Lateralisation
What is it?
The idea that different hemispheres have different specialisations.
Some mental process in the brain are mainly specialised to either the left or right hemisphere
The two halves are not exactly alike
Origins
Mark Dax was a French doctor in the early 1800s.
He found that patients lost power of speech as a result of brain damage but in every case there had been damage in the left hemisphere but not in the right.
He suggested that language was located in the left hemisphere → investigated experimentally in split-brain research
Split-brain research
Research that studies individual who have been subjected to the surgical separation of the two hemispheres of the brain as a result of severing the corpus callosum.
Originated from a treatment for severe epilepsy
Surgeons cut the bundle of nerve fibres that forms the corpus callosum - aim was to prevent the violent electrical activity that accompanies epileptic seizures crossing from one hemisphere to the other.
Meant psychologists could study the hemispheres separately.
Support for hemispheric lateralisation
Sperry 1968
- Carried out a combination of case studies and experiments.
- 11 patients had undergone split brain surgery for epilepsy.
- Control group was used who had no hemisphere disconnection.
- One experiment → participants covered 1 eye and looked at a fixed point on a screen. Pictures were projected onto the right or left of the screen → high speeds → no time for eye movements
S-
The two hemispheres must communicate
We need to talk about things that are experienced in the right hemisphere (e.g. facial recognition)
They are therefore connected to allow info received by one hemisphere to be sent to the other hemisphere
Corpus callosum (this is what is severed in split-brain research
Findings
If pictures = right visual field, all participants could say or write what it was without problem
If pictures = left visual field, participants couldn’t say or write what they’d seen
BUT they could select a corresponding object with their left hand, which represented what had been shown to their left eye (right hemisphere) even though they didn’t know why they had selected this object
Lateralisation
- Highlights that different areas of the brain specialise in different function.
- Left Hemisphere = receives visual information from the right visual field can convert sight into spoken written language.
- Usually information entering the right hemisphere can cross over to be processed in the left but this can't happen in split brains.
- Infomation going to the right hemisphere can not be converted to language.
- But the right hemisphere can still produce non-verbal responses.
Whilst these studies show that the brain had discrete regions with specific functions. It is important to understand that connectivity and communication is as important.