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Language and the brain: Aphasia and split-brain patients
You may perceive language as one big function the brain performs. Interestingly, though, it's divided into a lot of sub-functions. In this video, we'll discuss how your brain speaks and understands language and what happens when those functions are disrupted. First, let's go over some basic neuroanatomy. For about 90% of right-handed people, language functions are centralized in the left hemisphere of the brain. Lefties and ambidextrous folks are somewhat more likely to have language centralized in the right hemisphere, but still about 70% of them will have language centralized in the left hemisphere. Within whichever hemisphere is dominant, the two main areas associated with language are Broca's area, which helps us speak, and Wernicke's area, which helps us understand language. Broca's area is in the frontal lobe, usually the left frontal lobe, and it's responsible for language expression. When Broca's area is damaged, people tend to have trouble producing speech. Their words become halting or slurred. This is called non-fluent aphasia, or Broca's aphasia. I remember this by thinking that Broca's aphasia means "broken speech," and aphasia is just any type of disorder that involves language. When Wernicke's area, back in the temporal lobe, is damaged, you get Wernicke's aphasia, which is quite a different pattern of behavior than you get with Broca's apahasia. People have no trouble producing words-- in fact, words kind of just tumble out of them-- but the words that do come out don't make any sense. It's like listening to a bunch of nonsense sentences. People with Wernicke's aphasia, which is also sometimes called "fluent aphasia," can also have trouble understanding what other people say. And when both Broca's aphasia and Wernicke's aphasia are present, then you have something called "global aphasia," because it globally affects language instead of only affecting a subsection of it. Broca's area and Wernicke's area are connected in the brain by a bundle of nerve fibers called the arcuate fasciculus. One cool thing is that this loop is also found in deaf people who know sign language. So it's not specific to a spoken language, but the brain adapts to use whatever modality is necessary for communication. When this connection is damaged, people experience something called "conduction aphasia." Their ability to conduct information between listening and speaking is disrupted, which makes them unable to repeat things, even though they understand what's being said. Pretty crazy. And once you think about how many language-specific functions you do every day, you might wonder how many different types of aphasia there are. And the answer is "a lot." We have agraphia, which is the inability to write, anomia, which is the inability to name things, and other specific difficulties in reading, spelling, grammar, pronunciation-- all sorts of things you may not even realize that you do effortlessly with a healthy brain. But Broca's apahasia and Wernicke's aphasia are the most common. Language is just one example of how our brain works in general. Big tasks are subdivided into small tasks, which are then spread around in different parts of the brain. And this can actually be a good thing, because it means if you have very localized or very specific brain damage, then you probably won't completely lose your ability to perform some highly important function like communication. In fact, when functions are divided like that, it's easier for your brain to adapt. For example, when people have strokes that affect the left hemisphere of their brain, they may have trouble speaking or something right after the stroke, but over time and with proper therapy, some of those people are able to retrain other speech-related parts of their brain by creating new connections between neurons. Building these connections, in combination with some recovery of the originally damaged part of the brain, can help these people speak again with at least some degree of fluency. And the brain's ability to adapt and move functions to new parts is called neural plasticity, or synaptic plasticity. The neurons are plastic, or flexible enough, to learn new routes and connections, thus allowing undamaged parts of the brain to take over functions that the damaged parts previously performed. Even with perfectly functioning hemispheres, you might still have trouble naming objects. This can happen if communication between the hemispheres is disrupted by severing the corpus callosum, which is a band of nerve fibers that connects your brain's two hemispheres. This creates what's called a split-brain patient, because your brain is now split into two parts. People used to have to do this sometimes as a treatment for seizures, but in addition to helping with that problem, this surgery creates some pretty interesting side effects in terms of language. So assuming that language is centralized in the left hemisphere, this means that the right side of your brain can't connect to the language side. So anything that you perceive in the right side of your brain can't be named or dealt with in terms of language. Now, you may have heard before that your brain has what's called a contralateral organization, meaning that information that you perceive in your left visual field gets processed by the right side of your brain and vice-versa. So let's think about what that means for a split-brain patient. If you see an object on your left, and it gets sent to your right hemisphere, you won't be able to name it. You'd still be able to pick it up with your left hand, because again, your right hemisphere is controlling your left-side motor neurons, but you'd have to kind of turn your head so that the object would be in your right visual field before the language part of your brain would have any access to it. Now, when I say the right visual field, that doesn't mean just your right eye. It means the right side of your body, which you can view with half of each eye. So there you go. Lots of pretty cool implications for how our brain is organized and subdivided in terms of language.