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PubMed1995

The curvature of human arm movements in the absence of visual experience.

It has been suggested that the spatial path of the hand is an important controlled feature of normal human arm movements and that the desired path is a straight line through external space. Recent experiments have suggested that distortions in visual perception of external space may lead to errors in its representation and thus influence the curvature of movements. The movements of blind and normal blind-folded subjects were therefore compared in a task requiring point-to-point hand movements in six directions across a horizontal worktop. Movement curvature varied with direction in both groups but was significantly higher for the blind-folded control subjects. Thus, the normals' distorted visual experience of straight lines in some orientations may lead them to make curved movement paths. The perception of curvature was also tested in the two groups in a task in which they traced the curved edge of a ruler. The blind group were slightly better at this task, although the difference was not significant. We conclude that visual experience influences point-to-point hand movements, leading to higher curvature for movements made in the fronto-parallel plane by sighted subjects due to visual distortions. These data therefore support the hypothesis that the spatial path followed by the hand is influenced by sensory inputs and is a controlled feature of human reaching movements. The data argue against the hypothesis that movement curvature is a result of optimising only the dynamics of the limb control.

Adolescent↗
PubMed1996

Differential relation of discharge in primary motor cortex and premotor cortex to movements versus actively maintained postures during a reaching task.

The activity of cells in primary motor cortex (MI) and dorsal premotor cortex (PMd) were compared during reaching movements in a reaction-time (RT) task, without prior instructions, which required precise control of limb posture before and after movement. MI neurons typically showed strong, directionally tuned activity prior to and during movement as well as large gradations of tonic activity while holding the limb over different targets. The directionality of their movement- and posture-related activity was generally similar. Proximal-arm muscles behaved similarly. This is consistent with a role for MI in the moment-to-moment control of motor output, including both movement and actively maintained postures, and suggests a common functional relation for MI cells to both aspects of motor behavior. In contrast, PMd cells were generally more phasic, frequently emitting only strong bursts of activity confined mainly to the behavioral reaction time before movement onset. PMd tonic activity during different postures was generally weaker than in MI, and showed a much more variable relation with their movement-related directional tuning. These results imply that the major contribution of PMd to this RT task occurred prior to the onset of movement itself, consistent with a role for PMd in the selection and planning of visually guided movements. Furthermore, the nature of the relative contribution of PMd to movement versus actively maintained postures appears to be fundamentally different from that in MI. Finally, there was a continuous gradient of changes in responses across the rostrocaudal extent of the precentral gyrus, with no abrupt transition in response properties between PMd and MI.

Animals↗
PubMed1988

Goal-directed vestibulo-ocular function in man: gaze stabilization by slow-phase and saccadic eye movements.

Vestibular function was examined during passive head movements having profiles that approximated low-to-intermediate range of natural self-generated movements (10-220 degrees/s peak velocity, about 0.5 s duration). A seated subject looked at a point target on the wall, the lights were extinguished and the chair was briefly turned while the subject tried to "look" at the just-viewed point. The chair was stopped, the lights were turned on again and the target was re-fixated, if necessary. Ocular stabilization was characterized (1) by "net stabilization" that was due to the combined effects of both slow-phase and rapid (saccadic or quick-phase) eye movements, (2) by "cumulative-slow-phase stabilization" that was due to slow-phase eye movements, and (3) by "cumulative-saccadic stabilization" that was due to effects of all rapid eye movements. It was found that both slow-phase and saccadic eye movements tended to keep the eyes on the actual unseen target. During repeatedly applied head movements, net and cumulative-slow-phase stabilization tended to be almost perfect. However, the average magnitude of the error in net stabilization (i.e., deviation from perfection) was always less than the corresponding error in slow-phase stabilization. This occurred because in a given turn, saccadic movements tended to supplement deficient slow-phase movements and to decrement excessive slow-phases. In 4 of 5 subjects, cumulative-saccadic stabilization tended to equal the error in cumulative-slow-phase stabilization. All results were unaffected by head velocities up to +/- 220 degrees/s. It was concluded that these saccades tended to stabilize gaze (eye + head) in space during head movements in total darkness.

Eye Movements↗
PubMed1981

The composition of central programs subserving horizontal eye movements in man.

