Motor cortex in voluntary movements a distributed system for distributed functions pdf

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C RC P R E S S Boca Raton London New York Washington, D.C.

  The traditional metaphor of the piano presents the premotor cortex “playing” the uppermotoneuron keys of the primary motor cortex (M1), which in turn activate with strict point-to-point connectivity the lower motoneurons of the spinal cord. Furthermore, although more and more knowledge is accu-mulating, there is still an ongoing debate about what is represented in the motor cortex: dynamic parameters (such as specific muscle activation), kinematic param-eters of the movement (for example, its direction and speed), or even more abstract parameters such as the context of the movement.

EDITED BY

  The traditional metaphor of the piano presents the premotor cortex “playing” the uppermotoneuron keys of the primary motor cortex (M1), which in turn activate with strict point-to-point connectivity the lower motoneurons of the spinal cord. Furthermore, although more and more knowledge is accu-mulating, there is still an ongoing debate about what is represented in the motor cortex: dynamic parameters (such as specific muscle activation), kinematic param-eters of the movement (for example, its direction and speed), or even more abstract parameters such as the context of the movement.

Chapter 2 is to provide an overview of the contribution of functional magnetic

  Not only does fMRI reveal plausible brain regions for the control of localized effects, but the distribution of response foci and the correlation of effectsobserved at many different sites can assist in the guidance of detailed studies at the mesoscopic or microscopic spatio-temporal level. This biological control of a complex peripheral apparatus initially may appear unnecessarily com-plicated compared to the independent control of digits in a robotic hand, but can be understood as the result of concurrent evolution of the peripheral neuromuscularapparatus and its descending control from the motor cortex.

Chapter 8 , Riehle discusses the main aspects of

Chapter 9 , Jeannerod poses

  In the last chapter of Section II , the question of the role of the motor cortex in motor cognition. This revision of motor cortical functionoriginated from two main lines of research, dealing first with the plasticity of the somatotopic organization of the primary motor cortex, and second with its involve-ment in cognitive functions such as motor imagery.

Chapter 11 , Shadmehr, Donchin

  Many studies suggest that internal models are sensorimotor transformations that map a desired sensory state of the arminto an estimate of forces; i.e., a model of the inverse dynamics of the task. Results suggest that internal models are computed with bases that are directionally tuned to limbmotion in intrinsic coordinates of joints and muscles, and this tuning is modulated multiplicatively as a function of static position of the limb.

Chapter 12 , Padoa-Schioppa, Bizzi, and Mussa-Ivaldi

  Something akin to programming is achieved in nature by the biological mech- anisms of synaptic plasticity — that is, by the variation in efficacy of neural trans-mission brought about by past history of pre- and post-synaptic signals. The authors discuss the theoretical idea of internal models andpresent some evidence and theoretical considerations suggesting that internal models of limb dynamics may be obtained by the combination of simple modules or “motorprimitives.” Their findings suggest that the motor cortical areas include neurons that process well-acquired movements as well as neurons that change their behaviorduring and after being exposed to a new task.

Chapter 13 , Carmena and Nicolelis Copyright © 2005 CRC Press LLC

Chapter 14 , Pfurtscheller

  These studies have demonstrated that animals can learn to utilize their brain activity to control the displacements of computercursors, the movements of simple and elaborate robot arms, and, more recently, the reaching and grasping movements of a robot arm. For instance, the physiological and behavioral studies that provide evidence of differential involvement of each motorarea in the generation and control of movement are beyond the scope of this chapter.

V6 PE

  Based on these definitions, M1 is located in the anterior bank of the central sulcus and on the adjacent caudal portion of the precentral gyrus(Figure 1.1). (Adapted with permission from Reference 36.) 37 detailed equivalent of observations made initially by Ferrier who reported that inM1 “long-continued stimulation brings the hand to the mouth, and at the same time the angle of the mouth is retracted and elevated.” The interpretation of these complexmovements is limited by the fact that intracortical stimulation primarily activates 38,39 neurons trans-synaptically, and thereby enlarges its sphere of activation.

