Okay, well let's talk now about the frontal eye fields. So the frontal eye fields are a region here in the anterior part of the pre-motor cortex in the posterior part of the superior frontal and middle frontal gyri. So they're probably just outside of the pre-central gyrus. And a region that we called Brodmann's area 8. Now there is some concern to whether the frontal eye fields that have been described in non human primates are exactly in a location that we would predict in the human brain based on these non human primate studies. but I think this is a somewhat controversial question. Don't be surprised if some might say that the frontal eye fields are actually Brodmann area 6. Which is what we usually associate with the premotor cortex but the non hearing primary literature would put a bit more interior. And will be called Brodmann's area 8 which is localized right about here in the anterior part of the pre-motor cortex. So the frontal eye fields are important in directing our gaze towards a visual target that we actually choose to look at. So, we make a decision, we generate a motion plan, which is common to what is going on in the primary motor cortex and then we, we execute that plan. And the execution of the plan involves descending projections from the frontal eye fields to the ipsilateral superior colliculus. And to the contralateral gaze centers of the reticular formation. The paramedian pontine reticular formation in the pons for horizontal movement and a vertical gaze center in the midbrain. Or the mesencephalon for making vertical eye movements. Now the fact that the frontal eye fields projects to both the superior colliculus and the gaze centers in the brain stem suggests that there may be some redundancy. To the means by which circuits of upper motor neurons control eye movements. And the fact that may be the case. what we know from studying animal models, and even from studying clinical patients is that image to either the superior colliculus or the frontal eye fields can result in a deficit in saccadic eye movements. But over some period of time typically a couple of months that deficit seems to go away. And saccadic eye movements are now relatively normal. And so, it appears as though this redundancy allows the function of one structure to be compensated for by the increased capacity of the other to direct the positions of our eyes in the orbit. But nevertheless, there is an important distinction that I want to make between the operation of the fu-, of the frontal eye field and the superior colliculi. Concerning the frontal eye field, I want you to think of the intentional saccade that is, you actually intend to make a movement. I might ask you to look and see what time it is, and so you need to decide. Okay, where's my nearest clock and where do I need to look in order to see what time it is. So, you generate a shift of your gaze towards that new visual target, so that's a saccade that you tend to make. So that's a frontal eye field function, and the frontal eye in one hemisphere is going to drive your gaze to the opposite side. So that's important to keep in mind if you're in a clinical setting, and you're taking care of a patient that perhaps has had a stroke. In this anterior part of the premotor cortex. you may notice that this patient doesn't generate a lot of voluntary eye movements across the mid-line to the sine opposite the stroke. And that knowledge should inform you about how you approach that patient. Or it may help guide your interactions with perhaps friends and family that may be visiting that patient in the hospital, or wherever the care setting happens to be. So just be aware that the frontal eye field is directing voluntary shifts of attention across the mid-line to the opposite side of the visual field. Now the superior colliculi, on the other hand, they generate what we sometimes call express saccades. So these are saccades that are stimulated by some kind of sensory stimulus that we don't expect. We're not necessarily planning to make that movement. For example, imagine that you're outside as I am and it's spring time, and one common phenomenon that we have in Central North Carolina in the spring are thunderstorms. I, I don't think we're in for a thunderstorm today. But perhaps there might be, and it may be that the first sign of a thunderstorm that I would appreciate is a bolt of lightning, that might catch my attention off to one side. So I'm not expecting that. It just happens and my eye movement system just might be quick enough to make a saccadic eye movement in the direction of that lightning strike. To be able to catch just a glimpse of that striking visual, visual object. Well, that's an example of an expressed saccade. I didn't intend to make it, but the sensory stimulus gabbed by attention and induced a reflexive shift of my gaze in the direction of that stimulus. Well the same may be true if the first sensation concerning the impending thunderstorm, was not the sight of a lightning bolt, but the sound of thunder. If there should be a thunderclap off in the distance, I may find myself turning eyes and head and neck in the direction of that auditory stimulus. So that would be an example of the auditory pathway, providing input to the superior colliculus and generating the appropriate shift of eyes, head, neck. Well, let me conclude by simply stating that like all upper motor neuron systems, the frontal eye fields of the superior colliculus are subject to modulation. The circuity that runs through the basal ganglia. And even through the cerebellum in the basal ganglia, there are circuits that constitute their own functional stream that run through the caudate nucleus. And for the palatal portion of this circuit, the substantia nigra pars reticulata. And this circuit is important for modulating the expression of saccades. In order to make a saccade, we need to disinhibit the superior colliculus. We'll disinhibit the thalamic connection to the frontal eye field. In the same way that we talked about the role of the basal ganglia in disinhibiting other aspects of movement. Well, you can imagine that if you've got a problem, where there's insufficient inhibition coming out of the substantia nigra pars reticulata. We may be making eye movements all over the place because we can't control them. We can't suppress the unwanted movement. That very well may be the case, should there a problem in the circuitry of the substantia nigra pars reticulata. One might also have a Parkinsonian condition relative to the ability to generate spontaneous saccades and unfortunately that can be the case if there is insufficient. [INAUDIBLE] in operating in the circuity of the head of the circuit of the head of the cadillac nucleus. The cerebellum plays a role in modulating the output of these upper motor circuits. And sometimes those of that have a prescription for glasses or contact lens we have to make a change. And that change in prescription strength may mandate some kind of adjustment in the gain of our sensory mediated eye movement reflexes, like our vesicular oracular reflex. perhaps you've gotten a new pair of glasses and your optometrist or opthamologist had said, now be carful going down stairs. For, for a day or so until get used to it. Well, what that practitioner is really saying is give your cerebellar circuitry time to implement a, an adjustment. That will allow the gain the vestibulo ocular reflex to now be properly matched to. The magnification of the sensory world is passing through your lenses. Well it's a lot easier just to say, be careful on those stairs. But really, what's going on here is that the cerebellum is adjusting the gain of the vestibulo-ocular reflex. In order to make the appropriate amount of eye movement that's necessary to maintain fixation. And in the case of getting a new pair of glasses, the degree of magnification of that world may have been changed. Well, the basic point here is that the upper motor neurons that control eye movements in the frontal eye fields and the superior colliculus. Our subject to modulation by circuits of the basal ganglia in the cerebellum. Just like all the other upper motor neurons. And in fact, I think we've learned more about how all this works for the study of eye movements than we have for just about any other type of movement. Which again demonstrates the value of knowing something about the eye movement system, whether you're a neuroscientist or a clinician. eye movements are very important in helping us to diagnose neurological problems. And they're very instructive for understanding how sensory signals are turned into motor commands. Well, that's it for now, and if you're keeping pace in the order at which these videos are being presented to you, you'll notice that we're almost done with unit four. And what comes next in our final installment in our consideration of motor control is a session on the visceral motor system, otherwise known as the autonomic nervous system. So I'll look forward to talking to you then. And otherwise I hope you enjoy your studies thank you for your consistent efforts to date. I know it's been a long haul already, but we're almost done with unit four. We're almost done with our consideration of sensory motor function. And that will give us a little breathing room as the course concludes, thinking about. The changing brain across the lifespan, and finally, the basis of higher brain function and cognition. Okay? So, continue to, to work hard and study well, and I look forward to interacting with you in the discussion forum.
