[MUSIC] In the 150 years or so since Broca popularized the notion, that aphasia is a disorder of the left hemisphere. We've learned a great deal about how language is organized in the brain. And indeed as Broca and Dax realized it is organized largely in the left hemisphere. Let's look look at a simple circuit that supports language. Spoken language comes in to auditory cortex. It comes in to auditory cortex on both sides. But, on the left side, this information is processed both ventrally in the temporal lobe and in this area right here. You see this area right here is called the temporoparietal junction, or TPJ. And everything from this sensory processing of auditory information, this sensory processing of speech sounds, is going to end up in the temporoparietal junction. And then information goes into the frontal lobe. And I've drawn this in two different colors for a reason, which is that this dorsal pathway that comes into the frontal lobe is responsible for speech production. And this ventral pathway is responsible for speech comprehensive. Understanding speech, so let's try and understand a little bit about how that works. Sounds come in, and we have to evaluate these sounds on all sorts of different time scales. We have to understand [NOISE], [NOISE]. And then we have to put them together into phonemes. And then we have to put them together into syllables, SYLL-A-BLE. And then we have to put them into a chain of syllables, syllables. And then we're going to try and figure out what the heck does syllable mean? Well the job of this ventral pathway is simply to organize it and produce this interpretation of sound that is then sent to this area right here, the temporoparietal junction. And we can call the function of this is as a lexical interface. This is where we're going to get this sensory information and we're going to attach a meaning to it. We're going to attach a word, a meaning, a semantic meaning to this. This is the critical place. And from there, we can then go and also make speech. So from this lexical interface, we can make speech. Now, there are a lot of implications here which I want to point out. One is that let's say a person's born deaf, and they don't hear anything. This is not particularly useful so let's say that they either use sign language they're using visual input or they're using braille sensory input. Well as it turns out in both the situations of either sign language or braille, people use visual cortex. And they send that information Into the lexical interface. So this same, this lexical interface is the one place where we take sensory information we make semantic meaning out of it. So, how does that work? How do I know that? Well, one what that we know that is from this amazing singular case of a woman who was born blind and was a braille reader. She had a stroke in her visual cortex. Okay, the stroke was imaged and it was in her visual cortex. And she could no longer read braille. Now Braille is a somatosensory input. Why did she lose that ability if she loses her visual cortex? Because the somatosensory information in her was sent to the visual cortex. And this is where we get input for reading, for sign language, and as it turns out for Braille. Now, what's remarkable about her is that she had no problem detecting textures. She could say, that's a bump. But she could not make any meaning out of the Braille bumps, okay. So language comprehension, understanding language, is going to take place here. And as it turns out, there are two places where it can get its input from. Either auditory cortex, or visual cortex. Another very interesting thing that we can learn from this circuit is that if this is speech production, what about the person who is signing? Well, as it turns out, lesions in this area cause a problem with both speech production, the production of spoken words, and with signs. And what's really important there, is that, that shows you that this is a disorder of language. It is not a disorder of articulation. It's not a disorder of the throat. It is not a disorder of the larynx. It's not a disorder of moving your body. It is disorder of language. And no matter what the mode of language output is. Broca's area will affect it. What about the other major type of Aphasia? The other major type of Aphasia as you may have heard is called Wernicke's and it's also called a fluent Aphasia. And in general, lesions somewhere around here produce this fluent Aphasia. And in this situation the problem is more with comprehension. Production, speech production is fine but speech comprehension and reading comprehension are impaired. And, for example, the most subtle forms are going to affect the understanding of complicated sentences. So, for example, yesterday, two of my cats had a fight. Toola hit, I'm sorry. Minnie hit Toola, Minnie hit Toola. A person with Wernicke's or fluent aphasia might be able to understand that Minnie hit Toola. But would probably not be able to understand that Tula was hit by many. So these complicated grammatical constructions are much more difficult. Finally, why would we pair the sensory understanding of words with its production. And I think there are a few reasons. One, we have to learn language and we learn language from hearing it. So in order to seed our ability to talk, we have to be able to hear and comprehend speech. So putting these two circuits together, linking them, makes a lot of sense. And then, we learn new words. We learn new names, we learn new words and we might even learn a new language. Again, that's going to need, that's going to require linking this sensory input with this motor output. And one of the ways that we know that this is always operating Is that when people lose their hearing their speech production tends to degrade. So what that means is that everyday as we're talking we're listening to ourselves and grading ourselves and changing and we're re-learning and making sure. We're having quality control on our speech production by listening to ourselves. When we can't listen to ourselves speech production is going to degrade. So this is a really nice simple way to look at language. It's a devastating, to lose any facility with language. And the good news is that if one loses the facility due to a stroke, improvement is more the rule than the exception. [MUSIC]
