The two hemispheres of the brain are divided into the left and right hemisphere. The two hemispheres have different functions, this is known as lateralisation of function (specialisation of the hemisphere). For example, the right hemisphere processes info from the left half of body, whereas theleft hemisphere processes info from the right half of body. The corpus callosum is a bundle of nerve fibres which joins these two halves of the brain, so they are able to communicate with each other.
The following table highlights the function of each hemisphere:
As seen in the above table, information received by the right visual field is processed in the left hemisphere, whereas information received by the left visual field is processed by the right hemisphere.
In the past, the corpos callosum was sometimes severed (i.e. cut) by surgeons during surgery in order to improve symptoms of severe epilepsy which was uncontrollable by medication. This type of surgery is known as a commissurotomy.
Roger Sperry used these so-called split-brain patients within his famous studies carried out in the 1960s to determine whether the language function is localised in the left hemisphere. He was awarded the Nobel Prize in 1981 for his discoveries.
Broca’s Area and Wenicke’s Area: Back in the 1860, two neurologists (Paul Broca and Karl Wernicke) found that people who had damage to a particular area of the left side of the brain had speech and language problems. People with damage to these areas on the right side usually did not have language problems. This was the first real discovery that the different hemispheres have difference functions: These areas of the left side of the brain now bear their names: Broca’s area and Wernicke’s area.
To investigate the effects of hemisphere de-connection and show that each hemisphere has separate functions. (lateralisation)
Method & Design
A quasi experiment was used because the IV was naturally occuring. The IV was whether or not participants had undergone a surgical section of the corpus callosum as a treatment for epilepsy and the DV was their performance on a variety of tests.
The research could also be considered a case study because it involved the intensive study of 11 patients to investigate behavioural symptoms resulting from hemisphere deconnection.
It was also a snapshot study as each split brain patient and normal participant was tested at one point of time and their behaviour in relation to information presented to different visual fields and through touch alone was compared using one set of data.
The sample consisted of 11 people who were patients with epilepsy, who had undergone surgical section of the corpus callosum as treatment for epilepsy. Also, a control group of ‘normal’ people, who had not had the surgical section of the corpus callosum, were used for comparison. The sampling method was an opportunity sample.
Visual tasks: Patients had one eye covered in the study. They were told to focus on a central point on the screen and then an image would then be flashed onto the screen for 1/10 of a second or less. The time an image was exposed for was fast to prevent information from travelling to the other visual field and consequently the other hemisphere. Participants had to say what they saw or pick out an object from behind the screen. Hands and objects on table were kept out of sight. If a stimulus is presented to the left visual field of a split-brain patient they should not be able to name the stimulus.
Tactile tasks: tasks with objects were presented to one of the hands of the split-brain patients which was covered underneath a screen so they themselves could not see what they were doing. If the stimulus is presented to the left hand of a split-brain patient they should not be able to name it.
When an image was been identified and responded to in one visual field and was then presented to the other visual field the participant responded as if he/she had no recollection of the previous exposure.
Visual material presented to the right visual field could be described in speech and writing.
Visual material presented to the left visual field could not be identified in speech or writing but could be identified by pointing with the left hand.
When two different images were flashed simultaneously to the right and left visual fields, the participant could draw with his left hand what he saw in his left visual field but could not name it.
When two different words were flashed simultaneously to the right and left visual fields, the participant could select with their left hand the object from a collection of objects.
When two different words were flashed simultaneously to the right and left visual fields the participant could spell with his right hand and say the word.
Objects put in the right hand for identification by touch alone could be described in speech and writing.
Objects put in the left hand could not be identified in speech or writing but could be selected with the same hand from a grab bag.
These findings demonstrate that hemispheres do seem to have differing functions. Language is necessary to recognise and name objects.. Language is found in the left hemisphere of the brain so only objects presented to right visual field or right hand could be recognised. However, it is also evident that each hemisphere has its own memory. Finally, the corpus callosum section prevents hemispheres from communicating with each other.
Controls in the study: all participants had one eye covered, all participants were exposed to images for one tenth of a second, hands hidden from view via the use of the screen, participants were not allowed to speak unless spoken to
+ A quasi-experiment was used which allows researchers to investigate variables that are not able to be investigated normally.
+ The techniques Sperry developed allowed the functions of the two hemispheres to be studied in ways which had previously been impossible.
+ Both quantitative and qualitative data was recorded. Quantitative data allows the researcher to easily analyse data and draw conclusions, whilst qualitative data enabled us to consider the experiences of the split-brain patients.
+ The study was conducted under strictly controlled conditions and was therefore high in reliability
+ The snapshot design allows the researcher to compare individuals or groups at one period in time to see how they may be similar or different.
+ The opportunity sampling method is quick and easy to obtain participants, thus more economical
– A small, specific sample was used and thus the findings may not be representative of other people as there may have been individual differences influencing the results, therefore lacking reliability
– The quasi-experiment method tends to have less control as the IV is naturally occurring – for example Sperry was not able to test these participants prior to their operation and it is possible that their brain functioning may have been atypical (different from the norm) before the operation (epileptic). Thus drawing conclusions about the functions of the hemispheres in non-separated brains becomes more difficult.
– The techniques that Sperry developed artificially separate the visual and tactile information received by the individual. A person with a severed corpus callosum would be able to compensate in real life by using both eyes, therefore the study was low in ecological validity.
– Snapshot studies do not allow the researcher to discover whether results are due to the development of the behaviour or to individual differences.
– The study raises ethical issues as the participants may have become frustrated, embarrassed or distressed whilst completing the tasks asked of them.
– Opportunity samples tend to be biased and are therefore not representative of the general population.