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The corpus callosumThe band of nerve fibres connecting the brain's left and right hemispheres.Read more → matters because it is the main bridge connecting the brain's two hemispheres, and cutting it produces a strange, very specific pattern of symptoms. It is the largest white matterBrain tissue made mostly of the insulated, myelin-covered wires connecting nerve cells.Read more → commissure (a bundle of fibres connecting the left and right hemispheres) in the brain, allowing the two sides to share information constantly. For example, imagine information about an object seen in the left visual field: it normally crosses the corpus callosum to reach the language centres on the left side, so the person can name it. In practice, surgically cutting this structure, a callosotomy, was once used to treat severe epilepsy, producing the classic 'split-brain' syndrome first described by Roger Sperry.

In split-brain patients, information reaching the right hemisphere can no longer cross to the left hemisphere, where the main language centres sit in most people. An object shown only to the left visual field is processed by the right hemisphere, but the patient cannot name it, because naming requires the isolated left hemisphere's language areas. The right hemisphere can still show it understands, for example by pointing to the object with the left hand, since each hemisphere controls the opposite side of the body. Alien hand syndrome, where one hand acts seemingly on its own without the person feeling in control of it, can also follow corpus callosumThe band of nerve fibres connecting the brain's left and right hemispheres.Read more → damage, since the two hemispheres no longer coordinate voluntary movement smoothly. Agenesis of the corpus callosum is a congenital condition where the structure fails to develop at all, and can range from silent to associated with intellectual disability, depending on how well other pathways compensate. Take a split-brain patient shown a spoon only in their left visual field: they deny seeing anything when asked to name it, yet correctly pick out a spoon by touch using their left hand. In practice, this dissociationA disconnection between a person's thoughts, feelings, memories, or sense of identity, often as a response to stress.Read more → between naming and non-verbal recognition is the hallmark finding used to demonstrate split-brain syndrome.

• Which condition causes demyelination of the corpus callosumThe band of nerve fibres connecting the brain's left and right hemispheres.Read more → in people with chronic alcohol use?
• Which part of the corpus callosumThe band of nerve fibres connecting the brain's left and right hemispheres.Read more → carries fibres connecting the occipital lobesThe part of the brain at the back of the head, responsible for processing vision.Read more →?
• In roughly what proportion of right-handed people is language localised to the left hemisphere?

• Corpus callosum, the main commissure connecting the two hemispheres, unlike intrahemispheric tracts such as the arcuate fasciculus which connect regions within one hemisphere.
• Split-brain syndrome, an object in the left visual field cannot be named, whereas the same object is correctly identified by touch using the left hand.
• Alien hand syndrome, a hand acts without felt voluntary control after callosal damage, rather than the weakness seen with a corticospinal tract lesion.
• Agenesis of the corpus callosum, a congenital absence of the structure ranging from silent to disabling, unlike a callosotomy which is a deliberate surgical cut in an adult.

• Central pontine myelinolysis follows rapid sodium correction rather than affecting the callosum.
• Wernicke's encephalopathyAn acute brain condition from thiamine deficiency causing confusion, ataxia, and abnormal eye movements.Read more → affects the mammillary bodies and periaqueductal region.
• Korsakoff syndromeA chronic memory disorder, usually from thiamine deficiency, marked by severe short-term memory loss and confabulation.Read more → is the amnesic sequel of thiamine deficiency.
• Alcohol-related cerebellar degeneration affects the vermis and causes gait ataxiaLoss of full control of body movements, causing an unsteady, uncoordinated gait or actions.Read more →.
• The genu is anterior and carries prefrontal fibres.

• Localise the clue to the relevant anatomical level, cell type, pathway or molecular process.
• Map that structure or process to its normal function.
• Predict the deficit or clinical sign produced when it is disrupted.
• Reject options that belong to a neighbouring structure, pathway or level of explanation.