Well, we need to move on and consider one other domain of function associated with the temperal lobe and that would be language. again, another facinating topic. We could have a whole course just devolted to the brain and language. Well, in fact, we do. We have courses here at Duke University on this very topic. So my aim with you rather is to simply give you some of the basics that you need to understand functional localization in the human brain. Well, humor me for a moment, will you? And let's test your facility for the recognition of words, at least in presentation in English language form. So, let's read together the following text. According to research at Cambridge University it doesn't matter in what order the letters in a word are the only important thing is that the first and last letter be at the right place. The rest can be a total mess and you can still read it without a problem. This is because the human mind does not read every letter by itself but the word as a whole. Well for those of you who may not be native English speakers, perhaps you've struggled a little bit with this challenge. But you can create your own such text in your native language and demonstrate for yourself and your friends and your family really this amazing facility of the human brain. To take these arbitrary visual symbols and interpret them with meaning and semantic content. And to run them through our language circuit in a way that allows us to produce the articulated vocal sounds that are associated with these relatively arbitrary visual symbols. Well, this is evidence of the amazing facility of the human brain for manipulating symbolic representation derived from sensory stimuli and using it to communicate complex ideas and complex thought in the form of speech. So how does this work in the brain. Well, there are regions of our associational cortex primarily in the left hemishpere that are responsible for such amazing linguistic feats. Two regions in particular we need to discuss for just a few moments. One, we find associated with the lateral part of our temporal associational cortex. And this is named after a German physician, Carl Wernicke and most of us in the west will pronounce that as Wernicke, Wernicke's area. And it is often found in the superior temporal gyrus. Roughly the posterior 1 3rd of the superior temporal gyrus. But I think the reality is, is that there is not a single module in this lateral temporal cortex that we can identify as Wernicke's area. Rather I think we need to understand that there is probably vast network. Of nodal points that are critical for the recognition of these sensory stimuli and their interpretation their encoding in terms of semantic contact. So the actual critical nodes in this network. Maybe found in any one of a number of places. And indeed for those of you that are bilingual or multi-lingual, you may have distinct nodal points that represent meaning the language that is native to your personal life history. Some of you I'm sure can communicate using the symbols and gestures that we call American Sign Language. And for the representation of those sensory stimuli there may yet another set of key nodal points that establish an associational network here in this lateral temporal cortex. So in, in my view and I think this is the view of the neurosurgeons that actually map out these regions in the human brain. There's unlikely to be a single region that is consistently localized in every human brain that we can label as Wernicke's area. Rather at least the job of the neurosurgeon will be to figure out what are those key nodal points in the networks that represent the semantic knowledge that you operate with. that are essentially, untouchable from a neuro-surgical perspective that is. The key nodes that the surgeon is trying to identify so that she or he can preserve those nodal points during the course of an operation. So from this point I think this figure from our book is just a little bit misleading. I would suggest that there's not a single modual that we can vocalize and call Wenicke's area. Rather I believe that there is a more extended network in the associational cortex of the lateral temporal lobe that is processing these sensory symbols that are derived primarily through vision and auditory senses. And then interpreting them and applying semantic meaning to those sets of symbols. And this network then does the job of encoding the meaning of these symbols. And in terms of language function, this is where the understanding or the comprehension of language is built up in the human brain. So I think you can understand what might happen then if there would be some damage to key nodes in this lateral temporal associational network. One might still have the capacity to make speech but one would expect severe impairments in the ability to comprehend speech. Now our focus in this session is indeed on the temporal associational cortex. But I can't help but to jump across the lateral fissure and get into a inferior lateral region that we call Broca's Area. Broca's area as many of you know, is an area that is involved in the production of speech. Broca's area was named after Pierre Paul Broca who described a patient with damaged this part of the brain and in life following this injury, the patient was severely impared in his capacity to make speech. Now, we