Overview – Sperry (1968)

Sperry, R. W. (1968), Hemisphere deconnection and unity in conscious awareness. American Psychologist, 23, 723-733.

Sperry (1968) is a key study in the biological area of psychology that investigated whether the two hemispheres of the brain have different functions. Sperry studied people whose corpus callosum – the main bundle of nerve fibres that connects the two hemispheres of the brain – had been surgically severed as a treatment for severe epilepsy. By presenting information to one side of the brain at a time, Sperry investigated what each hemisphere could do when information couldn’t be transferred to the other hemisphere. The study provided evidence for lateralisation of function – that some functions are mainly carried out by one hemisphere rather than being equally distributed across both sides of the brain. The left hemisphere was particularly important for speech whereas the right hemisphere processes visual and spatial information and is able to perform some tasks without being able to describe its responses verbally.

You can jump to different sections of this article via the links below:

• Aim
• Method (sampling and procedure)
• Results
• Conclusions
• Evaluation and methodological issues
• Relation to biological psychology more broadly

Sperry (1968)

Aim

The study investigated how the two hemispheres of the brain function after the connection between them has been severed.

The brain is divided into two hemispheres. Some functions within the brain are lateralised, meaning that they are mainly controlled by one hemisphere. For example, language is mainly located in the left hemisphere (e.g. the Broca’s area is involved in speech production and the Wernicke’s area is involved in speech comprehension).

The hemispheres also control opposite sides of the body (this is known as contralateral control). So, for example, the left hemisphere controls the right side of the body and the right hemisphere controls the left side of the body.

The two hemispheres normally communicate via the corpus callosum – a large bundle of nerve fibres that connects the two sides of the brain. However, some people (split-brain patients) with severe epilepsy have this connection surgically severed in an operation called a commissurotomy. Severing the corpus callosum like this reduces the spread of abnormal electrical activity during seizures and so reduces the symptoms of epilepsy.

Sperry’s earlier research with split-brain animals provided evidence that the two hemispheres could learn and process information separately. For example, studies with cats and monkeys showed that a task could be learned by one hemisphere without the other hemisphere gaining access to the information. However, previous research in humans had produced mixed findings, with some studies suggested that severing the corpus callosum caused few noticeable behavioural effects. Sperry’s own research suggested that more detailed testing could reveal significant differences between the two hemispheres and he realised that studying these split-brain patients could reveal how independently the two hemispheres can function.

So the aim of Sperry’s study was to investigate whether each hemisphere has an independent stream of conscious awareness and its own separate memories that cannot be accessed by the other hemisphere. More broadly, the study aimed to investigate the lateralisation of brain function by examining what split-brain patients could do when information was presented to one hemisphere but not the other.

Method

Sperry used a quasi-experiment (i.e. an experiment where the researcher doesn’t randomly assign participants). The split-brain participants had already undergone surgery to sever their corpus callosum so as to control their epilepsy. Sperry compared the split-brain patients’ performance with people who had not undergone the surgery and did not have epilepsy.

The study also involved elements of case studies, with individual participants’ behaviour and abilities being examined in detail.

Participants and sampling

The main sample consisted of 11 split-brain patients.

All had suffered from severe epilepsy that had not responded adequately to drug treatment. They had therefore undergone surgery to sever the corpus callosum (commissurotomy).

• 2 participants had undergone the surgery some time before the experiments.
• 9 participants had undergone the surgery more recently.

The participants were therefore an opportunity sample. This means that the researchers used people who were already available because they had undergone the required surgery.

The study also included comparison participants who had not undergone commissurotomy and did not have epilepsy.

Procedure

Sperry used controlled procedures to present visual and tactile information to either the left or right hemisphere while limiting communication between the hemispheres:

• Visual tests: Participants maintained central fixation while stimuli were briefly presented to either the left or right visual field, allowing information to be directed primarily to the contralateral hemisphere.
• Tactile tests: Objects were placed in either the left or right hand while the hands were hidden from view, allowing the researchers to assess how information received through touch was processed by each hemisphere.

Visual investigations

Participants were asked to look at a fixation point in the centre of a screen. Images were then flashed very briefly to either the left or right of this point.

For example, if an image was shown in the right visual field, the information would mainly be processed by the left hemisphere (again, this is because the brain operates contralaterally – information to the left side goes to the right hemisphere and vice versa).

So, the participant might be shown the image above and the image of the car would be processed in the left hemisphere of their brain.

The images were shown for around one-tenth of a second or less. This was to prevent participants from moving their eyes towards the image and allowing the information to reach both hemispheres.

