Now as we've been discussing these important structures for the production and the understanding of human language, I've been highlighting the left hemisphere of the brain. And one final point that I'd like to leave with you is the lateralization of language function In the human brain. And this gets us into a broader discussion of cerebral dominance that is, again, unfortunately beyond the scope of medical neuroscience, but perhaps you'll be inspired to learn more about that topic on your own. But at least for language function it's important to emphasize that those functions that we associate with Wernicke's area and Broca's area, are lateralized to the left hemisphere for the vast majority of us. For those of you that are right handers, as am I, almost certainly, your functional Brocha's and Wernicke's areas are present in the left hemisphere. For those of you that are left handed, then the probability of your Broca's and Wernickes area being present in the left hemisphere is more like 80%. So, really, for the vast majority of people on the planet, these functional regions are present in the left hemisphere. But of course what you really care about is what is the situation in your own brain. Unfortunately, most of us don't get an opportunity to really confirm the organization of our language areas unless we've had a brain injury or unless we've be subject to a study that would reveal the organization of these parts of the brain in the two hemispheres. So, for the right handers out there, you can be quite confident that your function Broca's and Wernicke's area are lateralized to your left hemisphere. Now, the situation with left handers is, is a bit different. This remains an active area of, of investigation. It's not that left-handers have a reverse pattern of cerebral lateralization. It's that these lateralizations seems to be less severe, or perhaps less complete. So again, about 80% of left-handers have their language centers present in the left hemisphere. So you should be wondering well, okay if you're like most of us and you have Broca's and Wernicke's areas in your left hemisphere. What about the right hemisphere? Are there corresponding brain regions? And if so, what's their function? Well indeed there are corresponding brain regions that are present in the right hemisphere. that occupied the same position in the lateral temporal associational cortex. In the case of Wernicke's area, or the Inferior Frontal Gyrus in the case of Broca's area. But there may be differences in the organization of these networks. One very striking difference that's present in most of our brains concerns the superior surface of the posterior temporal lobe, a region that we call the temporal plane, or in Latin, the planum temporale. So, this posterior temporal plane, which is highlighted here in blue, is larger than the left hemisphere compared to the right hemisphere for most human beings. Now, a simple interpretation of this is perhaps this asymmetry in plantum temporale size is what instantiates our Wernicke's area in the left temporal associational cortex. Well, for a variety of reasons, we think it's not quite that simple. In fact the symmetry may be important for serving other aspects of auditory function. One intriguing finding now about a decade old, is the idea that for those individuals that have this amazing capacity for perfect pitch perception that their asymmetry is especially pronounced. So these would include certainly trained musicians, but not just skilled musicians. There are other people out there who have this capacity. And there are certainly world class musicians that lack this ability. but nevertheless, there seems to be something important about planum temporale asymmetry that is one means, perhaps of consolidating more circuitry for efficient processing in one hemisphere. Rather than more broadly distributing it across the hemispheres, and thereby mandating communication through the Corpus Callosum. Well, so, getting back to the functions then, of the right hemisphere, we do, in fact have corresponding temporal association of cortical regions and an Inferior Frontal Gyrus in the right hemisphere. And for those of us that are strongly lateralized to the left, our right hemisphere does contribute in important ways to language function just not in perhaps the most overt means that is accessible to simple clinical testing such as comprehension and speech production. Rather what we believe is that this right hemisphere, it's especially involved in the emotional aspects of vocal communication. We believe that the temporal associational cortex of the right hemisphere is important for interpreting the prosodic quality of speech. That is, the emotional content of speech. And likewise, in the right-sided language regions of the Inferior Frontal Lobe, we have networks that seem to be responsible for imbuing speech with emotion. So, the right hemisphere seems to be very important for that quality of communication. That relies more on emotion than it does on the semantic lexicon that we access as we produce speech. And lastly, one additional component of lateralization of function is worth mentioning, is that there seems to be gender difference, and the lateralization of language capacity in the two hemispheres. Women tend to be somewhat less severely lateralized compared to men. One reason why we believe this to be the case is that following a left middle celebral artery infarction, the capacity of the female brain to recover language function seems to be greater than it is in the male brain. Suggesting that the lateralization of language function in the female brain might be less complete than it is in the typical male brain. Now some have taken this idea to the extreme and have suggested that women simply are more flexible in their ability to mobilize resources that are distributed across the corpus callossum in the 2 hemispheres. I'm not so certain that we can generalize to that point yet. But it does seem to be a, an intriguing possibility that, with lateralization being less complete in the typical female brain. Perhaps that confers some advantage in accessing cortical circuitry via callosal connections that might mobilize additional resource to solve complex problems and to manage everyday life. So, you women out there, you may take some comfort in knowing that perhaps you have more ready access to the resources in your two hemispheres. And for the men out there well, some might say that we are stuck in one hemisphere. And, given the rate of loss of callosal fibers, we might imagine that our hemispheres are slowly drifting apart. As was the point of a cartoon I saw once. Well, I, I think that's going a little bit too far here. But, it is worth considering the degree of lateralization in the human brain. And there are other dimensions of lateralized behavior, we've talked about, to some extent. We, we talked early on about human handedness and how that might be reflected in the structure of the sensory and motor cortical regions. We've talked recently about the lateralization of attentional capacity to the right hemisphere. that would help us understand why the hemineglect syndrome is most commonly seen with right parietal damage affecting our ability to interact with all things left. And now we add language to the story of lateralization. Recognizing that, for most of us, our language function is rather severely lateralized to the left hemisphere. but there are still complimentary functions of the right hemisphere that are necessary to have a well rounded, integrated capacity to communicate effectively. Not just with the semantic content of her speech, but also with the emotions that are so critical for conveying genuine human to human interaction. Well there's so much more we could've said about human memory, about brain and language. I know we've only scratched the surface so I would encourage you if that is an area of special interest to you to pursue the studies through other resources, other means. Hopefully we'll have other kinds of online courses available that will allow you to pursue your specialized interest in these dimensions of cognitive neuroscience. But for our purposes here in medical neuroscience, I want to focus on the anatomical framework that I've tried to provide for you. So it's critical for you to understand the localization of the hippocampus and the parahippocampal gyrus, and the function of that brain network in acquiring and consolidating declarative memory. Likewise, it's critical that you be able to recognize the major language centers in the human brain. And the types of aphasias that result with localized focal injury to the lateral temporal associational cortex, and, to the Inferior Frontal Gyrus. So with those localizing points of emphasis in mind, I think we can wrap up now our session on the temporal associational cortex and anticipate our next time together. Where we will indeed continue to explore the associational cortex of the frontal lobe and consider yet additional dimensions of brain function that pertain to cognition. So I'll see you next time, across the lateral fissure in to the anterior part of the associational nework.
