Split-brain Research Into Hemispheric Lateralisation (AQA A-Level Psychology): Revision Notes
Split-brain Research Into Hemispheric Lateralisation
Key concepts
Hemispheric lateralisation refers to the concept that the brain's two halves operate differently, with specific mental processes and behaviours being primarily controlled by one hemisphere rather than both. Language processing provides a clear example, as it is both localised to particular brain regions and lateralised to one hemisphere.
Split-brain research encompasses a series of investigations that began in the 1960s and continue today. These studies examine people with epilepsy who underwent surgical separation of their brain hemispheres, providing researchers with a unique opportunity to explore how brain functions are lateralised.
The basic principle of brain organisation shows that the right hemisphere typically controls the left side of the body, whilst the left hemisphere controls the right side. This cross-lateral control is fundamental to understanding split-brain research findings.
Sperry's split-brain studies
Background and participants
Roger Sperry conducted groundbreaking experiments in 1968 that later earned him the Nobel Prize in 1981. His research involved a unique group of participants who had all undergone the same surgical procedure called a commissurotomy. During this operation, the corpus callosum and other tissues connecting the brain hemispheres were severed to control severe epileptic seizures.
This surgical intervention meant that the primary communication pathway between the hemispheres was eliminated. Consequently, Sperry could examine how specialised each hemisphere was for particular functions and whether they could operate independently.
Procedure
Sperry developed an ingenious experimental method to present visual information to just one hemisphere at a time. Images or words could be projected to either the right visual field (processed by the left hemisphere) or the left visual field (processed by the right hemisphere).
Experimental Setup: Visual Field Testing
Step 1: Participants focus on a central fixation point Step 2: Images are flashed for one-tenth of a second Step 3: Brief presentation prevents eye movement across the image Step 4: Information reaches only the intended hemisphere
In individuals with intact brains, the corpus callosum would immediately share this information between both hemispheres, but this was impossible in split-brain participants.
Key findings
Describing what you see
When objects were shown to the right visual field, participants could easily describe what they observed. However, when identical objects appeared in the left visual field, they typically reported seeing nothing at all.
This occurred because language processing is predominantly located in the left hemisphere. Images presented to the right visual field reach the left hemisphere, allowing verbal description. Conversely, images shown to the left visual field are processed by the right hemisphere, which lacks the language centres necessary for verbal reporting.
Recognition by touch
Although split-brain participants could not verbally identify objects presented to their left visual field, they demonstrated understanding through alternative methods. Using their left hand (controlled by the right hemisphere), they could select matching objects from a collection of different items.
Demonstration: Non-verbal Recognition
When shown a picture of a cigarette in the left visual field, participants selected an ashtray from the available objects. This indicated that the right hemisphere could process and understand visual information even without the ability to verbalise it.
Composite words
When different words were presented simultaneously to each visual field - such as 'key' on the left and 'ring' on the right - participants would use their left hand to select a key whilst verbally stating "ring". This demonstrated the right hemisphere's superiority in drawing tasks, as evidenced by the superior performance when using the left hand for visual-spatial activities.
Matching faces
The right hemisphere showed dominance in facial recognition tasks. When asked to match faces from a selection, participants consistently selected the face processed by the right hemisphere (presented to the left visual field), whilst ignoring the face presented to the left hemisphere. When composite faces were created using two different halves, participants could verbally describe one half whilst selecting a picture corresponding to the other half.
Evaluation
Strengths of the methodology
The split-brain studies employed highly specialised and standardised procedures that represented a methodological breakthrough. Sperry's technique of presenting visual information to specific hemispheric fields was particularly innovative. By requiring participants to maintain fixation on a central point whilst images flashed for extremely brief periods, he ensured that information could not spread across both visual fields.
This created well-controlled conditions that allowed systematic variation of procedures whilst ensuring only one hemisphere received information at any given time. The experimental design was crucial for isolating hemispheric functions.
Demonstrated lateralised brain functions
This research produced substantial evidence showing that the left hemisphere excels at analytic and verbal tasks, whilst the right hemisphere is superior at spatial tasks and music processing. The right hemisphere can only produce basic words and phrases but contributes emotional and holistic elements to language understanding.
Research findings suggest the left hemisphere functions as an analyser whilst the right hemisphere operates as a synthesiser - a key insight into brain processing mechanisms.
Issues with generalisation
The findings face limitations regarding their broader applicability. Split-brain participants represent an extremely unusual population - only 11 individuals participated across all experimental variations, all with histories of epileptic seizures. This medical condition may have caused unique brain changes that influenced the results.
Additionally, participants experienced varying degrees of hemispheric disconnection depending on their specific surgical procedures. The control group of 11 people without epilepsy history may have been inappropriate for comparison, as it differed from the experimental group in two important ways: epilepsy status and corpus callosum integrity.
A more suitable control group might have comprised epileptic individuals who had not undergone the operation, as epilepsy itself or anti-epileptic medications could have affected brain function independently of the surgical procedure.
Differences in function may be overstated
One consequence of Sperry's work has been the proliferation of popular psychology literature that oversimplifies hemispheric differences. Whilst 'verbal' and 'non-verbal' labels can occasionally summarise differences between hemispheres, modern neuroscientists recognise that actual distinctions are less clear-cut and more complex.
In typical brains, both hemispheres maintain constant communication during everyday tasks. Many behaviours traditionally associated with one hemisphere can be effectively performed by the other when circumstances require it. This flexibility demonstrates brain plasticity - the ability of neural pathways and synapses to change due to environmental factors or injury.
Studies with rats showed that when brain areas were damaged, surrounding cells underwent functional and structural changes, allowing them to assume the functions of damaged regions. This demonstrates the remarkable adaptability of neural systems.
Theoretical basis
Sperry's research sparked theoretical and philosophical discussions about hemispheric communication in normal functioning and the nature of consciousness. Some theorists, including Roland Pucetti (1977), proposed that the hemispheres are so functionally distinct they represent a form of duality in the brain - suggesting we essentially possess two minds, with this condition being emphasised rather than created in split-brain individuals.
In contrast, other researchers argue that rather than working independently, the hemispheres form a highly integrated system with both involved in most everyday tasks.
Key Points to Remember:
- Hemispheric lateralisation means certain brain functions are primarily controlled by one hemisphere rather than both
- Split-brain research studies people who had their corpus callosum surgically severed to treat severe epilepsy
- The left hemisphere specialises in verbal and analytical tasks, whilst the right hemisphere excels at spatial and holistic processing
- Sperry's findings showed that split-brain participants could describe objects in their right visual field but not their left visual field, demonstrating language lateralisation
- Research limitations include the unusual participant population and potential overgeneralisation of hemispheric differences to normal brain functioning