either the left visual field or the right visual field (although he was more fluent if information was presented in the left). It would appear that language is not lateralised entirely to the left hemisphere (Turk et al. 2002).
- If one hemisphere is damaged, undamaged regions on the opposite hemisphere can compensate. Danelli (et al. 2013) reported the case of EB, a 17-year-old Italian boy who had virtually his entire left hemisphere removed at the age of two and a half due to a huge benign tumour. EB’s language appeared almost normal in everyday life in terms of vocabulary and grammar. However, systematic testing revealed subtle grammatical problems as well as poorer than normal scores on picture naming and reading of loan words (words adopted from another language e.g. café). Language function can be largely preserved after removal of the left hemisphere in childhood, but the right hemisphere cannot provide, by itself, a perfect mastery of each component of language.
Split brain research
In the past surgeons have cut the corpus callosum in order to prevent the violent electrical activity caused by epileptic seizures crossing from one hemisphere to the other. Patients who underwent this form of surgery are often referred to as split-brain patients.
Sperry and Gazzaniga (1968) investigated split-brain patients. Information from the left visual field goes into the right hemisphere, whereas information from the right visual field goes into the left hemisphere. Because in split-brain patients the corpus callosum has been severed there is no way for the information presented to one hemisphere to travel to the other.
Patients are asked to stare at a dot in the centre of a screen and then information is presented in either the left or right visual field. They are then asked to make responses with either their left hand (right hemisphere), right hand (left hemisphere) or verbally (left hemisphere) without being able to see what their hands were doing.
They may be flashed an image of a dog in their right visual field and then asked what they have seen. They will be able to answer ‘dog’ because the information will have gone into their left hemisphere where the language centres are. If a picture of a cat is shown in their left visual field and they are asked what they have seen they will not be able to say because the information has gone into their right hemisphere, which has no language centres. However, they can draw a picture of a cat with their left hand because the right hemisphere controls
this hand.
Advantages of split brain research
+ Split-brain research has enabled discoveries of hemispheric lateralisation.
+ Experiments on split-brain patients are highly controlled and scientific.
Disadvantages of split brain research
- Split-brain patients have often had drug therapy for their epilepsy for much longer than others, which may affect the way in which their brain works. This means the findings of split-brain research cannot be generalised to the target population.
- Many studies using split-brain patients have as few as three participants, making it hard for results to be generalised to the target population.
- The data from this research is very artificial. In the real world a severed corpus callosum can be compensated for by the unrestricted use of both visual fields. This means the research lacks ecological validity.