now understand Broca's area really as part of a premotor cortical network. You recall, we described a mosaic of frontal cortical areas that are associated with the planning of movement. Especially movement that's directed away from personal space. So this aspect of, of planning for movement also applies to the governance of our vocal articulatory apparatus in our throat. Our larynx, our pharynxy the coordination of our breathing muscles. So, this is, in a sense, a premotor cortex that interacts closely with the nearby regions of the precentral gyrus that have upper motor neuronal control over our vocal motor apparatus. So from that standpoint we can understand Broca's area as a premotor cortex for vocal articulation. But it's not just for vocal articulation, Broca's area also seems to be involved with the production of written language. Which would have it affiliating with not just the lateral segment of the precentral gyrus but also the middle segment of the precentral gyrus that governs the movements of the distal upper extremities. The part of our body that we use to write with. So damage to this part of the inferior frontal lobe, specifically what we're referring to is the posterior one-third of the inferior frontal gyrus. And, there are various subdivisions of this part of the inferior frontal gyrus that we could identify. I think from neural surgical procedures, it seems clear that there may be, a set of nodal points within the networks of this posterior inferior frontal gyrus. That are critical for the expression of language. So again like Wernicke's area is not a single module that we can point to and identify. I think the same is likely to be said of Broca's area although the localization seems to be much more consistent from one person to the next. In our brains than would be Wernicke's area in the lateral temporal cortex. So here's roughly the location of Broca's area in the human brain and this is a region that if damaged can produce a severe impairment in the fluency of speech. With an understanding then that Broca's area can be conceived of as an essential premotor network for the production of speech. One can imagine that damage to Broca's area would lead to a severe inability to produce speech. Not just speech in oral form but also the written language will also be affected with damage to Broca's area. So it's critical for anyone headed towards a career in health professions. Or indeed practising in health professions to be able to differenciate between two forms of impairment that are associated with these critical language areas in the human brain. So an impairment of language function is called aphasgia and if the impairment is afflicting the lateral temporal cortex then we call this Wernicke's aphasia. If the injury is to the inferior frontal gyrus then the aphasia that we would expect is called Broca's aphasia. So let's differentiate these two conditions. So the key differentiation between these two forms of aphasia is that with injury to the inferior frontal gyrus in Broca's aphasia, comprehension is intact. But there is a severe impairment in the fluency of speech. Whereas with Wernicke's aphasia, the situation is somewhat reversed. There is fluency of speech. But the capacity for comprehension is severely impaired. Now these two parts of the brain. The lateral temporal associational cortex in the inferior frontal gyrus are interconnected by white matter structures that span this space within the brain. Between this temporal lobe around the lateral fissure of the brain and into the posterior part of the inferior frontal lobe. One can have damage of those white matter pathways. And see some combination of these deficits that reflect the failure of communication between this lateral/temporal associational cortex, and the inferior frontal gyrus. One might also have. Let's say a stroke involving the higher distribution of the middle cerebral artery and have a global aphasia that affects both the lateral temporal associational cortex and the inferior frontal gyrus. So while it's possible to have one type of aphasia or the other. One can see elements of both or one can see a global aphasia impacting all aspects of language function. Now since we're talking about aphasia and Broca's aphasia in particular, I just can't help. But to point you to a fantastic video that, was, juried and awarded, the grand prize in our inaugural, brain awareness week video contest sponsored by the society for neuroscience. And, this is a beautifully produced and just wonderfully rich. story told by our graduate student in the field of neuroscience from Australia. Her name is Shiree Heath and she's telling the story of her grandfather's stroke who suffered from a Broca's aphasia. So I would encourage you to just take a few minutes. Navigate to this page at brainfacts.org. And you can search aphasia or The Treasure Hunt, which is actually the title of this short video. And, I think you'll agree with the judges of this contest that, Sheree was. very much deserving of the grand price, given her insight. Given the accuracy of the neuroscience in her presentation. And of course her marvelous creativity with which she pulls all this together. And in the end you'll get a change to explore and reinforce some of what we've been talking about here recently about the language centers of the human brain.
