Hemispheric lateralisation
The idea that the two halves (hemispheres) of the brain are functionally
different and that certain mental processes and behaviours are mainly controlled by one
hemisphere.
Split-brain research
A series of studies, which began in the 1960s (and are still ongoing) involving
epileptic patients who had experienced a surgical separation of the hemispheres of the brain.
Sperrys surgery
● Sperry’s (1968) studies involved a unique group of 11 individuals, all of whom had undergone the same surgical procedure - an operation called a Corpus Callosotomy - in
which the corpus callosum and other tissues which connect the two hemispheres were cut down the middle in order to separate the two hemispheres and control frequent and severe epileptic seizures.
● This meant that for these split-brain patients the main communication line between the two hemispheres was removed.
-This allowed Sperry and his colleagues to see the extent to which the two hemispheres were specialised functions, and whether the hemispheres performed tasks independently of one another.
Sperrys procedure
● Sperry devised a general procedure in which an image or word could be projected to a patient’s right visual field (processed by the left hemisphere) and the same, or different, image could be projected to the left visual field (processed by the right hemisphere).
● In the ‘normal’ brain, the corpus callosum would immediately share the information between
both hemispheres giving a complete picture of the visual world. However, presenting the image to one hemisphere of a split-brain patient meant that the information could not be conveyed from that hemisphere to the other.
● This was a quasi-experiment
Findings
● The left hemisphere is more geared towards analytic and verbal tasks
● The right is more adept at performing spatial tasks and music
● Research suggests that the left hemisphere is the analyser whilst the right hemisphere is the
synthesiser.
Strengths of the Methodology
- The experiments involving split-brain patients made use of highly specialised and standardised procedures. Sperry’s method of presenting visual information to one
hemispheric field at a time was quite ingenious, especially for its time. - Typically, participants would be asked to stare at a given point, the ‘fixation point’, whilst one eye was blindfolded. The image projected would be flashed up for one-tenth of a second, meaning the split-brain patient would not 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 split brain procedure which has been
eveloped and refined over time through various case studies all of which have supported the theory of hemispheric lateralisation.
A03 - Generalisation
- Many researchers have urged caution in their widespread acceptance, as split-brain patients constitute such an unusual sample of people.
- There were only 11 who took part in all variations of the basic procedure, all of whom had a history of epileptic seizures. It has been argued that this may have caused unique changes in the brain that may have influenced the findings. It is also the case that some participants
had experienced more disconnection of the two hemispheres as part of their surgical procedure than others. - Finally, the control group Sperry used, made up of 11 people with no history of epilepsy, may have been inappropriate.
- This narrow field of participants questions the applicability and generalisability of the findings of split-brain research to the general population.
A03 - Alternative expanation
- Recent findings in the field of neuroscience shows that neuroplasticity takes place throughout the lifetime and sometimes as a result of brain injury.
- Cortical remapping can occur where corresponding dormant areas of the brain can take over functions after damage has occurred. This remapping can occur in a nearby area of the brain as well as opposing hemispheres.
- This shows that the brain function, and in this case lateralisation is not completely fixed, but it has the potential to adapt and change.
- This questions conclusiveness of the findings of Sperry as, as our knowledge of neuroplasticity increases we see the impact of events, trauma and the environment can
change our localised and lateralised brain function – so calling into question the biologically deterministic explanation of Sperry’s findings.
