Anatomical distribution of arm-movement-related neurons in the primate superior colliculus and underlying reticular formation in comparison with visual and saccadic cells

 We recorded from 389 ”reach” neurons (two monkeys) in the superior colliculus (SC) and underlying reticular formation (RF) or adjacent periaqueductal grey, whose activity was related to visually guided arm movements. Reach neurons were present from approximately 0.7 mm down to a depth of 6 mm below the surface of the SC (mean 3.7±1.3, n=389). Although this mean distribution was different from that of cells with visual (mean depth 1.7±1.4 mm, n=283) or saccadic responses (mean depth 2.0±1.4 mm, n=232), there was a large amount of overlap. Fifty-five per cent of all reach cells (213/389) were assumed to be located inside the SC. The others were considered to be located in the underlying RF. The characteristics of visual responses and saccadic bursts (e.g. response latencies, discharge rates, burst durations) of arm-movement-related neurons were not different from those of typical visual or saccade cells in the SC. Although reach neurons could be recorded in a large area of the SC, they were found more often in the lateral than in the medial parts (chi-squared=19.3, P<0.001). Possible pathways by which arm-movement-related neuronal activity in and below the SC might gain access to spinal motor structures are discussed. The location of arm-movement-related neurons described in this study is in accordance with the known target areas of skeletomotor-related corticotectal projections and with the sites of origin of tectofugal pathways. It is concluded that this population of reach cells is in a position to relay and transmit limb movement information to the spinal motor system, where it may influence (or interact with) motor commands coming from other motor centres. Received: 17 June 1996 / Accepted: 24 December 1996     

Activities of single precentral neurons of the monkey during different tasks of forelimb movements

The neuronal activity in the motor cortex of the rhesus monkey was investigated in three different tasks performed with finger, wrist, and arm movements. A total of 125 neuronal activities were analysed. They were classified into five groups in terms of muscular contractions provoked by intracortical stimulation; neurons related to contractions of finger, wrist, elbow, shoulder, or trunk muscles. The neuronal activities in three tasks performed with finger, wrist, or arm movements were investigated for each group. Most of the neurons related to the contractions of elbow, shoulder, or trunk muscles were associated solely with arm movement. Smaller numbers of neuronal activities changed their firing frequencies in association with two or three tasks. Neurons related to the contractions of finger and wrist muscles showed various firing patterns in the three tasks; some responded to a single task with wrist or arm movement, while others changed their activities in association with more than one task. The presence of multi-task related neurons is discussed with respect to the multisegmental termination of corticospinal axons in the spinal cord.     

The role of primate putamen neurons in the association of sensory stimuli with movement

Neuronal activity in the putamen of monkeys was recorded while they performed operantly conditioned body movements. Two categories of neurons were observed. Type I cells had tonic spontaneous discharges and responded to the sensory trigger stimuli for movements with excitation followed by inhibition or with only inhibition. These responses to the trigger stimuli disappeared when the conditioned movement was extinguished. Type II cells were characterized by phasic activity time-locked to the movement. Two subclasses of type II cells were observed. Type IIa cells exhibited phasic discharges before the first movement of a learned, repetitive sequence of arm or orofacial movements that were triggered by the sensory stimuli. Type IIb cells showed phasic activity modulation during each movement in one direction, either flexion or extension, in an unconditioned manner. Activity of the type IIa cells preceded the onset of EMG in prime mover muscles, while most type IIb cells were activated after the EMG had appeared. Thus, in both type I and type IIa cells the activity can be said to be behaviourally contingent. Type I cells show a movement contingent sensory response, and type IIa cells show movement-related activity that is contingent upon the triggering of the movement by a sensory stimulus.     

