The identification of coordination constraints across planes of motion

Two dominant coordination constraints have been identified during isofrequency conditions in previous work: the egocentric constraint, i.e., simultaneous activation of homologous muscle groups, and the allocentric constraint, i.e., moving the segments in the same direction in extrinsic space. To verify their generalization, bimanual drawing movements were performed in different planes of motion (transverse, frontal, sagittal, frontal-transverse) according to the in-phase and anti-phase mode along the X- and Y-axes. Convergent findings were obtained across the transverse, frontal, and frontal-transverse planes. The in-phase mode along both axes was performed most accurately/consistently, whereas the anti-phase mode resulted in a deterioration of the coordination pattern and this effect was most pronounced when the latter mode was introduced with respect to both dimensions. For sagittal plane motions, the in-phase mode was again superior but the second most optimal configuration was the anti-phase mode along both axes. This finding was hypothesized to result from the familiarity with the pattern since it resembles cycling behavior. It illustrates how cognitive mapping is superimposed onto the dynamics of interlimb coordination. Overall, these results support the presence of both the egocentric and allocentric constraint during bimanual movement production. Received: 21 August 1998 / Accepted: 12 February 1999     

Cyclic modulation of the H-reflex in a wrist flexor during rhythmic flexion-extension movements of the ipsilateral foot

 In 12 subjects, each sitting on an armchair with the right forearm prone, the H-reflex elicited in the resting flexor carpi radialis muscle underwent cyclic excitability changes correlated with rhythmic flexion-extension movements of the ipsilateral foot (frequency of oscillations between 1.5 and 2.5 Hz). During foot plantar flexion, the H-reflex underwent a clear-cut increase, the maximum facilitation falling, in most subjects, within the second half of that phase; then, a gradual reduction in size led the reflex amplitude back to the initial value at the end of foot dorsal extension. If present also when the wrist and the ankle are moved together, this facilitation should favour the in-phase (isodirectional) association between movements and, conversely, hinder the anti-phase coupling. Received: 16 June 1997 / Accepted: 10 July 1997     

Proprioceptive control of multijoint movement: bimanual circle drawing

Proprioception is used by the central nervous system (CNS) in the control of the spatial and temporal characteristics of single joint and multiple joint movement. The present study addressed the role of proprioception in the control of bilateral cyclical movements of the limbs. Normal blindfolded human subjects drew circles simultaneously and symmetrically with the two arms (16 cm diameter, 1 /s) upon two digitizing tablets. In selected trials, vibration (60–70 Hz) was applied to the tendon of the biceps and/or anterior deltoid muscles of the dominant arm to distort the proprioceptive information from muscle spindle afferents. One goal of this study was to identify whether tendon vibration influenced the spatial characteristics of circles drawn by the vibrated, dominant arm and the non-vibrated, non-dominant arm. A second goal was to determine the effect of vibration on the temporal coupling between the two arms during circle drawing. The results revealed that tendon vibration affected the spatial characteristics of circles drawn by the vibrated arm in a manner similar to that previously found for unilateral circle drawing. During bimanual circle drawing, vibration had only a minimal effect on the spatial characteristics of the non-vibrated, non-dominant arm. Temporal interlimb coupling was quantified by the relative phasing between the arms. Without tendon vibration, the dominant arm led the non-dominant arm. Vibration of the dominant arm increased the average phase lead. In a first control experiment, vibration of the non-dominant arm decreased the phase lead of the dominant arm, or even reversed it to a non-dominant arm phase lead. In a second control experiment, the subjects performed the bimanual circle-drawing task with vision of only the vibrated arm, in which case there was no spatial distortion of the circles drawn by the vibrated arm, but the phase relation between the two arms was still shifted as if vision were completely unavailable. It was concluded that, in bimanual movements such as these, the spatial and temporal characteristics of movement are controlled independently. Whereas the spatial characteristics of hand movement seem to be controlled unilaterally, the temporal characteristics of interlimb coupling appear to be controlled by proprioceptive information from both limbs, possibly by a proprioceptive triggering mechanism. Received: 29 November 1997 / Accepted: 16 February 1999     