1.2.1.2 Output of Single Corticomotoneuronal Cells

  On the other hand, the highly divergentprojections of many CM neurons are consistent with some of the more complex, multiple-joint movements observed with other variations of the intracortical stimu- 26,36 lation technique. Thus, adjustment of the parameters of intracortical stimulation may promote access to different structural features of the output organization of M1as well as other portions of the motor system.

1.2.1.3 Peripheral Input to M1

  The identification and characterization of the premotor cortex has been the subject 2,9,15,56–61 of some controversy and considerable revision over the last century. (See below.) On the lateral surface, attempts to define the boundaries of the premotor cortex using electrical stimulation or cytoarchitectonic criteria failed to produce a 9,61 consensus.

1.2.2.1 Identification by Direct Projections to M1

A more recent approach for determining the location of premotor cortex has been based on its neuroanatomical connections. The premotor cortex in non-human pri-mates has been operationally defined as consisting of those regions in the frontal lobe that have direct projections to M1 (For review see References 9,59,60,64–66.)According to this definition, the frontal lobe contains at least six spatially separate premotor areas (Figures 1.1 and

1.3 A ). For example, the arm representation of M1

  The PMv is located in the portion of area 6 that is lateral to the arcuate spur and extends rostrally into the posterior bank of the inferior limb of the arcuate sulcus. The PMd occupies the portion of area 6 that is medial to the fundus of the arcuate spur and caudal to the genu of the arcuate sulcus.

17 The portion of area 6 (area 6aB) that lies dorsal and anterior to the genu of

  An unfoldedmap of the frontal lobe shows the density of labeled corticospinal neurons after injections of a fluorescent tracer into the C7–T1 segments of the spinal cord. (Reproduced with permission from Reference 64.) 66.) Thus, the current definition of premotor cortex includes multiple premotor areas located in the caudal half of area 6 as well as in additional regions within the cingulate 9 sulcus that were historically considered part of the limbic cortex.

1.2.2.2 Somatotopic Organization Based on Connections with M1

  The somatotopic organization of the premotor areas has been evaluated based of 59,60,64,67–69,71–76,81,82 their projections to the arm, leg, and face representations of M1. In this map, the location of the arm representations in M1 and the premotor areas are based on the origin of neurons that project to upper and lower cervical segments.

1.2.2.3 Corticospinal Output

83 Russell and DeMyer first demonstrated that area 6 contributes about the same

  (See also References 86–93.) The distri- bution of corticospinal neurons in the premotor areas that projected to cervical segments of the spinal cord corresponded remarkably well to the distribution ofneurons in the premotor areas that projected directly to the arm representation inM1 (Figures 1.3A and 1.3B). After tracer injections con- fined to the cervical segments (arm representation), the percentage of the totalnumber of corticospinal neurons in the frontal lobe that originated in the premotor areas was always equal to or greater than the percentage of corticospinal neurons 60,84,85 in M1 (premotor mean = 56%, range 50–70%, n = 6).

1.2.2.4 Somatotopic Organization Based on Corticospinal

  Output: Forelimb and Hindlimb Representation Because cervical segments of the spinal cord are known to control arm movements and lumbosacral segments are known to control leg movements, the “arm” and the “leg” representations of a cortical area also can be identified on the basis of the origin of their projections to the cervical or lumbosacral segments of the spinal cord,respectively. The origin of corticospinal neurons in the premotor areas that projected to cervical or to lumbar segments of the spinal cord corresponded remarkably wellto the origin of neurons in the premotor areas that projected directly to the M1 arm 59,60,76,84,85 or to the M1 leg representations, respectively ( Figures 1.3A , 1.3B, and 1.4 ).

1.2.2.5 Somatotopic Organization Based on Corticospinal Output: Proximal and Distal Arm Representation

  Thus, the orientation of the body map in the CMAd is reversed compared to the one in the SMA, just as was predicted by the origin of corticospinal projections 85,108 to the cervical and lumbar segments (Figure 1.4). Arm movements were consistently 99 evoked in the rostral portion of the region corresponding to the CMAv ( Figure 1.5 )(see also Reference 109), but few penetrations have been made in the caudal portion 72,76,82,85,108 of the CMAv where a leg representation was reported to be located.