A quantitative analysis of pallidal discharge during targeted reaching movement in the monkey

Summary Neurons in the globus pallidus have been studied during reaching movements of the arm made at varying speeds. The reaching task is similar to one used in earlier experiments, in which disruption of normal pallidal output caused changes in movement time. The pallidal cells studied were those that showed task-related changes in activity and a modification of discharge when the arm was manipulated outside of the task. Neuronal discharge was assessed to evaluate two possible models, one in which the timing of task-related discharge varied as a function of movement time and the other in which the amplitude (mean firing rate) of the change in discharge varied as movement time varied. The relation between neuronal discharge and movement time was examined quantitatively on a trial-by-trial basis using a statistical algorithm that identified each phase of the change in neuronal discharge on each trial. A nonparametric statistic was used to determine the correlation between movement time and the duration or latency of changes in neural firing or the mean discharge during each phase of the cell's response. For fifty-five percent of the 40 neurons studied, the timing of the cell's response (duration or latency) varied as a function of movement time. For only 10 cells (25%) was there a significant correlation between movement time and the mean firing rate during one or more phases of the cell's response. Both timing and frequency modulation with movement time were limited to cells responsive to manipulation at the wrist or the shoulder.     

L-DOPA induced extracellular dopamine increases in the ventromedial hypothalamus affects food intake by chickens on a lysine-free diet

When a person standing upright raises an arm on cue, muscles of the left and right sides of the body exhibit changes prior to and specific to the responding arm. We had standing participants perform a visual lexical decision task (“is this letter string a word?”), responding yes by raising one arm and no by raising the other arm. We recorded onset of the arm movement and onset of electromyographic activity in thigh, trunk, and shoulder muscles. We observed the expected responding arm specificity and found that the onset difference favoring word decisions was evident in similar magnitude at all measurement sites, with the difference at the levels of thigh, trunk and shoulder muscles available 225, 189, and 120 ms, respectively, prior to its manifestation at the level of arm movement. We discuss including (a) whole body reaction time along with event-related potentials in determining the decision-response, brain–body temporal relation and (b) response execution along with response initiation in investigating mental chronometry.     

Superposition of independent units of coordination during pointing movements involving the trunk with and without visual feedback

Previous studies addressing the problem of the control of multiple degrees of freedom have examined the influence of trunk movement on pointing movements within the arm's reach. Such movements may be controlled by two functionally independent units of coordination (synergies): one involving only arm joints and producing the hand trajectory to the target (the transport synergy), and the other coordinating trunk and arm movements leaving the hand trajectory unchanged (the compensatory synergy). The question of whether or not this functional subdivision depends on visual feedback was addressed in the present study. We also tested whether or not the motor effects of different synergies are summated as independent components, a control strategy called "superposition." Finally, we investigated whether or not the relationship between different degrees of freedom within each synergy could be considered linear resulting in proportional changes in different joint angles. Seated subjects produced fast, uncorrected arm movements to an ipsi- or a contralateral target in the direction of +/-45 degrees to the sagittal midline of the trunk. Targets could be reached using the arm alone (control trials) or by combining the arm motion with a forward or backward trunk motion produced by hip flexion or extension (test trials), with and without visual feedback. The shape of the hand trajectory, its direction and tangential velocity, movement precision, joint angles and the sequence of the trunk and hand recruitment and de-recruitment were measured. In both visual conditions, the direction of the hand trajectory observed in control trials was generally preserved in test trials. In terms of sequencing, even in the absence of vision, the trunk movement was initiated before the onset of and outlasted the hand shift, indicating that the potential influence of the trunk on the hand movement was compensated by rotations in the elbow and shoulder joint. The analysis of other variables also implied that the effects of trunk recruitment on the hand trajectory were minor compared to those which could be observed if these effects were not compensated by appropriate changes in the arm joint angles. It was concluded that an arm-trunk compensatory synergy is present in pointing movements regardless of visual feedback. Principal component analysis showed that the relationship between elbow, shoulder and hip joint angles in individual arm and combined arm-trunk movements cannot be considered linear, implying that this relationship is adjusted according to the changing arm geometry. The changes in each arm joint angle (elbow, shoulder) elicited by a forward trunk bending in one block of trials were compared with those elicited by a backward bending in another block, whereas the hand moved to the same target in both blocks. These changes were opposite but of similar magnitude. As a result, for each moment of movement, the mean joint angle obtained by averaging across two directions of trunk motion was practically identical to that in control trials in which the trunk was motionless. It is concluded that the transport and arm-trunk compensatory synergies are combined as independent units, according to the principle of superposition. This principle may simplify the control of the coordination of a redundant number of degrees of freedom.     