Relative phase destabilization during interlimb coordination: the disruptive role of kinesthetic afferences induced by passive movement

The disruption of three patterns of two-limb coordination, involving cyclical flexion-extension movements performed in the same or in different directions, was investigated through application of passive movement to a third limb by the experimenter. The three patterns referred to the homologous, homolateral, and heterolateral (diagonal) limb combinations which were performed in the sagittal plane. The passive movement involved a spatiotemporal trajectory that differed from the movements controlled actively. Even though subjects were instructed to completely ignore the passive limb movement, the findings of experiment 1 demonstrated a moderate to severe destabilization of the two-limb patterns, as revealed by analyses of power spectra, relative phase, cycle duration, and amplitude. This disruption was more pronounced in the homolateral and heterolateral than in the homologous effector combinations, suggesting stronger coupling between homologous than nonhomologous limb pairs. Moreover, passive mobilization affected antiphase (nonisodirectional) movements more than inphase (isodirectional) movements, pointing to the differential stability of these patterns. Experiment 2 focused on homolateral coordination and demonstrated that withdrawal of visual information did not alter the effects induced by passive movement. It was therefore hypothesized that the generation of extra kinesthetic afferences through passive limb motion was primarily responsible for the detriment in interlimb coordination, possibly conflicting with the sensory information accompanying active movement production. In addition, it was demonstrated that the active limbs were more affected by their homologous passive counterpart than by their non-homologous counterpart, favoring the notion of specific interference. The findings are discussed in view of the potential role of kinesthetic afferences in human interlimb coordination, more specifically the preservance of relative phasing through a kinesthetic feedback loop.     

The influence of joint position on the dynamics of perception-action coupling

Six right-handed subjects performed rhythmic flexion and extension movements of the index finger in time with an auditory metronome. On each block of trials, the wrist of the response hand was placed in a extended, neutral or flexed position. In the flex-on-the-beat condition, subjects were instructed to coordinate maximum excursion in the direction of finger flexion with each beat of the metronome. In the extend-on-the-beat condition, subjects were instructed to coordinate maximum excursion in the direction of finger extension with each beat of the metronome. The frequency of the metronome was increased from 2.00 Hz to 3.75 Hz in 8 steps (8 s epochs) of 0.25 Hz. During trials prepared in the extend-on-the-beat pattern, all subjects exhibited transitions to either a flex-on-the-beat pattern or to phase wandering as the frequency of pacing was increased. The time at which these transitions occurred was reliably influenced by the position of the wrist. Four subjects exhibited qualitative departures from the flex-on-the-beat pattern at pacing frequencies that were greater than those at which the extend-on-the-beat pattern could be maintained. The time at which these departures occurred was not influenced by the position of the wrist. These results are discussed with reference to the constraints imposed on the coordination dynamics by the intrinsic properties of the neuromuscular-skeletal system. Received: 1 October 1997 / Accepted: 20 March 1998     

Bimanual coupling during the specification of isometric forces

The present study investigated the generalizability of the hypothesis of transient coupling during the preparation of bimanual movements (Spijkers and Heuer 1995) to the specification of isometric forces. In the first experiment we used the timed response paradigm (TRP) to examine the time course of the specification process. Subjects had to generate bimanual isometric force pulses while preparation time was controlled by the TRP. Target forces were weak (20% of maximal voluntary force, MVF) or strong (40% MVF) and assigned randomly to each hand. The first experiment revealed the predicted pattern of correlations between the peak forces but, because the subjects tended to delay responding when time for preparation was very brief, the time course of the specification process did not fully match expectations. In the second experiment we improved force–trajectory feedback and presented two initial cues that were expected to induce better preparation of the default force (30% MVF). Both changes were successful and the results further corroborate the transient-coupling hypothesis. Received: 15 October 1998 / Accepted: 04 June 1999     