ORTICOSPINAL ERMINATIONS

1.2.3 C T

1.2.3.1 Primary Motor Cortex

  (A complete discussion of this issue is 1 provided by Kuypers. ) Corticospinal efferents from M1 project to the intermediate zone (laminae V–VIII) of the spinal cord where interneurons that innervate moto-neurons are located as well as directly to the portions of the ventral horn where 1,97,121–129 motoneurons are located ( Figure 1.7 ). Compared to SMA terminations, M1 terminations are denser and more extensive in lamina IX, and extend further into the base of the dorsal horn.(Adapted with permission from Reference 9.) evidence suggest that the pattern of corticospinal terminations reflects the differential involvement of these cortical areas in motor output or in somatosensory processing.

1.2.3.2 Premotor Areas

  The relationship between the terminations of corticospinal efferents and the motor, sensory, and interneuronal systems of the spinal cord has been studied only for 97,128,141,142 premotor areas on the medial wall of the hemisphere. Although the terminations from the premotor areas were less dense over lamina IX than were those from M1, all of these studies had a consistent result: the terminationsof premotor areas that do overlap lamina IX were concentrated over the motor nuclei innervating muscles of the fingers and wrist.

1.3 CORTICAL INPUTS TO THE MOTOR AREAS

  We haveproposed that each cortical area in the frontal lobe that projects to the spinal cord could operate as a separate efferent system for the control of specific aspects of motor 59 behavior. To minimize the problems of cortical identification and strength of input, we focused our analysis on studies that examined the inputs to the arm representationof motor areas in macaque monkeys.

RIMARY OTOR ORTEX

  This input arises in area PE on the lateral surface of the postcentral gyrus and area PEip in the lateral portion of the dorsal bank of the intraparietal 59,68,71,73,76,147,152–155 sulcus. Lesion of the dorsal columns, a major ascending pathway to the parietal cortex, extinguishes the responsiveness of TABLE 1.2 Ipsilateral Cortical Input from the Parietal Lobe to M1, the Premotor Areas and the Pre-Premotor Areas Cortical Area M1 PMdc PMv SMA CMAd CMAv CMAr Pre-PMd Pre-SMA Superior Parietal Lobule Area 3a xx x xx x ?

V6A

  Removal of the primary and secondary somato- 5,158 sensory areas, as well as removal of the cerebellum, does not. Some authors have proposed that M1 receives input directly from a thalamic region innervated by aportion of the dorsal column pathway (for references and discussion, see Reference 5), 159 but firm anatomical evidence for such a neural circuit remains elusive.

2 Superior 3a, Premotor

  First, theSMA, of all the premotor areas, has the densest and most balanced reciprocal 58,59,68,72,81 58,161,162 caudal portion of the PMd, but only sparsely to its rostral portions. Fourth, the projections from the lateral motor areas (PMd,PMv, M1) to the cingulate motor areas, particularly the CMAv and the CMAr, tend 82,163,164 to be relatively weak.

1.3.2.2 Parietal Cortex

  Most of the parietal lobe input to the premotor areas originates from posterior portions of the 16 parietal lobe including the SPL (area 5, as described by Brodmann ), the inferior 16 parietal lobule (area 7, as described by Brodmann ) and the secondary somatosen- sory cortex (SII) ( Figure 1.8 , Table 1.2 ). These regions include the PEc on the most caudal portion of the superior parietal gyrus, area V6A buried on the anterior bank of the parietal occipital sulcus,area PGm on the medial wall just rostral to the parietal occipital sulcus and the posterior cingulate gyrus (CGp) ( Figure 1.1 ).

1.3.2.3 Pre-Premotor Cortex

  Three cortical areas — the preSMA, the prePMd, and the precentral opercular cortex 179 (PrCO) — reside at the junction between the prefrontal cortex and the premotor areas in the frontal lobe (Figure 1.1). These projections of area 46 to the premotor areas link the prefrontal cortex to cortical regions with direct access to the primary motor cortex and spinal mech-anisms of motor control.

UMMARY OF ORTICAL ONNECTIONS

1.3.3 S C C

  The second major conclusion from our analysis is that each of the nine cortical areas in the frontal lobe has a unique constellation of inputs from other areas of thecerebral cortex. Because M1 and all of the premotor areas project directly to the spinal cord, we have proposed that each premotor area may be a “nodal point forparallel pathways to the spinal cord.” As a consequence, each of these cortical areas may operate as a functionally distinct system that differentially generates and/or 59 controls specific aspects of motor behavior.

