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PubMed1976

Visual "closed-loop" and "open-loop" characteristics of voluntary movement in patients with Parkinsonism and intention tremor.

Normal voluntary movements are considered to be of two kinds, or to involve two components, (i) a ballistic or "open-loop" type, which are preprogrammed and executed without reference to current sensory information and (ii) a corrective or "closed-loop" type, whose course or termination are regulated by such information. In a previous paper it was suggested that Parkinsonism disrupted the first kind of movement, but intention tremor did not. In the present paper three experiments designed to test this hypothesis are described. Subjects were tested on an acquisition-tracking task using an oscilloscope display and joystick control, and measurements were made of the duration, velocity and error of their initial movements to acquire the target. Parkinsonian movements were found to be considerably different from normal in that (a) most movements by this group lasted longer than the reaction time for their initiation, as if including some secondary correction in their execution, (b) the rate of movement was not varied for different amplitudes (so keeping the duration fairly constant) as in normal subjects, but rather movements of all amplitudes were made at a constant slow rate, so that duration increased markedly with the larger steps, (c) error increased disproportionately as the velocity of movement increased; in particular any movements completed in one reaction time or less tended to be wildly inaccurate, (d) removing either the target or the response marker from the screen at the beginning of a movement had a significant effect, making it shorter in duration and smaller in amplitude than those usually produced with both markers visible all the time. Parkinsonian subjects showed no improvement in performance with repeated attempts at one movement over a whole sequence, so their deficit appears to be stable even after practice on a known fixed task. These results are interpreted as supporting the hypothesis that Parkinsonism interferes with the generation of accurate ballistic action which are characteristic of normal skilled movement. Tremor subjects in general resembled normal control subjects in their initial acquistion movements, but their accuracy was less with the larger steps.

Adult↗
PubMed2006

Heart rate response to respiratory events with or without leg movements.

STUDY OBJECTIVE: This study compares the heart rate responses to the termination of respiratory events, both with and without associated leg movements. METHODS: Heart rate was measured for 15 R-R intervals before (T-15 to T-1) and after (T+1 to T+15) the termination of respiratory events as a change from the baseline rate, defined as the average of 10 R-R intervals occurring before the termination of each respiratory event (T-15 to T-6). Individual heart rate changes of the 21 patients were then averaged separately for 10 respiratory events with and 10 without associated leg movements. SETTING: N/A. PARTICIPANTS: Twenty-one patients with obstructive sleep apnea who had respiratory events both with and without associated leg movements. INTERVENTION: N/A. RESULTS: Maximal heart rate rise for respiratory events with leg movements (7.9 beats per minute) was significantly greater than for respiratory events without leg movements (5.1 beats per minute) (p < .0001). The area under the curve for heart rate increase from T-5 to T+9 was 50.1% higher for respiratory events with leg movements than without leg movements. When respiratory events with and without accompanying leg movements were compared, there were no significant differences in mean duration of respiratory events, mean oxygen desaturation after respiratory events, mean duration of electroencephalogram arousal following respiratory events, or mean heart rate during the baseline period. Heart rate rise did correlate with duration of the leg movements (p < .001) in those respiratory events with leg movements. CONCLUSIONS: Cardiac activation is significantly greater when the termination of respiratory events is associated with leg movements compared to those without leg movements. This exaggerated heart rate response may be an independent consequence of the leg movements themselves, as other features of the respiratory events and associated arousal were not different in the two conditions.

Adult↗
PubMed1999

Effects of direction and curvature on variable error pattern of reaching movements.

A number of studies have analyzed various indices of the final position variability in order to provide insight into different levels of neuromotor processing during reaching movements. Yet the possible effects of movement kinematics on variability have often been neglected. The present study was designed to test the effects of movement direction and curvature on the pattern of movement variable errors. Subjects performed series of reaching movements over the same distance and into the same target. However, due either to changes in starting position or to applied obstacles, the movements were performed in different directions or along the trajectories of different curvatures. The pattern of movement variable errors was assessed by means of the principal component analysis applied on the 2-D scatter of movement final positions. The orientation of these ellipses demonstrated changes associated with changes in both movement direction and curvature. However, neither movement direction nor movement curvature affected movement variable errors assessed by area of the ellipses. Therefore it was concluded that the end-point variability depends partly, but not exclusively, on movement kinematics.

