9.2 - Sperry (1968): Split-brain Study
The concept of split-brain studies and the role of the corpus callosum
Split-brain studies explore how the brain functions when the two hemispheres are disconnected. This disconnection occurs through a surgical procedure known as a commissurotomy, often performed to treat severe epilepsy. The procedure severs the corpus callosum, a thick band of nerve fibres that normally allows communication between the left and right hemispheres of the brain.
Importance of the corpus callosum in brain function
- Information transfer - In a typical brain, the corpus callosum enables the exchange of information between hemispheres, ensuring coordinated processing and responses.
- Impact of severance - When the corpus callosum is cut, the hemispheres operate independently, preventing the transfer of information between them.
- Purpose of split-brain research - These studies aim to understand how each hemisphere processes information and whether they have distinct roles or independent awareness when disconnected.
Functional localisation and lateralisation in the brain
The brain is organised into specific regions with distinct functions, a concept known as functional localisation. Additionally, certain functions are predominantly handled by one hemisphere, which is referred to as functional lateralisation.
Key aspects of brain organisation
- Functional localisation - Different areas of the brain are responsible for specific tasks. For example, Broca's area, located in the frontal lobe, is crucial for speech production, while Wernicke's area, in the temporal lobe, is essential for language comprehension.
- Functional lateralisation - Some functions are specific to one hemisphere. Notably, Broca's and Wernicke's areas are typically found only in the left hemisphere, which often dominates language processing.
- Cross-body processing - The primary motor cortex in the frontal lobe controls movement on the opposite side of the body. For instance, the left hemisphere manages the right side of the body and vice versa.
Sperry's (1968) classic split-brain study and research methods
Roger Sperry's groundbreaking 1968 study focused on individuals who had undergone a commissurotomy to manage severe epilepsy. This research aimed to investigate how the disconnection of brain hemispheres affects perception, cognition, and behaviour, demonstrating that each hemisphere can function independently.
Methodological approach in Sperry's research
- Participants - The study involved 11 split-brain patients who had previously had their corpus callosum severed as a treatment for epilepsy.
- Type of experiment - Conducted as a quasi or natural experiment, as the condition (split-brain) was pre-existing and not artificially induced by the researchers.
- Visual testing method - Participants fixated on the centre of a translucent screen. Stimuli were projected for a brief duration of 1/10 of a second using a tachistoscope, a device for rapid visual presentation. Information presented to the left visual field was processed by the right hemisphere, and information in the right visual field was processed by the left hemisphere, regardless of which eye was used.
- Tactile testing method - Objects were placed in either the right or left hand behind a screen, so participants could not see them. The information from each hand was processed by the opposite hemisphere (right hand to left hemisphere, left hand to right hemisphere).
Key findings from Sperry's split-brain research
Sperry's experiments revealed striking differences in how the two hemispheres process and respond to information when disconnected. These findings highlight the independent capabilities and limitations of each hemisphere in split-brain patients.
Observations on visual and tactile processing
- Visual field recognition - Information shown to one visual field was only recognised when presented again to the same visual field, indicating no cross-hemisphere transfer.
- Verbal reporting of visual stimuli - Stimuli in the right visual field (processed by the left hemisphere) could be described verbally or in writing, while stimuli in the left visual field (processed by the right hemisphere) could not be described verbally but could be identified through non-verbal means, such as pointing.
- Simultaneous visual input - When different images were shown to each visual field at the same time, participants verbally reported only what was in the right field (left hemisphere), but when asked to draw, they depicted the image from the left field (right hemisphere).
- Tactile recognition - Objects placed in the right hand (left hemisphere) could be named and described verbally, while objects in the left hand (right hemisphere) led to guesses or a lack of awareness of the object.
- Hand-specific memory - Objects felt by one hand were only recognised again by the same hand, showing no information sharing between hemispheres.
- Independent hand selection - When different objects were placed in each hand simultaneously, each hand selected only the object it had felt, demonstrating independent processing by each hemisphere.
Conclusions on hemispheric independence in split-brain patients
Sperry's research provided profound insights into the nature of brain function in split-brain individuals. The conclusions drawn from these studies have significantly shaped the understanding of how the brain's hemispheres operate when disconnected.
Implications of split-brain findings
- Separate visual worlds - Split-brain patients appear to have two distinct visual inner worlds, with each hemisphere processing and perceiving information independently.
- Lack of cross-integration - Without the corpus callosum, there is no integration of information between the hemispheres, leading to isolated processing of stimuli.
- Independent consciousness streams - Each hemisphere in split-brain patients seems to maintain its own stream of consciousness, complete with separate memories, perceptions, and impulses, suggesting a dual awareness within one individual.