1.4 SUBCORTICAL INPUTS

  Because M1 and the premotor areas provide input to the basal ganglia via the 219,220 221–224 striatum and to the cerebellum via the pontine nuclei, these cortical areas have the potential to form “closed” loops with the basal ganglia and cerebellum 219,220,225 ( Figure 1.9 ). This rostral to caudal arrangement of the origin of projections to the leg, arm, and face representations in M1 corresponds well with the somatotopy 201,203,233–235 previously proposed for the dentate.

5 D

  Injection of virus into the arm representation of M1 labeled two dense clusters of neurons in the globus pallidus — one in the inner segment and another in the outer 229 segment (Figure 1.12A). Labeled neurons (dots) are indicated for two coronal sections separated by 0.5 mm.(Adapted with permission from Reference 229.) (B) Neurons in the GPi were labeled follow- ing injections of HSV1 into the arm representations of the SMA, M1, or PMv of monkeys.

1.5 SUMMARY AND CONCLUSIONS

  The output of the premotor cortex was viewed as being funneled to M1 which served as the final 1,4 common pathway for the central control of movement. Thus,each premotor area appears to have the potential to influence the control of movement not only at the level of the primary motor cortex, but also more directly at the level 59,60 of the spinal cord.

1. Kuypers, H.G.J.M., Anatomy of the descending pathways, in Handbook of Physiol-

  et al., Patterns of localization in precentral and “supplementary” motor area and their relation to the concept of a premotor area, Assoc. and Tanji, J., Supplementary and precentral motor cortex, contrast in responsiveness to peripheral input in the hindlimb area of the unanesthetized monkey,J.

I. Motor output organization, J. Neurophysiol., 48, 139, 1982

  W., The effects upon the activity of hand and forearm muscles of intracortical stimulation in the vicinity of corticomotorneurons in the conscious monkey, Exp. J., III et al., The relationship of corpus callosum connections to electrical stimulation maps of motor, supplementary motor, and the frontal eye fields in owlmonkeys, J.

II. Somatosensory input organization, J. Neurophysiol., 48, 150, 1982

  and Sakai, H., Exact cortical extent of the origin of the corticospinal tract (CST) and the quantitative contribution to the CST in different cytoarchitectonicareas. E., and Mushiake, H., Evidence for “output channels” in the basal ganglia and cerebellum, in Role of the Cerebellum and Basal Ganglia in Voluntary Movement , Mano, N., Hamada, I., and DeLong, M.

2 Resonance Imaging of

  In relation to findings from other techniques in theneurosciences, the authors of this chapter are tempted to acknowledge that the contribution to understanding the motor cortex that has come from neuroimaging issmall yet significant, in particular with respect to the human motor cortex. As a fourth constraint we will not consider in this review those many valuable studies that have integrated the imaging of motor cortex activation into a clinicalcontext, be that the issue of presurgical mapping or that of postlesional plasticity, or the influence of other disease conditions or pharmacological manipulations ontask-related motor cortex activity.

2.2 SIGNALS IN fMRI

  Apart from the problem of confidently relating the signal changes observed in fMRI to changes in a single physiological parameter, there remains the problem thatdeoxyhemoglobin is only an indirect index of neural activity. If one considers the huge neurochemical 15 and neurovascular heterogeneity in the central nervous system, the usual assump- tion of generic hemodynamic and metabolic response properties across different 18 brain regions needs to be taken with caution or even to be addressed analytically.

23 BOLD response. It is therefore not surprising that the findings reviewed in the following sections are almost entirely based on the positive BOLD response

  Over and above the uncertainties regarding the coupling between neural activity and blood flow and metabolism, and between blood oxygenation changes and fMRIsignal, there remain open questions as to the precise nature or component out of the orchestrated spectrum of neural activity that drives these effects. The sections thereafter deal with further aspects of the topic, such as acute (attentional) or long-term modulation of responses (learning), othersources of primary motor cortex activation than overt movement (sensation, imag- ery), and findings related to cognitive states such as motor intention and preparation.