Eye-hand coordination in uni- and bimanual goal-oriented tasks

 Two different drawer tasks were investigated with the aim of assessing the role of eye movements in well-coordinated hand movements. In an unimanual step-tracking task, which had a predictive and an unpredictive movement, a two-way repeated-measures ANOVA showed a significant effect of prediction on the onset of grip-force (GF) rate (300±39 ms for the predictive condition versus 394±53 ms for the non-predictive condition, P<0.0001). Correlation coefficients, computed from the eye and the hand movements were low for the right and the left hand. The saccade was more coupled with the visual step change than with the action of the hand per se. In a second bimanual pull-and-pick task, the instruction was to pull a drawer with the left hand from a closed position to a LED-cued open position and then to grasp and reinsert a small peg in the drawer with the right hand. Correlation coefficients, computed from the latencies of saccades and of the leading left hand or of the right hand, were significant in four of five subjects. Intermanual correlations were significant in all five subjects. In conclusion, we found that the initial saccade in the unimanual task was best related with the visual step change, but was poorly correlated with the pulling/pushing hand. In the bimanual task, a moderate, but significant temporal coupling between the eyes and hand events was observed. This coupling was, however, less tight than that between both hands. Received: 24 August 1998 / Accepted: 13 January 1999     

Bimanual coordination between isometric contractions and rhythmic movements: an asymmetric coupling

 Interactions between rhythmically moving limbs typically result in attraction to a limited number of coordination modes, which are distinguished in terms of their stability. In addition, the stability of coordination typically decreases with elevations in movement frequency. To gain more insight into the neurophysiological mechanisms underlying these stability characteristics, the effects of phasic voluntary muscle activation onto the movement pattern of the contralateral limb as well as onto the stability of interlimb coordination were examined. This was done in circumstances in which a minimal degree of movement-elicited afferent information was available to mediate the coupling influences. The task involved rhythmic application of isometric torque by one hand, while the other hand was moving rhythmically with unconstrained amplitude. The effects of two levels of applied torque, two coordination patterns (inphase and antiphase), and two movement frequencies were determined, both at the behavioural level (movement kinematics and kinetics) and the neuromuscular level (EMG). The isometric applications of torque clearly influenced the muscle-activation profile and movement pattern of the other limb, affecting both temporal variability and amplitude. Surprisingly, there were no differences between the two coordination patterns or between the tempo conditions. As such, the results did not conform to the Haken-Kelso-Bunz model for rhythmic movement coordination. These data suggest that the archetypal differences in stability of rhythmic bimanual coordination are contingent upon a correspondence between the limbs in terms of their respective tasks. This interpretation is elaborated in terms of the role of sensory feedback and the functional specificity of motor unit recruitment in rhythmic interlimb coordination. Received: 6 November 1998 / Accepted: 7 July 1999     

Control of reactive balance adjustments in perturbed human walking: roles of proximal and distal postural muscle activity

Studies on the proactive control of gait have shown that proximal (hip/trunk) muscles are the primary contributors to balance control, while studies on reactive balance control during perturbed gait, examining only activity in distal (leg/thigh) muscles, have shown that these muscles are important in compensating for a gait disturbance. This study tested the hypothesis that proximal muscles are also primary contributors to reactive balance control during perturbed gait. Thirty-three young adults participated in a study in which an anterior slip was simulated at heel strike by the forward displacement of a force plate on which they walked. Surface electromyographic data were recorded from bilateral leg, thigh, hip and trunk muscles. Kinematic data were collected on joint angle changes in response to the perturbation. The results did not support the hypothesis that the proximal muscles contribute significantly to balance control during perturbed gait. The proximal muscles did not demonstrate more consistent activation, earlier onset latency, longer burst duration or larger burst magnitude than distal muscles. Moreover, although proximal postural activity was often present in the first slip trial, it tended to adapt away in later trials. By contrast, the typical postural responses exhibited by young adults consisted of an early (90–140 ms), high-magnitude (4–9 times muscle activity during normal walking) and relatively long duration (70–200 ms) activation of bilateral anterior leg muscles as well as the anterior and posterior thigh muscles. Thus, postural activity from bilateral leg and thigh muscles and the coordination between the two lower extremities were the key to reactive balance control and were sufficient for regaining balance within one gait cycle. The adaptive attenuation of proximal postural activity over repeated trials suggests that the nervous system overcompensates for a novel balance threat in the first slip trial and fine-tunes its responses with experience. Received: 24 March 1997 /Accepted: 4 September 1997     

