Welcome back to unit six of medical neuroscience on complex brain functions. And in this tutorial, I'd like to talk to you a bit about the associational cortex that we find in the parietal lobe of the human brain. Our tutorial relates to two of our core concepts in neuroscience. We are talking about the associational cortex. And it's from the integrated action of these networks in the brain that intelligence arises, as the brain reasons, plans and solves problems. And, as I said last time, we wouldn't be here, and you certainly wouldn't be with me this deep in medical neuroscience, had it not been for the innate curiosity that your brain has given you. Which gives you a desire to understand how the world works, even the inner workings of the human brain itself. So, I appreciate your efforts and I certainly am right there with you in celebrating the curiosity that our brain has endowed us with. Okay, well, moving into our learning objective for today. there's just one objective. And that is for you to be able to discuss the major functions that are localized to the associational cortex of the parietal lobe. Well I'll remind you that when we are speaking of the associational cortex what we are referring to really, is the 75% or so of the cerebral mantle in each hemisphere that is beyond our primary sensory and motor region. So, for our purposes here, we are focusing in on the parietal lobe, which is posterior to the central sulcus, superior to the lateral fissure and then, bounded from the occipital lobe by this imaginary boundary. That we recognize as the parietal occipital sulcus emerges from the dorsal midline and extends down to this feature that we call the preoccipital notch. And the preoccipital notch is formed by. the tentorium as it impinges upon the developing ventral surface of the hemisphere. Well, so this is our parietal lobe, and specifically we're concerned with what we might call the posterior parietal lobe. Which is that territory that lies posterior to our primary somatic sensory region. Anatomically speaking, posterior to at least the crest of the postcentral gyrus, if not posterior to the postcentral sulcus. So if I could just show you an image of the human brain using my model here. Here again is our central sulcus. So this would be our postcentral gyrus. So we're talking about the formations of the posterior parietal lobe. And if you remember back from unit one, the fingers on the gyri experience? I suggested that you can conceptualize the parietal lobe as the cortex that's posterior to the central sulcus. So the space between my first and middle fingers is the central sulcus. My index finger would be the postcentral gyrus. So what we're talking about today basically is the cortex represented by my index and middle fingers of my left hand. The index finger is representing the superior parietal lobule, and the middle finger is representing the inferior parietal lobule. With the sulcus between these lobules being the intra-parietal sulcus. So, if you don't recall that aspect of human brain anatomy, why don't you spend a few minutes back in unit one and refresh your memory of these structures. Better yet maybe you can find a large piece of paper and a white board, and once again draw the lateral view of the human brain to make sure you're on board with these divisions of the parietal lobe that will be our focus in this session. Okay, well that is the focus for today from an anatomical point of view. Now let's talk about function. So, when we consider the functions of the posterior parietal cortex, the principal function that we want to think about is attention. So, recall from our brief survey of various components of cognition, we started with attention. This cognitive search light, this ability to illuminate a limited region of our world, and to focus on the information derived from that region. Now, when we speak specifically of the role of the parietal cortex in attention I want you to understand that I'm not claiming that all of attention is seated in the parietal cortex. that's not the case at all. Attention is a complex and multifaceted neuro-biological mechanism that is broadly distributed throughout the brain, rather the role of the parietal network. Seems to be to direct our attention to particular locations in the outside world or perhaps to the body surfaces as it interacts with that world. So it's really the direction of attention that is principally the function of the posterior parietal cortex and the associational networks of the slope. So let me give you an example of the role of the posterior parietal cortex in directing our attention. I'll show you some data that is illustrated in our textbook. So, this is figure 26.9. And it illustrates how attentional mechanisms in the posterior parietal cortex can increase the firing rates of neurons in response to stimuli that happen to be within that cognitive searchlight. That is, that happen to be the object of the subject's attention. So this is an experiment done in a non human primate model and a micro electrode is inserted into the posterior parietal cortex. And action potentials are being recorded, and what we find in panel A are post stimulus time histograms. reflecting the activity of these neurons when the monkey is instructed to either attend to the location of the stimulus or to not attend to the location of the stimulus. So, as we can see very clearly and definitively, when the animal is instructed to ignore the target stimulus, that is otherwise present within the receptive field, you'll notice that there's only a modest increase in activity. Compared to what happens when the animal is given the queue to attend to the target. Now there's a huge increase in activity for that location in the visual world that has captured this animal's attention. So, what we see by various behavioral measures Is that this animal is directing attention to a particular stimulus. And that attention can be measured in terms of the amount of time that the animal is dwelling on the target. As well as the amount of reward that the animal receives for doing the behavioral challenge that the experimenters have put forward. Now when we relate these behavioral metrics to neuronal firing what we discover is that neuronal firing is rather tightly related to the behavior as indicated by the amount of reward that the animal has received. And this simply indicates that attention can have an effect on the firing rates of neurons that are responding to sensory stimuli. And what attention does, is shifts in effect the dose response relationship, or the gain of that function. So with no particular direction of attention or, anti-attention, that is, directing the animal to ignore the stimulus, we imagine that this function would be much more flat. But what attention does is it increases the gain of this relationship between stimulus and neuronal response, or in this case. between the location of the stimulus and the behavior that's motivated by that stimulus. So, you know, I sometimes reflect upon this and my own behavior, and as an educator I get to meet, lots of interesting young people. And unfortunately I don't do very well remembering their names and associating their names with their faces. And, well sometimes we say, I'm just terrible with names, and don't hold this against me. But really, I look at these data and I think maybe I'm just not paying attention. And that's something I can do better, so I want to commit myself to paying more careful attention to being, as my communications colleagues like to say, to be in the moment, where our attention is focused on the task at hand. Be it a sensory task or a cognitive task, an emotional task, whatever it happens to be. I would encourage us all to be in the moment and to reflect upon how attention can change the gain of our response to a given set of stimuli. Well, the same basic effect can be seen within our early sensory cortex. And this is an example of how attention can modulate the tuning properties of neurons even in the early visual cortex. So what we're looking at here is a tuning function. So we call this an. tuning curve, in this case, for orientation of the visual stimulus. As you remember, neurons are selective for particular orientations of visual stimuli and particular directions of stimulus motion. Here, we're looking at the tuning for the orientation of the stimulus. And this neuron is tuned somewhere for probably a visual stimulus that is more or less vertical but perhaps tilted in the left root oblique direction. Now notice what happens when the animal is given a cue to pay attention to the orientation of a particular location of the visual world. What we see is for those neurons whose receptive fields are being stimulated, the tuning function gets much sharper. So this neuron is becoming more selective and more responsive as a function of this attentional signal. Now, what we don't really understand in full yet is well, where does this attentional signal come from? What's responsible for this effect where we see an increase in the response and the selectivity of neurons, even in early sensory cortex? Well, we imagine that this parietal associational network has a significant role to play. Perhaps there are feedback projections. From parietal cortex, into occipital cortex. Perhaps there are, systems that allow the parietal network to recruit other modulators of attention, such as those basal fore brain cholinergic neurons that we spoke about last time, or our biogenic amine systems in the brain stem. We don't quite yet know how all this works together, but we at least have a sense of who the major players are in mediating these attentional effects in sensory processing.
