So what is represented in the primary motor cortex? it doesn't seem to be a strict representation of the body and its muscles. So, I think what we can conclude then, based on the evidence to date, is that what's represented in the cortex is movement, not muscle. In fact, we might go so far as to hypothesize that truly whats represented is movement intention and I'll give you some examples of what I mean by that. Now we do know from a long history of investigation in this part of the brain, that there various dimensions of movement that are represented in the motor cortex. For example, the amount of force that we generate with muscle contraction. the directionality of movement if we're talking about moving an articulated muscular skeletal structure like the upper extremity. the amplitude of the movement. whether it's a small movement or a large movement. These features, these dimensions of movement seem to be encoded in our cortical network. but we also seem to have representation of movements that are ethologically meaningful. For example, the primary motor cortex seems to be concerned with coordinating movements of the hand and the lower part of the face. for example, moving the hand to the mouth. That's one aspect of movement that seems to be organized in the primary motor cortex. And perhaps that's one advantage of having the representation of the face adjacent to the representation of the hand in the body map. We also see evidence that organized in the motor cortex is representation of skilled manual behavior in the space that's right in, right in front of us. What we call central personal space. this might be very important for bringing objects in to our near visual field so that we can inspect and examine them. imagine the importance for example of having a careful look at a piece of fruit to make a judgement about whether it's truly palatable or not. Well, with all of this being represented in the primary motor cortex, we can make some predictions as to what we might find in a patience that has had damage to this part of the brain, or to the pathway that emanates from it. Generally speaking, what we find is that lesions of the primary motor cortex impair the fractionated or the skilled movements of our distal extremities in the lower part of our face. So, for example, some kind of fractionation of the fingers of the hand would be impaired with damage to the primary motor cortex. So I want to come back just for a minute and say more about my first point here, that what's encoded in the primary motor cortex may very well be the intention of the movement. So, I think a really interesting series of studies were performed by Michael Graziano and his colleagues where they were probing the movements that could be generated in the brain of a rhesus monkey when the motor cortex was stimulated. One of the interesting features of this study that I think it is, make it especially noteworthy, is that the duration of stimulation was longer than is typically done in the kinds of body mapping experiments that were done throughout the 20th century. rather than stimulating for just a very brief moment in time Dr. Graziano and his colleagues decided to extend the period of stimulation to a period a few seconds. Which was thought to mimic the more natural evolution of activity that emerges in the motor cortex during the performance of motor behavior. And what was found, was micro stimulation of certain sites in the motor cortex produced behaviors. Not just movements, but behaviors and here's one example. what we're looking at here, is a record of movement. With stimulation of the hand region, of the pre-central gyrus in this rhesus monkey. And the the blue the blue plus signs indicate the starting position of the hand, at the onset of stimulation. And then the red circles indicate the end position of the hand at the termination of the movement that was elicited with micro stimulation. And I think what's obvious here, is that at the particular site of stimulation, what was observed is, movement of the hand from peripheral regions. Be they to the left, or to the right of the mid line, it didn't seem to matter. And what we found here, is hand to mouth movement. So, where ever the starting position of the hand happened to be at the time of stimulation, it was brought towards the mouth. And it seems as though in this particular location, what's really important are the kinds of movements that would be needed for feeding. When Graziano and his colleagues moved the micro electrode in a slightly different direction in the primary motor cortex what was discovered is a site. Where the hand was moved not to the mouth, but more towards this central personal space. Where an animal could inspect through vision and presumably other such special sensory systems just what was being held in the hand. So these kinds of studies suggests that what's represented in the primary motor cortex is movement intention. Bringing the hand to the mouth or bringing the hand to central mid line for visual examination. So how might such movement be encoded in the firing patterns of neurons in the primary motor cortex? Well one idea might be, that for each particular movement that is illustrated here with the black line, there may be a neuron that is specifically tuned for the expression of that movement. Well that would be one idea, and it turns out that that is highly unlikely. Rather, what we find is encoding not by 1 neuron at a time, but by populations of neurons. And encoding for the various dimensions of movement that I alluded to earlier. The direction of movement. The, force that's generated. the amplitude of the movement. And I'd like to show you an additional experiment done some years earlier. that really made this point, that movement direction is encoded in the population activity of the motor cortex, not by any single neuron. So here's the simple experiment that's being illustrated here. There is a monkey that's trained to move a central joystick in the direction of some kind of target. And that target might be a green light that would eliminate at a particular location in this apparatus. So there's a particular directionality to the movement. And then question then is, well how does the motor cortex encode this directionality of movement towards a visual target. And the answer seems to be that the encoding is done at the level of the population. So what we're looking at here are series of raster plots that report the action potentials that were recorded during particular trials. So each row that we see here, we have one, two, three, four, five rows. These are five trials. And then each little tick mark that we see represents the occurrence of an action potential. Now, as the monkey moved the joy stick towards various visual targets and these various directions, what was generally observed in the firing of individual cells in the motor cortex is that they tended to respond very broadly. With movements directed towards the left and in the upward direction. So these movements that are represented by this yellow wedge of this circle here are those that led to an increase in the firing of action potentials around the time of the movement. And movement is represented by this red line here, which indicates at time 0, when the movement actually occurred. The firing of this cell seems to increase a few hundred milliseconds before the movement was made and continues during the course of the movement. Now notice what happened as the animal moved in the opposite direction. So as the animal moved broadly to the right, what we find is a suppression of activity. Just before and during the course of the movement. So there seems to be a very broad measure of tuning that can be reported at the individual neuronal level. In fact, if we look at the peak responses as the animal moves in different directions, what we see is a, is a very broad tuning function. And this is noteworthy, because it suggests that no one neuron encodes precisely the direction of movement. the responses, the contributions of any one neuron are simply too broadly tuned to achieve the functional goal. So, how must this happen? Well, the functional goal is achieved through encoding of direction in a population of neurons. So what was found as many neurons were recorded in the pre-essential [UNKNOWN] during directional movement, is that well no one neuron seem to be sufficiently tuned to produce the moment that a vector could be defined by the active population. That aligned quite precisely with the movement that was actually performed. So for example, as the monkey moved the joystick up into the right, we see this big red arrow that indicates the direction of the movement that was performed. there is a population of cells that are active. And if one then computes the vector average of their tuning functions during the execution of the movement, we find a vector average that aligns very nicely with the direction of movement. So, for each movement that was executed, there's a population of neurons that are contributing to the directionality of the movement that's actually expressed. It's as if there is some kind of ensemble code for the direction of movement, with many neurons contributing information from which the precise movement is governed. So this then, is an example of what we call a population code in the cerebral cortex. We find similar codes, we think, all over the cerebral mantle. with the idea being that for the most part single neurons are not sufficiently tuned to express behavior. But rather, the behavior is the product of an ensemble code reflecting the activities of the entire population of neurons.