Qualitative discrepancies between trunk muscle activity and dynamic postural requirements at the initiation of reaching movements performed while sitting

Reaching movements are associated with widespread, nonfocal muscle activity. That activity is often assumed to play a postural role. We tested this assumption for the trunk muscles at the initiation of reaching movements with the following question. Does initial trunk muscle activity play a dynamic postural role by resisting the segmental interactive effects of the arm movement on the trunk? Seated subjects performed bilateral reaching movements while target direction was systematically varied. Muscle activity was recorded from flexors and extensors of the trunk and shoulder. Trunk muscle activity was compared with trunk torques calculated from simulations of reaching movements in which the trunk was modeled to stay still. Recorded trunk muscle activity was in qualitative agreement with torque predictions for only some target directions, suggesting that the nervous system does not activate trunk muscles across all target directions to counteract postural disturbances at the initiation of reaching movements.     

Neuronal activity in cortical motor areas related to ipsilateral, contralateral, and bilateral digit movements of the monkey

1. Single cell activity was studied in the precentral (PCM), premotor (PM), and supplementary (SMA) motor cortex of the monkey to compare magnitudes of activity changes in relation to ipsilateral, contralateral, and bilateral digit movements. 2. Three Japanese monkeys were trained to press a small key with the right or left hand, or with both hands, in accordance with visual instruction signals given 2.6-5.4 s before a visual movement-trigger signal. Great care was taken to train the animal to use only the required part of the limb. As a result of extensive training, electromyographic (EMG) studies revealed that muscle activities before the key press were limited to the digit and hand muscles of the limb instructed to move. No overt increase or decrease in activity was detectable in the proximal limb or body muscles in relation to the key-press movements or instructions. 3. Even though the movement was thus limited to distal forelimb, distinct ipsilateral relationships were observed in 8.2% of the task-related PCM neurons. They changed their activity before ipsilateral and bilateral (but not before contralateral) key press. 4. A majority of the neurons recorded from the digit area of PCM (mostly limited to the anterior bank of the central sulcus) exhibited a contralateral relationship; namely the activity increased or decreased before the onset of the contralateral and bilateral key-press movements. In most of them, the magnitudes of the activity changes before the contralateral and bilateral movements were similar. 5. In proximal limb and trunk areas of PCM and also in the somatosensory cortex, no neurons were found to exhibit distinct relations to any of the key-press movements. 6. In both SMA and PM, a number of neurons exhibited relationships of the type never or only rarely observed in the primary motor cortex. Thirty-seven percent of SMA and 62% of PM neurons exhibited premovement activity changes before all of the key-press movements. The movement-specific type of activity was observed in 28% of SMA and 16% of PM neurons. In these neurons, the activity changes were observed in relation to only one of the right or left key-press movements or exclusively in relation to the bilateral key press. Neuronal activity resembling the majority of the PCM neurons (contralateral type) was observed in 31% of SMA and 13% of PM neurons. 7. Instruction-induced changes in activity were more often found in the secondary than in the primary motor area.(ABSTRACT TRUNCATED AT 400 WORDS)     

Distribution of eye- and arm-movement-related neuronal activity in the SEF and in the SMA and Pre-SMA of monkeys