Acquisition of anticipatory postural adjustments in a bimanual load-lifting task: normal and pathological aspects

Anticipatory adjustments of forearm posture are associated with a voluntary load-lifting movement in bimanual load-lifting tasks. Three aspects of these adjustments are analyzed: their goal, their central organization, and their acquisition. The goal of the anticipatory adjustment in this task is to minimize the perturbation of forearm posture that occurs during unloading. The central organization is based on two parallel controls responsible, respectively, for the lifting movement of the moving forearm and the anticipatory postural adjustment of the postural forearm, their coordination depending on a central timing signal. The acquisition of the anticipatory postural adjustment was tested using a paradigm where the voluntary movement performed by one hand triggered, via an electronic switch, the load release of the postural forearm. It was achieved after 40–60 trials and was not graded as a function of the voluntary movement parameters, but of the disturbance of the postural arm about to occur. The learned anticipation was not transferred when, after a first acquisition session with one forearm as the postural forearm, a second learning session was performed with the other forearm as the postural forearm. The acquisition was tested in Parkinsonian and in hemiparetic patients with capsular lesion. The highest acquisition deficit was observed in hemiparetic patients, when the contralateral forearm was the postural forearm; the deficit was less important when the ipsilateral arm was postural. Surprisingly, the anticipatory postural adjustments in hemiparetic patients were rather well preserved when the natural load-lifting task was tested. These results suggest that the basal-ganglia SMA circuit and M1 premotor areas are important in the acquisition process. Received: 20 October 1998 / Accepted: 24 March 1999     

Interlimb coordination in patients with Parkinson’s disease: motor learning deficits and the importance of augmented information feedback

 The basal ganglia have traditionally been associated with motor control functions and this view has prevailed since the late nineteenth century. Recent experimental studies suggest that this neuroanatomical system is also critically involved in motor learning. In the present study, motor learning/transfer capabilities were compared between patients with Parkinson’s disease and a group of normal elderly people. Subjects practiced a bimanual coordination task that required continuous flexion-extension movements in the transverse plane with a 90° phase offset between the forearms. During acquisition, augmented visual feedback of the relative motions was provided in real time. The findings revealed improvements in the bimanual coordination pattern across practice in both groups when the augmented concurrent feedback was present. However, when transferred to performance conditions in which the augmented information was withheld, performance deteriorated (relative to the augmented condition) and this effect was more prevalent in the Parkinson patients. More specifically, no improvement in interlimb coordination was observed under nonaugmented feedback conditions across practice. Instead, a drift toward the preferred in-phase and anti-phase coordination patterns was evident. The present findings suggest that Parkinson patients can improve their performance on a new motor task, but they remain strongly dependent on augmented visual information to guide these newly acquired movements. The apparent adoption of a closed-loop control mode is accompanied with decreases in movement speed in order to use the feedback to ensure accuracy. When the augmented feedback is withheld and the movement pattern is to be controlled by means of intrinsic information feedback sources, performance is severely hampered. The findings are hypothesized to indicate that learning/transfer is affected in Parkinson patients who apparently prefer some constancy in the environmental contingencies under which practice takes place. The present findings are consistent with the notion that the basal ganglia form a critical neuroanatomical substrate for motor learning. Received: 7 December 1995 / Accepted: 12 August 1996     