Adult↗
PubMed1993

Neural activity in the monkey anterior ventrolateral thalamus during trained, ballistic movements.

1. To examine the role of the anterior ventrolateral thalamic nucleus (VLa) in motor control, extracellular single-cell recordings were made from the VLa nucleus in three conscious monkeys performing visually triggered, rapid movements requiring flexion and extension of the wrist joint. The movement paradigms consisted of three components: an initial hold period, a ballistic movement, and a final hold period. 2. Cerebellar nuclear stimulation was used to physiologically identify thalamic neurons receiving input from the cerebellum. Neurons subsequently confirmed histologically as lying within the VLa nucleus were located anterior to those "driven" by cerebellar stimulation. 3. The activities of 261 VLa neurons displaying movement-related behavior were examined. In the absence of movement, the activity of these neurons ranged from 5 to 80 spikes/s, the majority of cells (222/261; 85%) firing between 10 and 30 spikes/s. Neural activity was not influenced by afferent input from skin, joint, or muscle but altered markedly when the animal engaged in active movement. 4. The relationship between neural activity and movement at a specific joint was determined in 189 of the 261 neurons. The majority of these neurons (161/189; 85%) modulated their activity in response to movement confined to a single joint; the remaining neurons displayed multijoint motor responses, their activity being best related to a motor act, such as grasping, reaching, or feeding. There was no clear segregation in the representation of body parts in the VLa nucleus to suggest the presence of clearly defined somatotopy. 5. The activities of wrist-related neurons (45 of the 261) were examined to determine the motor characteristics of the VLa nucleus. Three distinctive activity patterns were exhibited by these neurons during the performance of the movement paradigms: 1) a phasic burst of < 300 ms in duration (34/45; 76%); 2) a sustained change in neural activity lasting > 300 ms (typically up to 1,000 ms; 4/45; 9%); and 3) both a phasic and a sustained response, the sustained response occurring either prior or subsequent to the phasic response (7/45; 15%). 6. The movement-related activity of the wrist-related neurons occurred late, 52% firing after the onset of the forearm electromyogram (EMG). In 47% of the neurons, discharge rate was related to movement direction and, in a smaller proportion (13%), joint position. No relationship was found between neural activity patterns and amplitude of movement or velocity of movement. (ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

The learning of novel finger movement sequences.

1. Experienced typists typed phrases containing words in which one isolated letter was typed with one hand, while the remaining letters were typed with the contralateral hand. 2. The translational and rotational motion of the fingers and wrist of the right hand were obtained optoelectronically from the location of reflective markers placed on the fingers. 3. Midway through the experiment, the key corresponding to the isolated letter was physically switched with another key on the keyboard, and subjects typed the letter in its new location (for 140 trials). The letter "n," typed with the right index finger, was either switched with letters normally typed with the same finger (u), with a different finger but same hand (o), with the same finger of the left hand (v), or with a different finger of the left hand (w). 4. When the words were typed normally, the interkey intervals were relatively short, and the onset of movement of the right hand began before the preceding keypress with the left hand. Thus the movement of the two hands overlapped. Furthermore, the movement to the isolated key was highly stereotypical, with little trial-to-trial variability. 5. After the transposition of keys, there were prolongations in the interkey intervals, with the largest delay occurring directly before the typing of the transposed key. Switches between homologous fingers (involving mirror movements) delayed the onset of keypresses to a lesser extent than did other switches. With practice, these delays were reduced but never reached the control level. 6. After the keyswitch, the onset of movement to the isolated key did not occur on average until after the last keypress with the contralateral hand, except when the switch involved the use of homologous fingers. In the latter case, overlapping movement of the two hands was maintained. Thus the learning of a series of discrete movements does not necessarily require that each movement segment be performed sequentially. 7. After the transposition of keys, the movement pattern and time course to a given key were similar to the movement patterns for that key observed during control trials in all conditions. Thus the learning of a series of movements may involve the use of previously learned movements under new conditions. 8. The results suggest that typing movements may be organized at several levels, including the individual keystroke and word level.

Adult↗
PubMed1996

One-trial adaptation of movement to changes in load.