We analyzed neuronal activity in the supplementary eye field (SEF), supplementary motor area (SMA), and presupplementary motor area (pre-SMA) during the performance of three motor tasks: capturing a visual target with a saccade, reaching one arm to a target while gazing at a visual fixation point, or capturing a target with a saccade and arm-reach together. Our data demonstrated that each area was involved in controlling the arm and eye movements in a different manner. Saccade-related neurons were found mainly in the SEF. In contrast, arm-movement-related neurons were found primarily in the SMA and pre-SMA. In addition, we found that the activity of both arm-movement- and saccade-related neurons differed depending on the presence or absence of an accompanying saccade or arm movement. Such context dependency was found in all three areas. We also discovered that activity preceding eye or arm movement alone, and eye and arm movement combined, appeared more often in the pre-SMA and SEF, suggesting their involvement in effector-independent aspects of motor behavior. Subsequent analysis revealed that the laterality of arm representation differed in the three areas: it was predominantly contralateral in the SMA but largely bilateral in the pre-SMA and SEF.     

Neurons in the primate superior colliculus coding for arm movements in gaze-related coordinates

In the intermediate and deep layers of the superior colliculus (SC), a well-established oculomotor structure, a substantial population of cells is involved in the control of arm movements. To examine the reference frame of these neurons, we recorded in two rhesus monkeys (Macaca mulatta) the discharges of 331 neurons in the SC and the underlying mesencephalic reticular formation (MRF) while monkeys reached to the same target location during different gaze orientations. For 65 reach-related cells with sufficient data and for simultaneously recorded electromyograms (EMGs) of 11 arm muscles, we calculated an ANOVA (factors: target position, gaze angle) and a gaze-dependency (GD) index. EMGs and the activity of many (60%) of the reach-related neurons were not influenced by the target representation on the retina or eye position. We refer to these as "gaze-independent" reach neurons. For 40%, however, the GD fell outside the range of the muscle modulation, and the ANOVA showed a significant influence of gaze. These "gaze-related" reach neurons discharge only when the monkey reaches for targets having specific coordinates in relation to the gaze axis, i.e., for targets in a gaze-related "reach movement field" (RMF). Neuronal activity was not modulated by the specific path of the arm movement, the muscle pattern that is necessary for its realization or the arm that was used for the reach. In each SC we found gaze-related neurons with RMFs both in the contralateral and in the ipsilateral hemifield. The topographical organization of the gaze-related reach neurons in the SC could not be matched with the well-known visual and oculomotor maps. Gaze-related neurons were more modulated in their strength of activity with different directions of arm movements than were gaze-independent reach neurons. Gaze-related reach neurons were recorded at a median depth of 2.03 mm below SC surface in the intermediate layers, where they overlap with saccade-related burst neurons (median depth: 1.55 mm). Most of the gaze-independent reach cells were found in a median depth of 4.01 mm below the SC surface in the deep layers and in the underlying MRF. The gaze-related reach neurons operating in a gaze-centered coordinate system could signal either the desired target position with respect to gaze direction or the motor error between gaze axis and reach target. The gaze-independent reach neurons, possibly operating in a shoulder- or arm-centered reference frame, might carry signals closer to motor output. Together these two types of reach neurons add evidence to our hypothesis that the SC is involved in the sensorimotor transformation for eye-hand coordination in primates.     

Descending signals from the pontomedullary reticular formation are bilateral, asymmetric, and gated during reaching movements in the cat