Coordination in prehension Information-based coupling of reaching and grasping

Prehension involves the coordination of a reaching and a grasping movement, such that the hand opens and closes in tune with the transport of the hand to the object to be grasped. To investigate this coordination, we focused on the transition from hand opening to hand closing in the grasping component of prehension. Earlier research has suggested that the time taken to close the hand remains constant over varying reaching amplitudes. In the present experiment, in which subjects reached for objects at six different distances and for objects that moved away from them at three different, constant speeds, hand-closure time was found to vary as a function of experimental conditions. Moreover, initiation of hand closure did not occur at a constant value of the (perceptually available) first-order time remaining until contact with the object. However, the variations observed, occurring as a function of initial hand-object distance and object speed, could be accounted for by an abstract dynamical model of perceptually driven postural changes. Received: 25 July 1996 / Accepted: 9 October 1997     

Loading during the stance phase of walking in humans increases the extensor EMG amplitude but does not change the duration of the step cycle

 Prior work from mammals suggests that load experienced by extensor muscles of the hindlimbs (i.e. Duysens and Pearson 1980; Pearson and Collins 1993; Fouad and Pearson 1997) or cutaneous afferents from the plantar surface of the foot (Duysens and Pearson 1976; Guertin et al. 1995) enhances activity in extensor muscles during the stance phase, and delays the onset of flexor activity associated with the swing phase. The presumed functional significance of this phenomenon is that extensor activity of the supporting limb during walking can: (a) reinforce the supporting function in proportion to the load experienced, and (b) prolong the stance phase until unloading of the limb has occurred. Whether a similar functional role exists for load-sensitive afferents during walking in the human is unknown. In this study, the effect of adding or removing a substantial load (30% of body weight) at the centre of mass was studied in healthy adult human subjects. Loads were applied near the centre of mass to avoid the need for postural adjustments which might confound the interpretation of the results. Subjects walked on a treadmill with either: (a) a sustained increase or decrease in load, or (b) a sudden unexpected increase or decrease in load. In general, subjects responded to the changes in load by changing the amplitude of the extensor electromyographic (EMG) bursts. For example, with sudden unexpected additions in load, the average increase in amplitude was 40% for the soleus across the stance phase, and 134% for the quadriceps during the early part of the stance phase. Extensor EMGs increased with both sustained and sudden increases in load. Extensor EMG durations also increased (average increase in duration of 4% for soleus with sudden loading, and 7% for sustained loading). Cycle duration hardly changed (average increase of 0.5% with both sudden and sustained loading). These results differ from those of infants subjected to a similar perturbation during supported walking. A large change in timing (i.e. an increase in the duration of the stance phase by 30% and the step cycle by 28%) was seen in the infants, with no change in the amplitude of the EMG burst (Yang et al. 1998). These results suggest that the central nervous system can control the timing and amplitude of extensor EMG activity in response to loading independently. Maturation of the two components most likely occurs independently. In the adult, independent control of the two components may provide greater flexibility of the response. Received: 28 April 1998 / Accepted: 3 September 1998     

Gaze effects in the cerebral cortex: reference frames for space coding and action

Visual information is mapped with respect to the retina within the early stages of the visual cortex. On the other hand, the brain has to achieve a representation of object location in a coordinate system that matches the reference frame used by the motor cortex to code reaching movement in space. The mechanism of the necessary coordinate transformation between the different frames of reference from the visual to the motor system as well as its localization within the cerebral cortex is still unclear. Coordinate transformation is traditionally described as a series of elementary computations along the visuomotor cortical pathways, and the motor system is thought to receive target information in a body-centered reference frame. However, neurons along these pathways have a number of similar properties and receive common input signals, suggesting that a non-retinocentric representation of object location in space might be available for sensory and motor purposes throughout the visuomotor pathway. This paper reviews recent findings showing that elementary input signals, such as retinal and eye position signals, reach the dorsal premotor cortex. We will also compare eye position effects in the premotor cortex with those described in the posterior parietal cortex. Our main thesis is that appropriate sensory input signals are distributed across the visuomotor continuum, and could potentially allow, in parallel, the emergence of multiple and task-dependent reference frames. Received: 21 September 1998 / Accepted: 19 March 1999     

The development toward stereotypic arm kinematics during reaching in the first 3 years of life

