- Cognitive Psychology
- Developmental Psychology
- Individual Differences
- Physiological Psychology
- Social Psychology
This study was reported in 1968 and previously Sperry had carried out research on animal brains where he had discovered that a number of functions are ?hard wired? into the nervous system and cannot be learned.
The split-brain technique of severing the corpus callosum was first investigated by Sperry on cats and monkeys before being carried out on people.
If like me you are left handed a good tip is that when you are first trying to understand this study just assume that everybody is right handed including you. Trying to reverse everything to make it fit a left handed person can be confusing. After you understand the study don?t forget to remind yourself that you are left handed as you may have an accident.
Lateralisation of function refers to the uneven distribution of tasks carried out by the hemispheres. Lateral means side. Any function, e.g. language, which is found on one side of the brain, is called a lateralised function.
In every day life split brain patients do not usually notice any differences because the problems described in this study only occur when visual material is displayed very briefly. The problems are overcome because a split brain patient will move their eyes and the image will be spread to both hemispheres.
Another way that split brain patients overcome their hemisphere deconnection is by saying something out loud so that the information is shared between right and left hemispheres.
Sperry (split brain)
Sperry, R.W. (1968) Hemisphere deconnection and unity in consciousness.
In this study Roger Sperry investigated the functions of separated hemispheres of the human brain. The experiments were undertaken on people who had already had their corpus callosum severed as treatment for severe epilepsy.
Human brains are divided into hemispheres ? a left hemisphere and a right hemisphere. Although these halves look similar they have very different functions. This division of tasks between the hemispheres is called lateralisation of function. For example, the major functions of language are carried out in the left hemisphere (for right handed people).
The table below gives a summary of some of the functions you need to know for this study.
The hemispheres are joined at their base by commissural fibres, which allow the hemispheres to communicate internally. The commissural fibres are bundled into structures called commissures (these include the corpus callosum).
These fibres are sometimes cut by surgeons to reduce the effects of epilepsy. Epileptic seizures are caused by millions of brain cells firing excessively. When someone has a grand mal (major epileptic seizure) they can have uncontrollable movements and eventually a loss of consciousness. Many of these seizures can involve both hemispheres and it was noticed by surgeons that if the hemispheres were separated the seizures could be contained in one half of the brain, therefore causing less damage to the person. Such surgery however was only used as a last resort if medication could not control the epilepsy.
Roger Sperry was able to take the opportunity of studying patients with hemisphere deconnection in order to determine whether there are differences between the two hemispheres of human brains.
The table above points out that information from the left visual field is processed in the right hemisphere and vice versa. The diagram below shows how this works.
If a person is looking straightforward everything to the left of their nose is the left visual field and everything to the right is their right visual field. Everything in the persons left visual field is received to the right of their retina and then via the optic chiasma the information goes to the right hemisphere.
The aim of this study was to investigate the effects of hemisphere deconnection and to show that each hemisphere has different functions.
The participants were 11 ?split-brain? patients, that is, they were patients who had undergone disconnection of the cerebral hemispheres. The participants had all undergone hemisphere deconnection because they had a history of advanced epilepsy which could not be controlled by medication.
The method used was a natural (also called quasi) experiment. The quasi-experiments involved comparing the performance of the 11 participants on various tasks with the performance of people with no inter-hemisphere deconnection. The independent variable was therefore the whether a person had hemisphere deconnection or not and the dependent variable was the participants performance on the tasks.
The study also makes use of the case study method. The case studies were in-depth investigations of the 11 participants.
Sperry used a number of ingenious tasks in order to investigate lateralisation of brain function. The tasks were carried out in laboratory conditions, using specialised equipment and were highly standardised. The tasks all involved setting tasks separately to the two hemispheres.
One of the tasks used to send information to just one hemisphere involved asking participants to respond to visual information. This involved blindfolding one of the participant?s eyes and then asking them to fixate with the seeing eye on a point in the middle of a screen. The researchers would then project a stimulus on either the left or right hand side of the fixation point for less than 1/10 of a second. The presentation time is so small to ensure that the participant does not have time for eye movement as this would ?spread? the information across both sides of the visual field and therefore across both sides of the brain.
As language is processed in the left hemisphere, when a stimulus is presented to the left visual field of a split-brain patient they should not be able to name the stimulus.
Another of the tasks used to send information to just one hemisphere involved asking patients to respond to tactile information. This involved presenting a stimulus to one of the hands of a split-brain patient so the participant could not see the stimulus and then asking the participant to name it. If the stimulus is presented to the participant?s left hand the participant should not be able to name it.