We examined the contribution of neurons within the pontomedullary reticular formation (PMRF) to the control of reaching movements in the cat. We recorded the activity of 127 reticular neurons, including 56 reticulospinal neurons, during movements of each forelimb; 67/127 of these neurons discharged prior to the onset of activity in the prime flexor muscles during the reach of the ipsilateral limb and form the focus of this report. Most neurons (63/67) showed similar patterns and levels of discharge activity during reaches of either limb, although activity was slightly greater during reach of the ipsilateral limb. In 26/67 cells, the initial change in discharge activity was time-locked to the go signal during reaches of either limb; we have argued that this early discharge contributes to the anticipatory postural adjustments that precede movement. In 11/26 cells, the initial change in activity was reciprocal for reaches with the left and right limbs, although activity during the movement was nonreciprocal. Spike-triggered averaging produced postspike facilitation or depression (PSD) in 12/50 cells during reaches of the limb ipsilateral to the recording site and in 17/49 cells during reach of the contralateral limb. Some cells produced PSD in ipsilateral extensor muscles before the start of the reach and during reaches made with the contralateral, but not the ipsilateral limb; this suggests the signal must be differentially gated. Overall, the results suggest a strong bilateral, albeit asymmetric, contribution from the PMRF to the control of posture and movement during voluntary movement.     

Cerebellar neuronal activity related to whole-arm reaching movements in the monkey

1. Three monkeys were trained to make whole-arm reaching movements from a common central starting position toward eight radially arranged targets disposed at 45 degrees intervals. A sample of 312 cerebellar neurons with proximal-arm receptive fields or discharge related to shoulder or elbow movements was studied in the task. The sample included 69 Purkinje cells, 115 unidentified cortical cells, 65 interpositus neurons, and 63 dentate units. 2. The reaching task was divided into three movement-related epochs: a reaction time, a movement time, and holding over the target. All neurons demonstrated significant changes in discharge during one or more of these three epochs. Almost all of the cells (95%) showed a significant change in activity during the movement, whereas 68-69% of the cells showed significant changes from premovement activity during the reaction time and holding periods. 3. During the combined reaction time-movement period, 231/312 cells were strongly active in the task. Of these, 151 cells (65.4%) demonstrated unimodal directional responses. Sixty-three had a reciprocal relation to movement direction, whereas 88 showed only graded increases or decreases in activity. A further 37 cells (16.0%) were nondirectional, with statistically uniform changes in discharge in all eight directions. The remaining 43 cells (18.6%) showed significant differences in activity for different directions of movement, but their response patterns were not readily classifiable. 4. The proportion of directional versus nondirectional cells was consistent across the four cell populations. However, graded response patterns were more common and reciprocal responses less common among Purkinje and dentate neurons than among unidentified cortical cells and interpositus neurons. 5. The distribution of preferred directions of the population of cerebellar neurons covered all possible movement directions away from the common central starting position in the horizontal plane. When the preferred direction of each cell in the sample population was aligned, the mean direction-related activity of the cerebellar population formed a bell-shaped tuning curve for the activity recorded during both the reaction time and the movement, as well as during the time the arm maintained a fixed posture over the targets. A vector representation also showed that the overall activity of the cerebellar population during normal reaching arm movements generated a signal that varied with movement direction. 6. These results demonstrate that the cerebellum generates a signal that varies with the direction of movement of the proximal arm during normal aimed reaching movements and is consistent with a role in the control of the activity of muscles or muscle groups generating these movements.     

Arm reaching improvements with short-term practice depend on the severity of the motor deficit in stroke

The effects of short-term, constant practice on the kinematics of a multi-joint pointing movement were studied in the hemiparetic arm of 20 chronic patients with unilateral left cerebro-vascular accident (CVA) and in 10 age- and sex-matched healthy individuals. Practice consisted of a single session of 70 pointing movements made with the right arm. Movements were made from a target located beside the body to one in the contralateral workspace, in front of the body. Vision of the final hand position was allowed after every 5^th trial. At the beginning of practice, stroke patients made slower, less precise and more segmented movements, characterised by smaller active ranges of elbow and shoulder motion, disrupted elbow–shoulder coordination, as well as greater trunk movement compared with healthy subjects. With practice, healthy subjects and some patients made faster and more precise movements. These tendencies were revealed only after many repetitions (up to 55 for those with severe hemiparesis), whereas changes in healthy individuals occurred after fewer trials (approximately 20). In addition, the patients decreased movement segmentation with practice. In healthy subjects, faster movement times may be attributed to better shoulder/elbow movement timing in the first half of the reach, whereas improvement of precision was not correlated with any changes in the movement variables. In patients, improvements were accomplished differently depending on arm motor severity. For some patients with mild-to-moderate clinical symptoms, practice resulted in better timing of shoulder/elbow movements with less trunk rotation in middle to late reach. Patients with more severe impairment also improved shoulder/elbow movement timing in mid-reach but used more compensatory trunk rotation. The results suggest that even one session of repetitive practice of a multi-joint pointing task leads to improvements in movement performance-based outcome measures, but the mechanisms of improvement may vary with the individual's level of motor impairment.     