1. We analyzed the rapid adaptation of elbow movement to unexpected changes in external load conditions at the elbow joint. The experimental approach was based on the lambda model, which defines control variables (CVs) setting the positional frames of reference for recruitment of flexor and extensor motoneurons. CVs may be specified by the nervous system independently of the current values of output variable such as electromyographic (EMG) activity, muscle torques, and kinematics. The CV R specifies the referent joint angle (R) at which the transition of flexor to extensor activity or vice versa can be observed during changes in the actual joint angle, theta, elicited by an external force. The other CV, the coactivation (C) command, instead of a single transition angle, defines an angular range in which flexor and extensor muscles may be simultaneously active (if C > 0) or silent (if C < 0). Changes in the R command result in shifts in the equilibrium state of the system, a dynamic process leading to EMG modifications resulting in movement or isometric force production if movement is obstructed. Fast movements are likely produced by combining the R command with a positive C command, which provides movement stability and effective energy dissipation, diminishing oscillations at the end of movement. 2. According to the model, changes in the load characteristic (e.g., from a 0 to a springlike load) influence the system's equilibrium state, leading to a positional error. This error may be corrected by a secondary movement produced by additional changes in R and C commands. In subsequent trials, the system may reproduce the CVs specified after correction in the previous trial. This behavior is called the recurrent strategy. It allows the system to adapt to the new load condition in the subsequent trials without corrections (1-trial adaptation). Alternatively, the system may reproduce the CVs specified before correction (invariant strategy). If the movement was perturbed only in a single trial, the invariant strategy allows the system to reach the target in the subsequent trials without corrections. 3. To test the assumption on the dominant role of the recurrent strategy in rapid adaptation of movement to new load conditions, we performed experiments in which subjects (n = 6) used a pivoting manipulandum and made fast 60 degrees movements to a target. After a random number of trials (5-10) with no load, we introduced opposing (experiment 1), assisting (experiment 2), or randomly varied opposing or assisting loads (experiment 3) for 5-10 trials before unexpectedly switching loads again (14-18 switches in total). The opposing or assisting torque was created by position feedback to a torque motor and was a linear function of the displacement of the manipulandum form the initial position (springlike load). Subjects were instructed to correct positional errors as soon as possible to reach the target. The EMG activity of two elbow flexors (biceps brachii and brachioradialis) and two elbow extensors (triceps brachii and anconeus), elbow position, velocity, and torque were recorded. Kinematic and EMG patterns were compared with those obtained in similar experiments in which subjects were instructed not to correct errors. 4. In 94% of the trials in which a change in the load occurred, the primary movement was in error and was followed by a corrective secondary movement. In primary movements, both the phasic and tonic levels of EMG activity as well as the kinematics were load dependent, implicating reflex and intramuscular mechanisms in the adaptation of muscle forces counteracting external loads. These mechanisms, however, were not sufficient to eliminate positional errors. 5. An undershoot error occurred in trials with an opposing load after those with no load or in trials with no load after those with an assisting load. After adaptation to a new load condition, a sudden return to the previous load condition resulted in an error of the oppo

Adult↗
PubMed2005

Computer-assisted kinematic evaluation of induced compensatory movements resembling lameness in horses trotting on a treadmill.

OBJECTIVE: To characterize compensatory movements of the head and pelvis that resemble lameness in horses. ANIMALS: 17 adult horses. PROCEDURE: Kinematic evaluations were performed while horses trotted on a treadmill before and after shoe-induced lameness. Lameness was quantified and the affected limb determined by algorithms that measured asymmetry in vertical movement of the head and pelvis. Induced primary lameness and compensatory movements resembling lameness were assessed by the Friedman test. Association between induced lameness and compensatory movements was examined by regression analysis. RESULTS: Compensatory movements resembling lameness in the ipsilateral forelimb were seen with induced lameness of a hind limb. There was less downward and less upward head movement during and after the stance phase of the ipsilateral forelimb. Doubling the severity of lameness in the hind limb increased severity of the compensatory movements in the ipsilateral forelimb by 50%. Compensatory movements resembling lameness of the hind limb were seen after induced lameness in a forelimb. There was less upward movement of the pelvis after the stance phase of the contralateral hind limb and, to a lesser extent, less downward movement of the pelvis during the stance phase of the ipsilateral hind limb. Doubling the severity of lameness in the forelimb increased compensatory movements of the contralateral hind limb by 5%. CONCLUSIONS AND CLINICAL RELEVANCE: Induced lameness in a hind limb causes prominent compensatory movements resembling lameness in the ipsilateral forelimb. Induced lameness in a forelimb causes slight compensatory movements resembling lameness in the ipsilateral and contralateral hind limbs.