2.3 LATERALIZATION AND HANDEDNESS

  This corresponds to the decussation of the pyramidal tract as the main output of M1 and mirrors the clinical deficit observedafter lesions of this tract or its cortical origin. This does not clarify the source of ipsilateral deactivation but it suggests that the functional meaning may be to reduceactivity in the hand contralateral to the one executing the task.

40 In a similar vein, Solodkin at al. found that left- and right-handed subjects had

  This view would be compatible withthe observation that while the location of the dominant contralateral M1 focus is not mirrored in the ipsilateral cortex, the ipsilateral activations do in part mirror theminor contralateral foci. The interpretation of the various findings discussed above is stuck at the level of anatomical analysis, which is still not detailed enough to allow for the confidentdiscrimination between effects in M1 and those in the premotor areas.

2.4 SOMATOTOPY

  In a more explicit andexperimental way, the pioneering work on electrical stimulation during open brain surgery established the notion of somatotopy in the human motor cortex, i.e., thesystematic and orderly representation of the body along the medio-lateral extent of the precentral gyrus. In textbooks, this is usually represented as the so-called homun-culus of the primary sensory and motor cortices, with the knee bent approximately into the interhemispheric fissure and the more cranial body parts rolled out laterallyalong the convexity, with the exception of an inverted and thus upright face repre- sentation.

47 Penfield and Rasmussen reported similar observations and also emphasized that

“in most cases movement appears at more than one joint simultaneously.” They stated that their cartoon only referred to those rare cases when movement appearedat only one joint, although they also stated that grouped responses often involved neighboring fingers. The two issues raised above were readdressed in an fMRI study performed by 48 one of the authors at considerably higher spatial resolution than was the case with

45 Sanes and colleagues. We found that any type of hand or finger movement tested

  Although each of these studies added some aspect of refinement or some degree of more detailed quantifi-cation and thus further corroborated the experimental proof and characterization of somatotopy in the motor hand representation, none of these studies advanced ourunderstanding from a functional perspective of why this should be the case. In the visual system for instance, a dotthat is present in one spot of the visual field is not present in another spot, and accordingly the processing of this information may be aided by spatially separatingthe neural populations that code for these different spots in the visual field.

62 Brecht et al. ). A classical debate in motor control research has been whether

  The observation of such cases is in accordance with our general reasoning here because the radialnerve innervates muscles that implement extension across several joints of the arm(elbow, wrist, phalanges), and the synergy of these movements in natural contexts may result in a higher degree of interaction of the underlying neural units. Following the general reasoning of optimizing interaction by minimizing the associative fiber path lengths, one would expect a bias to clusterthose neural units in the primary motor cortex that may engender, for instance, thumb movements (and concomitant sensory thumb stimulation) in spatial proximity tothose neural units in M1 that will receive and process this sensory stimulation and thus be informative for optimizing the muscular activity pattern.

68 Rosén and Asanuma observed that in roughly half of the units that could be driven

  In conclusion, one could say that somatotopy in the motor cortex codes which body part will show the effect of movement, and that demonstrating a somatotopydefined in this way already points to the role of somatosensory somatotopy as its functional source. This view obviously still represents a gross simplification, but it can also account for the progressively blurred or even functionally redefined observations ofsomatotopy in motor areas that are upstream from the primary motor cortex or that pertain to other motor circuits than the corticospinal tract and contribute to the 72–76 planning, initiation, and execution of voluntary movements.

2.5 MOTOR RESPONSE PROPERTIES ATE AND OMPLEXITY FFECTS

2.5.1 R C E

  One experiment involved repetitive move- 77 ments of the index finger at 1, 2, and 3 Hz, the other flexion–extension movements 78 of digits 2 to 5 at rates of 1, 2, 3, 4, or 5 Hz. For instance, while the primary motor cortex displayed significant rate effects, it did not appear to be affected by the complexity of the temporal movement sequenceinduced by a fixed cueing sequence.

82 Mayville et al. asked their subjects to perform either synchronized (on the beat)

  Again, M1 only showed a rate effect but, in contrast with many other motor areas, was not sensitive to the manip-ulation of complexity. and Nakai et al., the predictability of cueing did not because each of the sequences was perfectly regular.