Activity patterns of upper arm muscles in relation to direction of rapid wrist movement in man

Summary Adjustment of arm posture associated with rapid wrist movements was studied by EMG analysis. Seven healthy adults, seated and holding their right arm with the shoulder in a neutral position with the elbow in 90° flexion and the wrist position neutral, were instructted to flex or extend the wrist as fast as possible. To examine whether the activity patterns of the upper arm muscles were related to the prime mover or the direction of the movement in space, the forearm was in two postures, supinate and pronate. The surface EMGs of biceps brachii, brachialis, triceps brachii and the prime movers were recorded along with the angular displacement of the wrist. The sequences of the upper arm muscle activities changed in relation to the direction of the movement. The earliest activities of the upper arm muscles were considered to counteract the dynamic perturbation induced by the rapid wrist movement. The onsets of the earliest activity of the upper arm muscles preceded the movement onset by 50–60 ms. These results revealed that the activity patterns of the arm muscles associated with the rapid wrist movements were functionally compatible with the anticipatory postural adjustment and were controlled according to the direction of the movement in space.     

Postural configuration does not alter unperturbed or perturbed reach movement kinematics

This study investigated whether postural configuration has a significant effect upon the kinematics of arm movements when humans performed unconstrained reach movements to visual targets. Eight subjects were required to reach to static visual targets (unperturbed REACH movements) or correct reach movements when the position of a target unexpectedly changed during the execution of a planned movement (perturbed reaches, or online corrections, OC). Subjects were required to execute REACH and OC movements in sitting and standing (STAND) positions. The height of the targets, distance from the right shoulder (acromion) and eccentricity in terms of the body midline were standardized between the two postural conditions before movements begun. Unperturbed REACH movements were executed to a central target placed at 130 % of outstretched arm length, along the midline (0°). Perturbed (OC) movements involved subjects initiating an arm movement to the 0° target upon its illumination. Two hundred milliseconds after the onset of the hand movement, the 0° target was extinguished and the target at 60° to the right of the midline (still at 130 % outstretched arm distance) illuminated. Subjects had to correct their reach movements online to the new target. Results demonstrated that, despite evident differences in postural kinematics between the four experimental conditions (e.g. pelvis obliquity and trunk/pelvis rotation), postural configuration had little or no effect upon the endpoint kinematics of the finger. Most importantly, the STAND position, with its greater postural constraints, did not affect the time taken to initiate an OC, nor did it lengthen the time taken to complete the REACH or OC movements. Our results suggest, therefore, that postural constraints are accounted for by the central nervous system when executing complex arm reaching movements.     

Experimental muscle pain changes feedforward postural responses of the trunk muscles