Animals↗
PubMed1996

Properties of eye movements induced by activation of neck muscle proprioceptors.

BACKGROUND: We have previously reported that neck muscle vibration can induce visual illusory movements and eye movements under subdued ambient illumination in human subjects with the head and body fixed. The subjects viewed a fixation light with one eye and the other eye was covered. The eye movements were recorded with an infrared system mounted in goggles attached to the head of the subject. METHODS: In order to further examine the properties of these cervico-ocular reactions and reveal any artefacts in the eye movement recordings, we have studied (1) the effect of increasing the ambient light, which made visual illusory movements disappear, (2) the timing between the illusory movement and the eye movement in subdued light, (3) the effect of viewing the target through a pin-hole, which would reveal artefacts due to head movement, and (4) the effect of mounting the eye movement recording system on the head support, which would allow recording to the absolute eye position change. RESULTS: The results of these experiments showed no significant difference from those that were reported previously. Eye movements of about the same amplitude were induced in both eyes under all conditions, and there was no time difference in the occurrence of visual illusory movements and eye movements. CONCLUSION: The results of this study confirm that neck muscle vibration can induce eye position changes. This seems to confirm that the proprioceptive messages originating in the neck muscles are processed together with visual information of eye position in determining gaze direction.

Adult↗
PubMed2004

Electrical stimulation of rhesus monkey nucleus reticularis gigantocellularis. I. Characteristics of evoked head movements.

The nucleus reticularis gigantocellularis (NRG) receives monosynaptic input from the superior colliculus (SC) and projects directly to neck motor neuron pools. Neurons in NRG are well situated to play a critical role in transforming SC signals into head movement commands. A previous study of movements evoked by NRG stimulation in the primate reported a variety of ipsilateral and contralateral head movements with horizontal, vertical and torsional components. In addition to head movements, it was reported that NRG stimulation could evoke movements of the pinnae, face, upper torso, and co-contraction of neck muscles. In this report, the role of the rhesus monkey NRG in head movement control was investigated using electrical stimulation of the rostral portion of the NRG. The goal was to characterize head movements evoked by NRG stimulation, describe the effects of altering stimulation parameters, and assess the relative movements of the eyes and head. Results indicate that electrical stimulation in the rostral portion of the NRG of the primate can consistently evoke ipsilateral head rotations in the horizontal plane. Head movement amplitude and peak velocity depend upon stimulation parameters (primarily frequency and duration of stimulation trains). During stimulation-induced head movements the eyes counter-rotate (presumably a result of the vestibulo-ocular reflex: VOR). At 46 stimulation sites from two subjects the average gain of this counter-rotation was -0.38 (+/-0.18). After the end of the stimulation train the head generally continued to move. During this epoch, after electrical stimulation ceased, VOR gain remained at this reduced level. In addition, VOR gain was similarly low when electrical stimulation was carried out during active fixation of a visual target. These data extend existing descriptions of head movements evoked by electrical stimulation of the NRG, and add to the understanding of the role of this structure in producing head movements.

Animals↗
PubMed1997

Slow eye movements.

Monkeys and humans are able to perform different types of slow eye movements. The analysis of the eye movement parameters, as well as the investigation of the neuronal activity underlying the execution of slow eye movements, offer an excellent opportunity to study higher brain functions such as motion processing, sensorimotor integration, and predictive mechanisms as well as neuronal plasticity and motor learning. As an example, since there exists a tight connection between the execution of slow eye movements and the processing of any kind of motion, these eye movements can be used as a biological, behavioural probe for the neuronal processing of motion. Global visual motion elicits optokinetic nystagmus, acting as a visual gaze stabilization system. The underlying neuronal substrate consists mainly of the cortico-pretecto-olivo-cerebellar pathway. Additionally, another gaze stabilization system depends on the vestibular input known as the vestibulo-ocular reflex. The interactions between the visual and vestibular stabilization system are essential to fulfil the plasticity of the vestibulo-ocular reflex representing a simple form of learning. Local visual motion is a necessary prerequisite for the execution of smooth pursuit eye movements which depend on the cortico-pontino-cerebellar pathway. In the wake of saccades, short-latency eye movements can be elicited by brief movements of the visual scene. Finally, eye movements directed to objects in different planes of depth consist of slow movements also. Although there is some overlap in the neuronal substrates underlying these different types of slow eye movements, there are brain areas whose activity can be associated exclusively with the execution of a special type of slow eye movement.

