The human brain is a complex organ, controlling bodily functions and thoughts. It comprises two distinct halves, the cerebral hemispheres, which are symmetrical. Each hemisphere is specialized for certain functions, with the left side generally controlling the right side of the body and the right side controlling the left. Normally, these hemispheres work in concert, sharing information to produce a unified experience and coordinate actions.
The Corpus Callosum: A Bridge Between Hemispheres
Connecting the two cerebral hemispheres is the corpus callosum, a bundle of nerve fibers. This large white matter structure, located beneath the cerebral cortex, facilitates communication and information exchange between the brain’s left and right sides.
This bridge allows nerve signals to move between hemispheres, coordinating sensory information, motor control, and higher cognitive functions like memory and language. This exchange ensures the two halves of the brain operate as a single unit, enabling complex thought and coordinated actions. Different regions of the corpus callosum transfer specific types of information, such as visual, auditory, and somatosensory signals.
Why Sever the Bridge? Medical Indications
Severing the corpus callosum, known as a corpus callosotomy, is performed to treat severe, intractable epilepsy. This surgery is a last resort for individuals whose seizures do not respond to medication or other therapies. It aims to prevent or reduce the spread of epileptic activity from one hemisphere to the other.
Many patients undergoing a corpus callosotomy experience generalized seizures, particularly atonic seizures (“drop attacks”), where they suddenly lose muscle tone and collapse. By disconnecting the corpus callosum, the surgery limits seizure impulses to one side of the brain, reducing the severity and frequency of these widespread seizures and lowering injury risk. It does not stop seizures entirely, but rather contains them to one hemisphere.
The Direct Impact: Disconnected Hemispheres
Severing the corpus callosum directly disrupts communication between the two cerebral hemispheres. This separation leads to “split-brain syndrome” or callosal syndrome. Each hemisphere functions independently but can no longer share information or coordinate processes.
One hemisphere becomes unaware of the sensory input, thoughts, or actions processed by the other. Information presented exclusively to one side cannot be directly accessed by the other, leading to unique cognitive and behavioral manifestations. Though patients often appear normal in daily tasks, this separation results in distinct changes in perception and cognitive function.
Manifestations in Perception and Action
Interhemispheric disconnection from a corpus callosotomy manifests in various behaviors and cognitive functions. For visual processing, information from the left visual field is processed by the right hemisphere, and from the right visual field by the left. A split-brain patient shown an image only in their left visual field may not be able to verbally name it, because the visual information goes to the right hemisphere, which typically lacks strong language centers, and cannot be transferred to the language-dominant left hemisphere.
Motor control can also be affected, leading to independent hand actions. For example, a patient might try to button a shirt with one hand while the other simultaneously tries to unbutton it, a phenomenon sometimes called alien hand syndrome.
Language and speech can also be impacted. While the left hemisphere is generally dominant for language, information processed by the non-dominant right hemisphere may be difficult to verbalize. Similarly, if a split-brain patient touches an object with only their left hand without visual cues, they may struggle to name it. This is because tactile information from the left hand goes to the right hemisphere and cannot be relayed to the left hemisphere’s language center.