We recorded reaching movements from nine infants longitudinally from the onset of reaching (5th postnatal month) up to the age of 3 years. Here we analyze hand and proximal joint trajectories and examine the emerging temporal coordination between arm segments. The present investigation seeks (a) to determine when infants acquire consistent, adult-like patterns of multijoint coordination within that 3-year period, and (b) to relate their hand trajectory formation to underlying patterns of proximal joint motion (shoulder, elbow). Our results show: First, most kinematic parameters do not assume adult-like levels before the age of 2 years. At this time, 75% of the trials reveal a single peaked velocity profile of the hand. Between the 2nd and 3rd year of life, “improvements” of hand- or joint-related movement units are only marginal. Second, infant motor systems strive to obtain velocity patterns with as few force reversals as possible (uni- or bimodal) at all three limb segments. Third, the formation of a consistent interjoint synergy between shoulder and elbow motion is not achieved within the 1st year of life. Stable patterns of temporal coordination across arm segments begin to emerge at 12–15 months of age and continue to develop up to the 3rd year. In summary, the development toward adult forms of multijoint coordination in goal-directed reaching requires more time than previously assumed. Although infants reliably grasp for objects within their workspace 3–4 months after the onset of reaching, stereotypic kinematic motor patterns are not expressed before the 2nd year of life. Received: 10 April 1996 / Accepted: 28 May 1997     

Integration of the hand in postural reactions to sustained sideways force at the pelvis

 In order to investigate the potential hand contribution to sideways balance, sideways pushes to the right, which subjects resisted using either the lower limbs (”hip only”) or the lower limbs assisted by the right upper limb (”hand and hip”), were delivered to the pelvis. Analysis of force and electromyogram recordings from the legs, arm and hand in the hand-and-hip condition showed a close co-ordination of upper and lower limbs in terms of mean latencies and amplitudes. However, trial to trial fluctuations of forces generated by the hand and leg did not correlate, suggesting parallel pathways under central co-ordinative control. Received: 6 August 1998 / Accepted: 11 December 1998     

Coordination between posture and movement in a bimanual load-lifting task: is there a transfer?

The present experimental series was designed to test the possibility that an anticipatory postural adjustment learned during the performance of a bimanual load lifting task may be transferred between the upper extremities. Eight seated subjects were asked to maintain horizontally one forearm (postural arm) loaded with a 1kg load, which was fixed to the arm by means of an electromagnet. The unloading was triggered either by the experimenter pressing a switch (control) or by the subjects making a voluntary movement with their other arm (moving arm). In the latter case, the subject lifted a 1-kg load resting on a force platform with the moving hand, and the switching off was triggered when the force level reached a threshold of 0.5 kg. The maximum amplitude (MA) and the maximum velocity (MV) of the postural forearm elbow joint rotation occuring after the unloading were measured at each trial. The learning process was estimated by performing a regression analysis on each series of trials, using an exponential model, and from the intercept of the regression curve with the ordinate. 1. During the original learning session (three series of 20 trials), a decrease in MA and MV was found to occur both within the series and between the series during a session. 2. After the initial learning session, the sides of the postural and moving arm were interchanged to test whether any transfer had occurred. The first series of trials in the second session (transfer) and the last series of trials in the original learning session were compared and found to be significantly different in terms of the intercept (seven subjects in the case of MA, five subjects in the case of MV) and the slope (five subjects), indicating a lack of transfer. 3. The data recorded during the second transfer learning session indicated that learning occurred in all eight subjects in the case of MA and in six subjects in the case of MV. It was observed that the original learning session did not facilitate the second one. 4. The lack of transfer of the anticipatory postural adjustment observed in this task is discussed with reference to the data in the literature.     

