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The ability to understand language and produce speech is associated with several areas of the cerebral cortex. Basically, spoken language is first perceived in the auditory cortex, while written text, or sign language, is processed in the visual cortex. This information is then sent to the Wernicke’s area, in the temporal lobe, where it is matched against the person’s vocabulary stored in the memory. This is where meaning is assigned to words and language comprehension is achieved. The signals are then transmitted via a bundle of nerve fibers, known as the arcuate fasciculus, to Broca’s area in the frontal lobe. Broca’s area is responsible for production of speech. Output from Broca’s area goes to the motor cortex which controls muscle movements necessary for speech.
A language disorder caused by brain damage is called aphasia. Lesions in the Wernicke’s area cause sensory, or receptive, aphasia. Wernicke’s aphasics have trouble understanding language, whether it is spoken or written, but have NO motor problems. They can speak at a fluent pace but their speech is often INcoherent. It can be described as a strange mixture of words that may sound like complete sentences but makes no sense and has nothing to do with the subject of conversation.
Patients with lesions in the Broca’s area, on the other hand, CAN understand language, but have difficulties speaking. They talk slowly, searching for words, forming INcomplete sentences with poor syntax, but usually manage to say important words to get their message across.
In the early days, research of language pathways was based mainly on studying patients who had a specific language deficit that could be associated with a specific brain damage. Nowadays, advanced brain imaging techniques allow mapping, in real time, the areas of the brain that are activated when a person carries on a specific task. Thanks to these techniques, a THIRD area is found to be essential for language comprehension: the inferior parietal lobule. This lobule is not only connected to both Wernicke’s and Broca’s, but also to the auditory, visual, and somatosensory cortical areas. The inferior parietal lobule is therefore perfectly wired to perform a multimodal, complex synthesis of information; it can process and connect different word elements such as the sound of the word with the look and feel of the object.
The languages centers are usually located in ONLY ONE hemisphere – the “dominant” hemisphere of the brain, which is the LEFT side in RIGHT-handed people. The corresponding areas in the right hemisphere are responsible for the emotional aspect of language. Lesions in the right hemisphere do NOT affect speech comprehension or formation but result in emotionless speech and inability to understand the emotion behind the speech such as sarcasm or a joke. The right hemisphere may also develop to take over the MAIN language functions if the left side is damaged in early childhood. This phenomenon is known as neuroplasticity.