Many studies have identified changes in trunk muscle recruitment in clinical low back pain (LBP). However, due to the heterogeneity of the LBP population these changes have been variable and it has been impossible to identify a cause-effect relationship. Several studies have identified a consistent change in the feedforward postural response of transversus abdominis (TrA), the deepest abdominal muscle, in association with arm movements in chronic LBP. This study aimed to determine whether the feedforward recruitment of the trunk muscles in a postural task could be altered by acute experimentally induced LBP. Electromyographic (EMG) recordings of the abdominal and paraspinal muscles were made during arm movements in a control trial, following the injection of isotonic (non-painful) and hypertonic (painful) saline into the longissimus muscle at L4, and during a 1-h follow-up. Movements included rapid arm flexion in response to a light and repetitive arm flexion-extension. Temporal and spatial EMG parameters were measured. The onset and amplitude of EMG of most muscles was changed in a variable manner during the period of experimentally induced pain. However, across movement trials and subjects the activation of TrA was consistently reduced in amplitude or delayed. Analyses in the time and frequency domain were used to confirm these findings. The results suggest that acute experimentally induced pain may affect feedforward postural activity of the trunk muscles. Although the response was variable, pain produced differential changes in the motor control of the trunk muscles, with consistent impairment of TrA activity.     

Role of primate basal ganglia and frontal cortex in the internal generation of movements

Summary This study is a part of a project investigating neuronal activity in the basal ganglia and frontal cortex and describes externally and internally induced preparatory activity in the supplementary motor area (SMA), which forms a closed neuronal loop with the striatum. Monkeys made self-initiated arm reaching movements toward a constant target in the absence of phasic external stimuli. In separate blocks of trials, animals performed in a delayed go no-go task in which an instruction cue prepared for subsequent movement or no-movement to a trigger stimulus. A total of 328 neurons were tested in the delay task. Of these, 91 responded transiently to the instruction light with a median latency of 262 ms. Three quarters of these responses were restricted to the instruction preparing for arm movement, as opposed to with-holding it, and thus may be involved in movement preparation processes. Sustained activation during the instruction-trigger interval was found for 67 neurons and occurred nearly exclusively in movement trials. Activation usually increased gradually after the cue and ended abruptly upon movement onset and thus could be related to the setting and maintenance of processes underlying the preparation of movement. Time-locked responses to the trigger stimulus were found in 38 neurons and were usually restricted to movement trials (median latency 80 ms). Activity time-locked to movement execution occurred in 67 neurons, beginning up to 252 ms before movement onset. A total of 266 neurons were tested with self-initiated arm movements. Of these, 43 showed premovement activity beginning 610–3030 ms before movement onset (median 1430 ms). The activity increased slowly and reached its peak at 370 ms before movement onset. It ended before movement onset or continued until the arm began to move or reached the target. This activity appears to reflect neuronal processes related to the internal generation of movements. Two thirds of activations preceding self-initiated movements occurred in neurons not activated before externally instructed movements, suggesting a selectivity for the internal generation process. Activity related to the execution of self-initiated movements occurred in 67 neurons: it began during and up to 420 ms before movement onset and was usually not associated with pre-movement activity. Most of these neurons were also activated with stimulus-triggered movements, suggesting a lack of selectivity for the execution of self-initiated movements. In comparison with the striatum, more SMA neurons showed preparatory activity preceding externally instructed movements (transient 27% vs 16%, sustained 20% vs 12%) and self-initiated movements (16% vs 11%). Whereas transient responses showed similar latencies and durations in the two structures, sustained preparatory activity preceding externally instructed or self-initiated movements began and reached its peak earlier in SMA compared to striatal neurons. However, due to the long durations, sustained activation largely overlapped in the two structures, and thus essentially occurred simultaneously. Instruction-induced or internally generated preparatory activity may originate outside of the SMA and striatum or may derive from activity reverberating in cortico-basal ganglia loops, possibly in conjunction with other, closely associated cortical and subcortical structures. These data would favor a conjoint role for SMA and striatum in the internal generation of individual behavioral acts and the preparation of behavioral reactions.     