Coordination between posture and movement in a bimanual load lifting task: putative role of a medial frontal region including the supplementary motor area

Summary The aim of the present experimental series was to investigate the role of the medial frontal region including the supplementary motor area in the coordination between posture and movement in a bimanual load lifting task. The seated subject was instructed to maintain in a horizontal position one forearm (postural arm) which was loaded with a 1 kg weight. The unloading was performed either by the experimenter (imposed unloading) or by a voluntary movement of the other arm (voluntary unloading). In normal individuals, with the voluntary unloading, the movement control was accompanied by an anticipatory adjustment of the postural forearm flexor activity, which resulted in the maintenance of the forearm position despite the unloading. The anticipatory postural adjustments were impaired in 4 out of 5 patients with unilateral lesion of the SMA region; the defect was observed mainly when the postural forearm was contralateral to the lesion. No change in the anticipatory postural adjustment was observed in one patient with complete callosal section. This finding indicates that the coordination between the posture and movement in this task is not organized through callosal fibers linking the cortices on both sides but rather at a subcortical level. The anticipatory postural adjustments were abolished in two patients with spastic hemiparesis when the postural forearm was the spastic arm. It is suggested that the SMA region contralateral to the postural forearm, together with other premotor or motor areas, may select the circuits responsible for the phasic postural adjustments which are necessary to ensure postural maintenance, whereas the motor cortex contralateral to the voluntary movement controls both the movement and, via collaterals, the preselected circuits responsible for the associated postural adjustments.     

Differential localisation of the metabotropic glutamate receptor mGluR1a and the ionotropic glutamate receptor GluR2/3 in neurons of the human cerebral cortex

Specimens of human cerebral cortex were obtained during neurosurgical operations and studied by immunocytochemistry and electron microscopy, using antibodies to the metabotropic glutamate receptor subunit mGluR1a and the ionotropic glutamate receptor GluR2/3. A small number of non-pyramidal neuronal cell bodies were labelled for mGluR1a. Double immunolabelling with mGluR1a and GluR2/3 showed that most pyramidal cell bodies were labelled for GluR2/3 but not for mGluR1a. Despite the non-colocalisation of these two receptor subtypes in cell bodies, however, many dendrites and dendritic spines were double-labelled for mGluR1a and GluR2/3 at electron microscopy. As there is evidence that most neurons positive for GluR2/3 are pyramidal cells, this suggests that mGluR1a is present in dendrites of pyramidal neurons, despite absent or low levels of immunoreactivity in their cell bodies. Received: 5 May 1997 / Accepted: 24 July 1997     

The influence of peripheral load on resetting voluntary movement by cortical stimulation: importance of the induced twitch

 We studied the effects of changes in loading torque on the effectiveness of magnetic cortical stimulation in evoking phase resetting of voluntary wrist movement. Nine normal subjects were studied (five on two occasions), while making rhythmical movements of the right wrist, at their preferred rate, against extension torque loads of 0.35 Nm, 0.26 Nm and 0.18 Nm, flexion torque loads of 0.09 Nm and 0.18 Nm and without external load. The position records of individual trials were used to measure the effectiveness of resetting (resetting index: the slope of the phase-response curve) and the ”null phase”, the phase to which the trials were being reset. The loading torque had a strong influence upon both the resetting index and the null phase, generated by a constant intensity of cortical stimulation such that the largest resetting indices were obtained for movements made against the largest extension torque load (mean resetting index 0.72). The degree of resetting and null phase were related to the mean amplitude and direction of the first poststimulus position peak, which in turn was largely determined by the twitch induced by the cortical shock. The timings of the averaged poststimulus position peaks following the first were simple multiples of the prestimulus movement period. Our results indicate that loading conditions profoundly influence the effectiveness of magnetic cortical stimulation in resetting a voluntary movement and that these effects appear to be largely explicable by the changes in the muscle twitch evoked by the stimulus with the different loads. We suggest that the magnetic shock is therefore unlikely to reset voluntary movement by an action directly upon the motor programme. We propose that the main method by which magnetic cortical stimulation resets repetitive wrist movement is indirect: normal generation of repetitive wrist flexion and extension is disrupted by the cortical shock, following which afferent information related to the twitch induced is able to reset the movement. Received: 2 September 1996 / Accepted: 3 April 1997