84 Dassonville et al. found that activity was related to reaction time and thus was

  For a hemodynamic signal, as in fMRI, one can expect two types of rate effect on the cortical response focus: (1) the amount of signal change in a given clusterof voxels could increase, and (2) the number of voxels that constitute this cluster at a given significance threshold of signal change could increase. As another movement parameter apart from rate, amplitude has been found to correlate positively with the BOLD fMRI response in M1 and, as a more indirectsign, with the significance levels for responses in the SMA proper and in the premotor 25 and postcentral areas, the insula, and the cerebellum.

ORCE FFECTS

2.5.2 F E

  reported increases of both activation volume and amplitude (percent signal change) in the left M1, and also, to a less reliableextent, in the right M1 and the supplementary motor area. This expands the findings 79 94 from previous studies using index flexion and squeezing at different force levels, where the effect of force on M1 activity was mainly reflected in the activation volumeand not, or not so much, in the signal change within given voxels.

95 Dai et al. had also found that increased force of hand squeezing translated into a

  Precision as opposed to power grip involved less activity in the primary motor cortex, but stronger bilateral activations in the ipsilateral ventral premotorareas, the rostral cingulate motor area, and at several locations in the posterior parietal and prefrontal cortices. In subsequent studies, the same group showed that,in contrast to the behavior during the power grip, the activity in the contralateral primary sensorimotor cortex, as well as in the inferior parietal, ventral premotor,supplementary, and cingulate motor areas, increased when the force of the precision grip was lowered such that it became barely sufficient to hold a given object without 98,99 letting it slip.

2.6 SENSATION AND ATTENTION

  A final residual Despite these open questions, a rather global view on attentional effects in the motor system thus strongly resembles that in the visual system, where attentionaleffects can be detected by functional neuroimaging as early as in the lateral genic- ulate nucleus, but where they become more pronounced the deeper one advances 106 into the hierarchically organized processing levels. As opposed to the visual system, however, it remains much more obscure in the motor system whether the attentional modulations observed should be consideredthe neural correlates of attention-dependent changes in motor behavior.

2.7 IMAGERY

  The movement-predominant activity included the primary sensory and motor areas, theparietal operculum, and the anterior cerebellum, which had little imagery-related activity (–0.1~0.1%), and the caudal premotor areas and Brodmann area 5, which One of us used a different approach for dissociating the effects of motor imagery 114 and actual movements during fMRI measurements. Nonetheless, there is now a consensus that the primary visual cortex can participate in imagery, and this may depend on Motor imagery can be carried out in a predominantly visual mode (imagining seeing one’s own hand moving) or in a kinesthetic mode (imagining the proprio-ceptive sensations one would experience if one moved the hand in the mentally simulated way).

2.8 PREPARATION, READINESS, AND OTHER TEMPORAL ASPECTS

  By correlating the dif- ference in fMRI response onset of pairs of regions (visual cortex–supplementarymotor area; supplementary motor area–primary motor cortex) with the reaction times on a subject-by-subject basis, the authors showed that reaction time differences couldbe predicted by BOLD delays between SMA and M1, but not between V1 and SMA. This role seems to fit into the more general perspective of the organization of the parieto-frontal system, with parietal areas 141,149 involved in evaluating the potential motor significance of sensory stimuli, frontal 150,151 areas involved in preparing movements as a function of their probability, and 114,139 central regions focused on executing the actual movement.

2.9 PLASTICITY AND MOTOR SKILL LEARNING

  The authors reported specific learning-related increases in the BOLD signal in the ventral sector of the precentral gyrus, in the region containing a motor 164 representation of the face. Yet, in addition to providing some sort of overview, we hopethis chapter can help achieve a better assessment of the strengths and limitations of magnetic resonance as a tool in the neurosciences in general and in the study of However, the spatiotemporal response function, i.e., the dispersion of the fMRI response in time and space, poses the most relevant limitation for this type ofrecording.

IOMECHANICAL ACTORS

3.2.1 B F

  The tendons of flexor digitorum profundus (FDP) to the four different fingers also are interconnected in the palm, both by thin sheets of inelastic connective tissue and 18 by the origins of the lumbrical muscles. In humans performing a brisk extensionof the little finger, a simultaneous rise in tension in the tendon of abductor pollicis longus (APL) often can be palpated; presumably the contraction of APL counter- balances the wrist torque produced by the extrinsic finger muscles used to extend the little finger, EDC and extensor digiti quinti (EDQ).