Animals↗
PubMed2000

Wireless optoelectronic recordings of mandibular and associated head-neck movements in man: a methodological study.

Human mandibular movements in space are the result of combined motions of the mandible and the head-neck. They can be simultaneously monitored by an optoelectronic recording technique via markers at different locations on the mandible and on the head. Markers can be attached to the teeth or to the facial skin. Mandibular movements relative to the head can be calculated by one- or three-dimensional (1D and 3D, respectively) mathematical compensation for head movements. The present study analysed mandibular and associated head movements during maximal jaw opening-closing tasks in 10 healthy subjects using a wireless 3D optoelectronic movement recording system. The study aimed to: (i) estimate the soft tissue related displacement of skin-attached markers at different locations on the face; (ii) compare 1D with 3D mathematical compensation for associated head movements; (iii) evaluate the influence of marker location on the recorded head and mandibular movement amplitudes; and (iv) compare skin-attached markers with teeth-attached markers with regard to temporal estimates of recorded mandibular and head movements. Markers were attached to the upper and lower incisors and to the skin of the forehead, nose-bridge, nose-tip and chin. Soft tissue related displacement of skin-attached markers varied between locations. The displacement for the chin marker was larger than that of other markers. The least displacement was found for the nose-bridge marker. However, relative to mandibular and head movements, respectively, the displacement of the chin marker was of the same order as that of the nose-bridge marker. The temporal estimates were not significantly affected by displacement of the skin-attached markers. Markers at different locations on the head and the mandible registered different amplitudes. The mandibular movement patterns calculated by 1D and 3D compensation were not comparable. It is concluded that markers attached to the chin and the nose-bridge can be reliably used in temporal analyses of mandibular and head movements during maximal jaw opening-closing. With certain limitations, they are acceptable for spatial analyses. Selection of method of marker attachment, marker location, and method of compensation for associated head movements should be based on the aim of the study.

Adult↗
PubMed2001

Functional-rhythmical coupling of head and mandibular movements.

It is known that small head movements accompany the movements of the jaw during mastication; however, it is unknown whether these movements occur rhythmically and synchronously. The objective of this study was to determine whether there exists a functional coupling between the head and mandibular movements. Four healthy male adults (mean age 25.5) with normal occlusion and without TMD history were selected as subjects. Using the Trimet system, we measured tridimensionally both the movement of the head and the mandible by tracking upper and lower incisal points, respectively, during tapping movements with different opening range and frequency, then analysed the vertical component of these movements. The upper incisal point moved in opposite direction to the mandible in all tapping strokes in all subjects, during opening the head moved in a cranial direction and during closing in a caudal direction; the incidence rate for this concomitant movement was 98%, implying that the head moves periodically and rhythmically, as the mandible does. The cycle time of these coincident movements showed a correlation coefficient of 0.94. Moreover, the vertical range of head movement was within 10% of the jaw's movement. From these results we concluded that, at least during teeth tapping, the head moves in rhythmical coordination with mandibular movement.

Adult↗
PubMed1988

The coexistence of bradykinesia and chorea in Huntington's disease and its implications for theories of basal ganglia control of movement.