The researchers did the same thing but for the other side. So, they would show the image in the participant’s left visual field, which meant it would mainly be processed by the right hemisphere.

The upshot of these tests was that they enabled Sperry to show images to one hemisphere of participants’ brains only and test the effects.

After the images were presented, the researchers assessed the participant’s response using different methods. Participants could be asked to verbally identify or describe what they had seen, or to draw the object using their left or right hand while unable to see their hand. They could also be asked to select or point to the object from a range of alternatives. This enabled the researchers to compare what participants could report verbally with what they could demonstrate through non-verbal responses.

The researchers also presented two different images simultaneously. For example, an apple could be shown in the left visual field while a car was shown in the right visual field.

Again, after displaying these images, the researchers would ask the participant to identify what they saw – whether verbally, by drawing it, or by pointing from a range of options.

Tactile investigations

The researchers also investigated whether each hemisphere could identify objects through touch.

To test this, objects were placed into the participant’s left or right hand while their hands were hidden from view. This prevented them from using vision to identify objects that were being placed in their hands. They were also asked to remain silent unless the researcher asked them a question. This was important because speech could allow information processed by the left hemisphere to be communicated to the right hemisphere.

Like with the visual tests, the participant was then asked to:

• Identify the object verbally
• Draw the object
• Find the object from an array of other objects

Objects were also sometimes placed in both hands at the same time.

Tests of the right hemisphere

The researchers carried out additional tests to investigate whether the right hemisphere could perform more complex tasks even though it had limited access to speech.

These included:

• Matching objects that were semantically related
• Solving simple mathematical problems
• Sorting objects according to shape, size, or texture
• Responding to emotionally meaningful images presented to the right hemisphere

Results

The main results showed that the two hemispheres could perform some tasks independently when the corpus callosum had been severed.

When different images were presented simultaneously to the two visual fields, participants could verbally report information presented to the right visual field/left hemisphere, but were generally unable to verbally report information presented to the left visual field/right hemisphere. However, when asked to draw the objects using the left hand, they could often reproduce information that had been presented to the left visual field/right hemisphere. This demonstrated that information could be processed by the right hemisphere even when it could not be reported verbally.

The tactile tests produced similar findings. Participants were generally better able to verbally identify objects placed in the right hand/left hemisphere, whereas objects placed in the left hand/right hemisphere could be identified through non-verbal responses such as pointing or selecting the correct object from an array. This indicated that the right hemisphere could process tactile information even when the participant could not verbally describe the object.

Below is a table summarising these results and what they imply:

The tests also showed that the right hemisphere could do more than simply process basic visual information:

• Participants could match objects that had similar meanings. For example, a picture of a wall clock could lead them to select a toy wristwatch by touch.
• The right hemisphere could perform simple mathematical tasks.
• The left hand could sort objects according to characteristics such as shape, size, and texture.
• Participants sometimes giggled or looked embarrassed when an emotionally meaningful image was presented to the right hemisphere even though they could not explain what they had seen.

This suggested that the right hemisphere could process information, solve some problems, and produce emotional responses. It just couldn’t communicate these responses verbally.

Conclusions

Sperry concluded that the two hemispheres have specialised functions but normally work together through the corpus callosum. When the corpus callosum was severed, the hemispheres could operate relatively independently, showing that information processed by one hemisphere was not always available to the other.

The findings provided strong evidence for lateralisation of function, showing that different mental functions are more strongly associated with different hemispheres.

Verbal and non-verbal processing

The findings suggested that the two hemispheres have different strengths in processing information. The left hemisphere is dominant for verbal processing and the right hemisphere is better able to process information non-verbally, including visual and spatial information.

Information presented to the right visual field or right hand was processed primarily by the left hemisphere, which allowed participants to verbally describe or identify the information. In contrast, information presented to the left visual field or left hand was available primarily to the right hemisphere. As such, participants could not normally verbally report this information. However, they could demonstrate knowledge of it through non-verbal responses such as drawing an object with the left hand or selecting and matching objects.

This suggests that the right hemisphere processes and responds to visual-spatial information without relying on language whereas the left hemisphere is stronger at verbal processing and communication.

Lack of cross-integration

Sperry’s findings also suggested that there was a lack of cross-integration between the hemispheres after the corpus callosum was severed. Information processed by one hemisphere was not automatically available to the other hemisphere.

For example, information presented to the right hemisphere could influence the participant’s behaviour without being available for verbal report by the left hemisphere. Similarly, the left hemisphere could verbally report information that the right hemisphere had not received. This suggests that the corpus callosum normally plays an important role in integrating information between the two hemispheres.