EUROMUSCULAR OMPARTMENTALIZATION

3.2.2 N C

  Mechanical separation of the FDP and EDC tendons would not result in greater independence of the fingers if all the motoneurons of these muscles still acted as asingle pool. FDPr is activated during flexion of the index or middle finger but not during flexion of the ring or little finger, whereasFDPu is activated during flexion of the little or ring finger, but not during flexion 27 of the index finger.

ENTRAL OUPLING

  Indeed, axons from single M1 neurons have beenshown to branch within the cervical enlargement of the spinal cord to innervate 31 multiple motoneuron pools, and the physiological effects of such branching have been shown with spike-triggered averaging to be present in the EMG activity of 32,33 multiple finger muscles. The motor unit territories also have become partially selective: the red and orange motoneurons innervate muscle fibers to the left, and the green and blue motoneurons innervate muscle fibers to the right, with a central region ofoverlap.

ORTICAL RGANIZATION AND EUROMUSCULAR

VOLUTION

3.3.1 C O N E

  The sets of M1 neurons that provideinput to two muscles acting on the digits and wrist also are intermingled in the physical space of the cortex. Consequently, the neurons that provide input to anygiven muscle are spread over a relatively large cortical territory (typically a few millimeters in diameter in nonhuman primates) and the territory providing input toone muscle overlaps extensively with the territory providing input to other mus- 38,39 cles.

ONTROL OF NDIVIDUATED OVEMENTS

3.3.2 C

  I M WITHOUT OMATOTOPY S If we accept the notion that movements of different fingers are not controlled simply through a somatotopic map of the hand in M1, we then must ask how M1 mightcontrol individuated finger movements. For this purpose, cluster analysis has the advantage of being an objective method of searching for similar membersof a population, without making assumptions as to what the function of any group The view of M1 activity during individuated finger movements that has devel- oped up to this point appears chaotic.

ORTICOMOTONEURONAL ETWORK OFIVERSE LEMENTS

3.3.3 A C N D E

  One might think that the neurons with firing ratemodulations most similar to the EMG amplitude modulations have the strongest connections to the motoneuron pool, but the degree of neuron–EMG activity simi-larity seems to have little correlation with the strength of the neuron–EMG connection as measured by SpikeTA. In particular, this pruned network reconstruction of EMGassumes (1) that SpikeTA effects represent a constant input to motoneuron pools, the strength of which does not fluctuate during different movements or task timeperiods; (2) that the effects of M1 neurons on EMG activity sum linearly; and (3) thatM1 neurons provide sufficient input to the motoneuron pools to create the pattern of EMG activity observed.

VOLUTION OF THE OTOR ORTEX

3.3.4 E M C

  Paralleling the greater biomechanical and muscular independence of the digits, somatotopic gradients with stronger representation of the radial digitslaterally and the ulnar digits medially are found in the human M1 hand representation. Because of biomechanical coupling and muscle structure, the vast majority of hand and finger movements, eventhe highly individuated movements used in fine motor tasks, require the simultaneous control of multiple muscles, some moving the intended digit or digits and othersstabilizing the other digits and the wrist.

3.4 CONCLUSIONS

  Neurons activeduring various finger movements, with outputs to various subsets of finger muscles, are intermingled with one another, such that the cortical territory representing anyparticular body part, muscle, or movement overlaps extensively with the territory representing any nearby body part. This work was supported by R01-NS27686 and R01-NS36341 from the National Institute of Neurologic Disorders and Stroke and BCS-0225611 from the National Science Foundation of the UnitedStates of America.

1. Gorska, T. and Sybirska, E., Effects of pyramidal lesions on forelimb movements in the cat, Acta Neurobiol. Exp., 40, 843, 1980

  H., Gardinier, J., and Liu, J., Tension distribution to the five digits of the hand by neuromuscular compartments in the macaque flexor digitorum profundus,J. M., The effects of muscimol inactivation of small regions of motor and somatosensory cortex on independentfinger movements and force control in the precision grip, Exp.

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