Investigation of motor function in a group of 17 patients with Huntington's disease reveals that, in addition to the chorea that many patients exhibit, defects in voluntary motor performance also are evident. Fast simple wrist flexion movements to 15 degrees or 60 degrees were slower, and individual movements showed greater variability than seen in normal subjects. This bradykinesia was most pronounced in those patients who were akinetic and rigid, but also was seen in those with chorea alone; bradykinesia was independent of the drug treatment that the patients were receiving (and was therefore not due to drug-induced parkinsonism). The electromyographic activity of the agonist muscles during such simple but slow movement differed from that seen in Parkinson's disease. The performance of complex movements revealed further deficits. Some patients were unable to combine two movements in a simultaneous or sequential movement task of squeezing the hand and flexing the elbow. Those who could perform these complex movements exhibited slowing of the velocity of the movement and prolongation of the interval between movements. These abnormalities were present in patients with chorea who were not taking neuroleptic drugs. It is argued that they represent an abnormality of motor programming of complex movements, over and above the defect in executing simple movements. The long latency stretch reflexes in wrist flexor muscles and flexor pollicis longus were reduced or absent, but this did not correlate with changes in motor performance, or with the reduced size of the early components of cortical sensory evoked potentials. Bradykinesia is thus shown to be an integral component of the motor disorder of Huntington's disease, in addition to the chorea. The coexistence of bradykinesia and chorea in this illness is compatible with current theories of the role of the basal ganglia in the control of movement.

Adult↗
PubMed2003

Neural basis of visually guided head movements studied with fMRI.

We used event-related fMRI to measure brain activity while subjects performed saccadic eye, head, and gaze movements to visually presented targets. Two distinct patterns of response were observed. One set of areas was equally active during eye, head, and gaze movements and consisted of the superior and inferior subdivisions of the frontal eye fields, the supplementary eye field, the intraparietal sulcus, the precuneus, area MT in the lateral occipital sulcus and subcortically in basal ganglia, thalamus, and the superior colliculus. These areas have been previously observed in functional imaging studies of human eye movements, suggesting that a common set of brain areas subserves both oculomotor and head movement control in humans, consistent with data from single-unit recording and microstimulation studies in nonhuman primates that have described overlapping eye- and head-movement representations in oculomotor control areas. A second set of areas was active during head and gaze movements but not during eye movements. This set of areas included the posterior part of the planum temporale and the cortex at the temporoparietal junction, known as the parieto-insular vestibular cortex (PIVC). Activity in PIVC has been observed during imaging studies of invasive vestibular stimulation, and we confirm its role in processing the vestibular cues accompanying natural head movements. Our findings demonstrate that fMRI can be used to study the neural basis of head movements and show that areas that control eye movements also control head movements. In addition, we provide the first evidence for brain activity associated with vestibular input produced by natural head movements as opposed to invasive caloric or galvanic vestibular stimulation.

Acoustic Stimulation↗
PubMed2002

Comparing brain activation associated with isolated upper and lower limb movement across corresponding joints.

It was shown recently that functional activation across brain motor areas during locomotion and foot movements are similar but differ substantially from activation related to upper extremity movement (Miyai [2001]: Neuroimage 14:1186-1192). The activation pattern may be a function of the behavioral context of the movement rather than of its mechanical properties. We compare motor system activation patterns associated with isolated single-joint movement of corresponding joints in arm and leg carried out in equal frequency and range. Eleven healthy volunteers underwent BOLD-weighted fMRI while performing repetitive elbow or knee extension/flexion. To relate elbow and knee activation to the well-described patterns of finger movement, serial finger-to-thumb opposition was assessed in addition. After identifying task-related voxels using statistical parametric mapping, activation was measured in five regions of interest (ROI; primary motor [M1] and somatosensory cortex [S1], premotor cortex, supplementary motor area [SMA] divided into preSMA and SMA-proper, and cerebellum). Differences in the degree of activation across ROIs were found between elbow and knee movement. SMA-proper activation was prominent for knee, but almost absent for elbow movement (P < 0.05); finger movement produced small but constant SMA-proper activation. Ipsilateral M1 activation was detected during knee and finger movement, but was absent for the elbow task (P < 0.05). Knee movement showed less lateralization in M1 and S1 than other tasks (P < 0.05). The data demonstrate that central motor structures contribute differently to isolated elbow and knee movement. Activation during knee movement shows similarities to gait-related activation patterns.

Adult↗
PubMed2000

Compatibility between observed and executed finger movements: comparing symbolic, spatial, and imitative cues.