Two streams of consciousness?

The findings led Sperry to propose a more radical interpretation: the two hemispheres could potentially support two relatively independent streams of consciousness.

Each hemisphere could process its own perceptions, memories, and responses, which would normally be integrated with information from the other hemisphere via the corpus callosum. However, in split-brain patients, the severed corpus callosum could no longer provide this normal integration. This led Sperry to suggest that a split-brain patient could potentially be understood as having two relatively independent minds within one body.

However, this does not mean that split-brain patients literally experienced themselves as two completely separate people. Instead, Sperry’s findings suggested that consciousness and mental processing could be partly divided between the two hemispheres when normal communication between them was disrupted.

Evaluation

Research methods and techniques

Sperry used a quasi-experiment in a highly controlled laboratory setting. This allowed him to investigate what happened when communication between the hemispheres had been surgically disrupted.

• Standardised procedure: The researchers carefully controlled how information was presented. Images were flashed very briefly and participants were asked to fixate on a central point. Their hands were also hidden from view and they were asked not to speak unless instructed. These controls helped ensure that information was directed to the intended hemisphere.
• Useful comparison: The split-brain participants were compared with people who had not undergone commissurotomy and did not have epilepsy. This helped Sperry identify effects associated with the split-brain condition.
• Ethical: A quasi-experiment enabled Sperry to investigate the effects of a severed corpus callosum without deliberately causing this condition in participants. It would be massively unethical to randomly assign healthy participants to have their corpus callosum surgically severed solely for the purposes of an experiment. Instead, Sperry studied patients who had already undergone the surgery as a treatment for severe epilepsy. This meant that the research could investigate the effects of split-brain surgery without exposing participants to unnecessary surgical risks.
• Lack of control: However, because Sperry used a quasi-experiment, he could not randomly assign participants to the split-brain condition. As such, the split-brain participants’ unusual responses to the tests could have been caused by something else other than the severing of the corpus callosum. For example, the effects could have been the result of the epilepsy itself, other effects of the surgery, or individual differences between patients.
• Complex interpretation: Sperry inferred the function of each hemisphere from participants’ behaviour rather than directly measuring brain activity. For example, the inability to verbally name an object was taken as evidence that the right hemisphere had limited access to language. However, differences in behaviour do not necessarily show exactly what is happening in the brain. In contrast, modern techniques such as fMRI can measure changes in brain activity directly while participants perform a task. This would arguably provide stronger evidence about which brain regions are involved in particular behaviours.

Population and sampling

The study involved only 11 split-brain patients.

• Rare sample: Split-brain patients are extremely unusual. Studying people who had actually undergone commissurotomy gave Sperry access to a very valuable and difficult-to-obtain sample.
• Detailed investigation: The small sample allowed Sperry to examine individual participants in considerable detail rather than relying only on group averages.
• Very small sample: Only 11 split-brain participants were studied. This makes it difficult to generalise the findings to all people.
• Unusual participants: The participants were not typical members of the population. They had severe epilepsy, had received medication and had undergone major brain surgery. These factors could have affected their brain function.
• Individual differences: Sperry noted that there were striking differences and even exceptions between some participants. This suggests that lateralisation may vary between individuals.

Types of data

Sperry’s research involved both qualitative and quantitative data but most emphasis was placed on the qualitative descriptions of individual patients’ performances. The number of participants, task outcomes, correct/incorrect responses, etc. can be treated quantitatively.

• Quantitative data: Researchers recorded quantitative data such as whether participants correctly identified, matched, selected, or drew an object during different tasks. However, relatively little of this quantitative data was reported in detail, with greater emphasis placed on individual participants’ performances.
  • More precise: Quantitative data provides numerical evidence that can make it easier to identify patterns and compare participants’ performances.
  • Easier to compare: Numerical results could show how consistently a finding occurred across the participants.
  • Limited detail: However, numerical data alone would ignore the individual differences and unusual responses observed in the split-brain patients.
• Qualitative data: The researchers recorded detailed observations of participants’ behaviour and responses.
  • Rich detail: The detailed observations showed clear patterns in what each hemisphere could do. For example, participants could draw an object shown to the left visual field even when they could not name it.
  • Useful for case studies: Detailed information about individual participants helped reveal differences between people and allowed Sperry to identify unusual exceptions.
  • Less precise: Qualitative descriptions do not clearly show how common each finding was. It would have been useful to report numerical data showing how many of the 11 participants showed each pattern.

Representativeness and generalisability

The study provides important information about split-brain patients but has limitations when applying the findings to the wider population.