It is also possible to present Auditory (sound) and olfactory (smell) stimuli to one side of the brain using various methods of blocking the unused ear or nostril.
Below is a summary of some of the main results
When participants were presented with an image in one half of their visual field and then presented with the same image in the other half of the visual field they responded as if they had never seen the image before. If the same image was presented in the original visual field the participants were able to recognise the image as one they had seen before.
Participants were not able to give a description of an image that was presented to the left hand side of the visual field. The image was either not noticed or just appeared as a flash. Although they could respond non-verbally by pointing with their left hand to a matching picture or selecting an object presented among a collection of other pictures and objects. This of course only works with right-handed participants.
If two symbols were presented simultaneously, one on either side of the visual field (e.g. a dollar sign on the left and a question mark on the right) and the participant was required to draw with their left-hand (shielded from their own view) what they had seen, they would draw the left visual field symbol (a dollar sign). If they were required to say what they had just drawn, the participant would say by name, the right visual field symbol (a question mark).
Objects put in the participants hand for identification by touch could be described or named in speech or writing if they were in the right hand but if placed in the left hand, the participant could either only make wild guesses or even appeared to be unaware that anything at all was present. However, if the object was taken from the left hand and placed in a ?grab bag?, or was scrambled among other test items, the participant was able to search out and retrieve it with their left hand.
An interesting example of lateralisation of function is when two different objects were placed in each hand at the same time and then removed and hidden for retrieval in a scrambled pile of test items. Each hand hunted through the pile and searched out its own object. During the search each hand was seen to explore, identify and reject the item for which the other hand was searching. Although the performance of ?normal? participants would be slowed down by the competing demands of the tasks, the people with hemisphere deconnection could actually perform these double tasks in parallel, as quickly as they could perform one of the tasks on its own. It is worth noting though that even though Sperry showed that split-brain patients were better at completing such highly unusual tasks that this would have no advantage in the real world.
Through the case studies Sperry found that the hemisphere deconnection did not appear to affect the patients? intelligence (as measured by an IQ test) or their personality. The effects of the surgery did seem to have affected the patients in that they had short-term memory deficits, limited concentration spans and orientation problems.
Sperry argued that his studies give considerable support to his argument of lateralisation of function. That is, that different areas of the brain specialise on different tasks, such as the left hand side being responsible for language.
He also went on to argue that each hemisphere has its own perceptions and memories and experiences.
Evaluation of the Method/Procedure
A strength of Sperry?s procedure was that by using a mixture of quasi-experiments and clinical case studies, he was able to combine qualitative and quantitative approaches. The quasi-experiment is a quantitative method of data collection, that is, it provides information in the form of numbers and frequencies, and so can be easily analysed statistically. The information that is gathered is regarded as fairly reliable but not very valid. On the other hand, the case study is a qualitative method of data collection which is concerned with describing meaning. It is argued that what the case study loses on reliability it gains in terms of validity. Thus, such a combination of methods allows for the collection of statistically reliable information to be enhanced by information about the research participants? explanations.
A major criticism of the procedure was Sperry?s sample. 11 participants is a very small sample, however Sperry may not have had any control over this - there may not be very many split-brain patients available to study. The small sample also enabled Sperry to gain more in-depth data
The 11 split-brain patients were lumped together as the experimental group, but some of the patients had experienced more deconnection than others. We also cannot be sure how long each of the participants had experienced ineffective drug therapy which could have been affecting the findings.
The comparison group used by Sperry, was people with no inter-hemisphere deconnection, it could be argued that a much more valid group would be epileptic people who had not had their hemispheres deconnected.
A further weakness of the study relates to ecological validity. The findings of the study would be unlikely to be found in a real life situation because a person with severed corpus callosum who had both eyes would be able to compensate for such a loss.
Evaluation of Explanation
Sperry points out that his studies demonstrate that there is lateralisation of function.
However, not all psychologists agree. For example some psychologists argue that the two hemispheres do not function in isolation but form a highly integrated system. They argue that most everyday tasks involve a mixture of ?left? and ?right? skills, (e.g. in listening to speech we analyse both the words and the pattern of intonation) thus, rather than ?doing their own thing? the two hemispheres work very much together.
There is also some evidence of gender differences whereby women show less lateralisation than men. So, the left-right specialisation is most prevalent in men than women.
Sperry, R.W. (1968) Hemisphere deconnection and unity in consciousness. American Psychologist, 23, 723-33.