Unraveling the interaction between pathological upper limb synergies and compensatory trunk movements during reach-to-grasp after stroke: a cross-sectional study

The aim of the present study was to identify how pathological limb synergies between shoulder and elbow movements interact with compensatory trunk movements during a functional movement with the paretic upper limb after stroke. 3D kinematic joint and trunk angles were measured during a reach-to-grasp movement in 46 patients with stroke and 12 healthy individuals. We used principal component analyses (PCA) to identify components representing linear relations between the degrees of freedom of the upper limb and trunk across patients with stroke and healthy participants. Using multivariate logistic regression analysis, we investigated whether component scores were related to the presence or absence of basic limb synergies as indicated by the arm section of the Fugl-Meyer motor assessment (FMA). Four and three principal components were extracted in patients with stroke and healthy individuals, respectively. Visual inspection revealed that the contribution of joint and trunk angles to each component differed substantially between groups. The presence of the flexion synergy (Shoulder Abduction and Elbow Flexion) was reflected by component 1, whereas the compensatory role of trunk movements for lack of shoulder and elbow movements was reflected by components 2 and 3 respectively. The presence or absence of basic limb synergies as determined by means of the FMA was significantly related to components 2 (p = 0.014) and 3 (p = 0.003) in patients with stroke. These significant relations indicate that PCA is a useful tool to identify clinically meaningful interactions between compensatory trunk movements and pathological synergies in the elbow and shoulder during reach-to-grasp after stroke.     

Motor control of voluntary trunk movements in standing

The pattern of activity in different trunk muscles during voluntary trunk movements was studied in the standing position in man. The electromyographic activity from ventral and dorsal trunk muscles on the left and right sides were recorded together with the movements in the sagittal and frontal planes (Selspot optoelectronic system). Movement direction, amplitude, velocity and initial posture were varied. In all movements there was a basic pattern of alternation between antagonist muscle groups. Fast movements were initiated by a sharp burst of activity, whereas slow flexions and side bendings resulted from a decrease in antigravity muscle activity. Movement amplitude was related to the magnitude of the initiating burst, and also to the time of onset of antagonist muscle activity with a braking effect. The contribution of passive internal forces in the braking of a movement was indicated by the myoelectrical pattern of activity, particularly in slow large side bendings, where ipsilateral activity was present at the end of the movement. Sagittal movements starting at different initial trunk inclinations resulted in shifts in onset time and duration between antagonist muscles. The observed modifications are specific adaptations of the motor program to balance changes in mechanical conditions, such as angular acceleration, moment arm for the gravitational force, and intrinsic forces of active and passive structures surrounding the spine and pelvis. In conclusion, the present results demonstrate that trunk movements are generated and controlled by specific patterns of muscle coordination.     

Changes in the degree of motor variability associated with experimental and chronic neck–shoulder pain during a standardised repetitive arm movement

The aim of the present study was to investigate the effect of experimental and chronic neck–shoulder pain on the magnitude of cycle-to-cycle variability of task timing, kinematics and muscle activation during repetitive arm movement performed for 3 or 5 min. In an experimental part, acute muscle pain was induced in healthy subjects by intramuscular injection of hypertonic saline in trapezius (n = 10) and infraspinatus (n = 10) muscles. In a clinical part, workers with (n = 12) and without (n = 6) chronic neck–shoulder pain were compared. Cycle-to-cycle standard deviations of task duration, arm and trunk movement in 3D and surface electromyographic (EMG) root mean square activity were computed to assess the degree of variability. The variability in task timing increased in presence of both experimental and chronic pain (P < 0.05) compared with non-painful conditions. Experimental pain increased the variability of the starting position of the arm (P < 0.05), the arm range of motion (P < 0.01), the arm and trunk movement area (P < 0.01) and the acceleration of the arm (P < 0.01). In the chronic pain condition, the variability of arm and trunk acceleration (P < 0.01) and EMG activity (P < 0.05) was decreased compared with healthy controls. These results indicate that pain alters the magnitude of motor variability, and that the transition from acute to chronic pain is accompanied by changes in motor patterns. Experimental pain likely resulted in a quest for a motor solution reducing nociceptive influx, while chronic pain was characterised by a diminished motor flexibility.