• Consistent patterns: Similar patterns were observed across participants. This provides some evidence that the findings reflect genuine differences in hemispheric function rather than unusual responses from one or two individuals.
• Abnormal sample: All of the split-brain participants had severe epilepsy and had undergone major brain surgery. Their brains may therefore function differently from those of people without these conditions.
• Small sample: The sample of 11 people was too small to assume that all people would show exactly the same pattern of lateralisation.
  • However, split-brain patients are rare, so obtaining a larger sample would have been difficult.
• Individual differences: The exceptions found within the sample further reduce confidence that the findings apply equally to everyone.

Ethical issues

• Informed consent: The participants had already undergone surgery for medical reasons, but they consented to take part in the research testing.
• Deception: There was no obvious need to deceive participants about the purpose of the behavioural tests.
• Protection from psychological harm: The experimental tasks were not designed to cause physical harm or serious psychological distress. The researchers mainly asked participants to identify, draw or sort objects.
• Right to withdraw: Participants could take part in the research after their medical treatment and were not required to undergo the surgery specifically for the experiment.

Overall, the research itself appears to have involved relatively few serious ethical concerns. The major ethical issue concerns the medical context of the participants rather than the experimental tasks themselves.

Validity

• Internal Validity: Do the findings accurately show differences between the two hemispheres?
  • High control: The researchers carefully controlled the presentation of visual and tactile information. This made it more likely that the observed differences were caused by which hemisphere received the information.
  • Brief visual presentation: Flashing images for a very short time reduced the possibility that participants could move their eyes and allow information to reach the other hemisphere.
  • Hands hidden: Keeping the hands out of sight prevented participants from using visual information to solve the tactile tasks.
  • Other effects of epilepsy and surgery: The participants had severe epilepsy and had undergone major brain surgery. These factors could have affected their performance independently of the severed corpus callosum.
• Ecological Validity: Do the findings translate to real-life behaviour?
  • Artificial tasks: Participants completed unusual laboratory tasks involving flashing images, hidden hands and objects presented in carefully controlled conditions. These situations are not typical of everyday life.
  • Compensation in everyday life: In normal situations, people can use both eyes, speech and other forms of communication. This can allow information to be shared between the hemispheres in ways that were deliberately prevented during the experiment.
  • Purpose of the research: However, low ecological validity was not necessarily a major weakness because Sperry was specifically trying to investigate what the hemispheres could do under controlled conditions. The aim was not to reproduce normal everyday behaviour.
  • Tactile tasks: Some of the tactile tasks were more similar to everyday behaviour because people regularly identify objects by touch without looking at them.

Reliability

• Internal Reliability: Is the procedure and measurement consistent across participants in this particular study?
  • Standardisation: The researchers used carefully controlled procedures for presenting visual and tactile information. This made the testing consistent across participants.
  • Clear procedures: Participants were given the same basic instructions, such as fixing their eyes on the central point and keeping their hands hidden.
  • Individual differences: Sperry reported substantial variation between participants, including some striking exceptions. This suggests that not every participant showed exactly the same response.
• External Reliability: Can the study be replicated at a different time and with different groups to produce consistent results?
  • Highly controlled procedure: The use of a fixation point, brief visual presentations, hidden hands and standardised tasks makes the basic procedure reasonably easy to repeat.
  • Consistent general pattern: Similar patterns of lateralisation were found across several participants. This suggests that some findings are reliable.
  • Rare sample: Split-brain patients are extremely uncommon, so obtaining a comparable sample for an exact replication would be difficult.
  • Exceptions: Differences between participants make it difficult to know whether every finding would be reproduced in another group of split-brain patients.

Ethnocentrism

Sperry doesn’t specify the participants’ cultural backgrounds. However, the participants were all patients receiving neurosurgical treatment in the US and so it is reasonable to infer that the sample was predominantly Western.

• Limited sample: A sample of only 11 participants – likely all from Western backgrounds and being treated in a Western medical context – would in many contexts not be sufficient to draw universal conclusions.
• Biology and brain structures are universal: However, the functions Sperry investigated – such as speech and visual processing – are based on the structure and functioning of the human brain. The basic organisation of the brain and its major structures (including the two hemispheres and corpus callosum) is broadly shared across humans regardless of culture. As such, there is limited evidence that Sperry’s findings were specifically ethnocentric. 

Evaluation summary table

Relation to biological psychology more broadly

Sperry (1968) is an important study in biological psychology because it provided evidence that different functions are associated with different areas or sides of the brain. It also showed how brain structure can be investigated by studying changes in behaviour.

The table below summarises how Sperry compares with other core biological psychology studies and evaluates his contribution to the area: