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Language and the brain: Aphasia and split-brain patients

Video transcript

you may perceive language as one big function the brain performs interestingly though it's divided into a lot of sub functions this video will 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 with in 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 nonfluent 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 Verna Keys area back in the temporal lobe is damaged you get Verna keys aphasia which is quite a different pattern of behavior than you get with Broca's aphasia 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 Verna Keys aphasia which is also sometimes called fluent aphasia can also have trouble understanding what other people say and when both Broca's aphasia and Verna gives 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 aphasia and verdict is 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 a 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 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 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 thick band of nerve fibers that connects your brains to 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 on 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 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