Intuitively, one can assume that imitating a movement is an easier task than responding to a symbolic stimulus like a verbal instruction. Support for this suggestion can be found in neuropsychological research as well as in research on stimulus-response compatibility. However controlled experimental evidence for this assumption is still lacking. We used a stimulus-response compatibility paradigm to test the assumption. In a series of experiments, it was tested whether observed finger movements have a stronger influence on finger movement execution than a symbolic or spatial cue. In the first experiment, we compared symbolic cues with observed finger movements using an interference paradigm. Observing finger movements strongly influenced movement execution, irrespective of whether the finger movement was the relevant or the irrelevant stimulus dimension. In the second experiment, effects of observed finger movements and spatial finger cues were compared. The observed finger movement dominated the spatial finger cue. A reduction in the similarity of observed and executed action in the third experiment led to a decrease of the influence of observed finger movement, which demonstrates the crucial role of the imitative relation of observed and executed action for the described effects. The results are discussed in relation to recent models of stimulus-response compatibility. Neurocognitive support for the strong relationship between movement observation and movement execution is reported.

Cues↗
PubMed1994

Optimal control of antagonistic muscle stiffness during voluntary movements.

This paper presents a study on the control of antagonist muscle stiffness during single-joint arm movements by optimal control theory with a minimal effort criterion. A hierarchical model is developed based on the physiology of the neuromuscular control system and the equilibrium point hypothesis. For point-to-point movements, the model provides predictions on (1) movement trajectory, (2) equilibrium trajectory, (3) muscle control inputs, and (4) antagonist muscle stiffness, as well as other variables. We compared these model predictions to the behavior observed in normal human subjects. The optimal movements capture the major invariant characteristics of voluntary movements, such as a sigmoidal movement trajectory with a bell-shaped velocity profile, an 'N'-shaped equilibrium trajectory, a triphasic burst pattern of muscle control inputs, and a dynamically modulated joint stiffness. The joint stiffness is found to increase in the middle of the movement as a consequence of the triphasic muscle activities. We have also investigated the effects of changes in model parameters on movement control. We found that the movement kinematics and muscle control inputs are strongly influenced by the upper bound of the descending excitation signal that activates motoneuron pools in the spinal cord. Furthermore, a class of movements with scaled velocity profiles can be achieved by tuning the amplitude and duration of this excitation signal. These model predictions agree with a wide body of experimental data obtained from normal human subjects. The results suggest that the control of fast arm movements involves explicit planning for both the equilibrium trajectory and joint stiffness, and that the minimal effort criterion best characterizes the objective of movement planning and control.

Arm↗
PubMed1989

Saccadic reaction times, eye-arm coordination and spontaneous eye movements in normal and MPTP-treated monkeys.

The oculomotor performance of monkeys was investigated before and after destruction of nigrostriatal dopamine neurons by MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). Stimulus-triggered saccades and their relationships to arm movements were measured in a reaction time task. Spontaneous eye movements were recorded while monkeys sat in a primate chair and looked around the laboratory without performing any task. In the reaction time task, saccades and arm movements were commonly triggered by the rapid, visible and audible opening of a small food-containing box which was located at a constant position in front of the animal at eye level. Median saccadic latencies ranged from 68 to 118 ms in intact animals. Saccades were followed by onset of electromyographic (EMG) activity in the extensor digitorum communis and the biceps brachii, the prime mover muscles for the following arm reaching movement. Latencies of stimulus-triggered saccades showed an absence of linear relationship to EMG or arm movement reaction time in intact animals (correlation coefficients of 0.15-0.56). This suggests that eye and arm movements were initiated independently from each other in this experimental situation. Treatment with MPTP resulted in 98-99.5% loss of striatal dopamine in both monkeys. This induced a 29-93% increase in saccadic latency in the reaction time task. The sequential occurrence of saccade, EMG activity and arm movement in each trial was preserved, although intervals between onset of saccades and onsets of EMGs and arm movements were prolonged by 53-173% and 33-679% respectively. Onsets of individual saccades remained uncorrelated with onsets of EMG activity or arm movement. Spontaneous eye movements were strongly reduced in frequency and amplitude after MPTP. Administration of the dopamine precursor L-Dopa increased spontaneous eye movements for less than two hours. The severe deficits in stimulus-triggered and spontaneous saccadic eye movements are oculomotor components of hypokinesia arising after MPTP-induced lesions of the nigrostriatal dopamine system in primates. The data are further evidence for a role of midbrain dopamine neurons in behavioral responsiveness and spontaneous activity.

Animals↗