A hypothesis is presented which describes, in biomechanical terms, the central programs underlying horizontal eye movements in man. It is suggested that eye movements are produced by means of programmed shifts of the so-called invariant muscle characteristics (static force vs angle phi of gaze). These shifts lead to a change of the equilibrium point resulting from the interaction of agonist and antagonist muscles and, as a consequence, to movement and the attainment of a new position of gaze. A reciprocal or a coactivation command to agonist and antagonist muscles occurs when their characteristics shift with respect to the coordinate phi in the same or opposite directions, respectively. It is proposed that during pursuit and saccadic eye movements a superposition of the both central commands occurs. During a saccade, the reciprocal command develops evenly up to a certain level. The initial and final levels of the reciprocal command dictate the respective position of gaze and therefore the size of the saccade. The coactivation command develops to a maximum level and is slowly switched off when the new position of gaze has been achieved. The magnitude of the coactivation command seems to be not connected with an absolute position of gaze. It provides probably a stability of the movement and, in particular, prevents overshoot and oscillation during the saccade. The same timing of these commands occurs during pursuit movements, but the magnitude of the coactivation command and the rates of the development of the both commands are less in this case and correlate with the velocity of the movement. This hypothesis enables the tension changes in the muscle during saccadic and pursuit movements to be simulated in qualitative accordance with unique experimental data obtained by Collins et al. (1975). The functional significance of superposition of these motor commands and similarity in the efferent organization of eye and limb movements are discussed. Analysis of limb movements in man and animals has allowed one to formulate some concepts concerning the motor control. For instance, it has been suggested and experimentally confirmed that central commands are adequately expressed in terms of shifts of muscle static length - force characteristics and specify an equilibrium point resulting from the interaction of agonist and antagonist muscles (Asatryan and Feldman, 1965; Felman, 1966a, 1974, 1979, 1980a, b; Bizzi et al., 1976; Kelso, 1977; Polit and Bizzi, 1978, 1979; Houk, 1979; Kelso and Holt, 1980). Experimental observation have also shown that two central commands, i.e. reciprocal and unidirectional activation of agonist and antagonist muscles are usually combined by the nervous system in a proper manner depending on the motor task (Feldman, 1979, 1980a, b). The present, theoretical report is designed to show that these concepts are consistent with available experimental data concerning oculomotor control.

Eye Movements↗
PubMed1995

Role of agonist and antagonist muscle strength in performance of rapid movements.

Six subjects performed rapid self-terminated elbow movements under different mechanical conditions prior to, and 5 weeks after an elbow extensor strengthening programme. Despite the large difference in the strengths of elbow flexors and extensors, the pretest did not demonstrate significant differences between the movement time of flexion and extension movements performed under the same mechanical conditions. The results obtained in the posttest demonstrated a decrease in movement time (i.e. an increase in movement speed) in both elbow flexion and extension movements under some mechanical conditions. In addition, flexion movements demonstrated a relative increase in the acceleration time (acceleration time as a proportion of the movement time). It was concluded that the strength of both the agonist and antagonist muscles was important for the performance of rapid movements. Stronger agonists could increase the acceleration of the limb being moved, while stronger antagonists could facilitate the arrest of the limb movement in a shorter time, providing a longer time for acceleration.

Adult↗
PubMed1992

Electromyographic studies of neck muscles in the intact cat. I. Patterns of recruitment underlying posture and movement during natural behaviors.

Natural head movements in alert, unrestrained cats were studied using video-filming, videofluoroscopy and electromyographic (EMG) recording methods. In each cat, up to sixteen neck muscles or neck-muscle compartments were implanted with recording electrodes. Patterns of muscle recruitment were examined during systematically-selected behavioral epochs in which the cat held a range of stationary postures, and when it performed volitional and exploratory behaviors such as flexion-extension or turning, grooming, eating, or headshaking. Patterns of muscular activity were interpreted with reference to simultaneous video images of head and neck movements. In separate, videofluoroscopic analysis, flexion-extension movements were examined to gain insight into the underlying movements of the skull and cervical vertebrae. These and other movements were found commonly to depend upon changes in joint angles between lower as well as upper cervical joints. Stationary postures in which the neck was held vertically were consistently associated with tonic EMG activity in only two long dorsal muscles, biventer cervicis and occipitoscapularis. Less consistent activity was also present in dorsal intervertebral muscles crossing lower cervical joints. When the neck was held horizontally, the long dorsal muscles increased their EMG activity and moderate activity was also recorded in deeper intervertebral and suboccipital muscles. When flexion-extension occurred around upper cervical joints, greatest activity was recorded in rectus capitis posterior and complexus, but when it involved the lower cervical joints, large changes in EMG activity could also be detected in biventer cervicis, occipitoscapularis, and the intervertebral muscles crossing lower cervical joints. During specialized, sagittal-plane movements such as grooming, well-defined patterns of synergy could be recognized that varied according to the degree of involvement of upper and lower cervical joint-sets. Movements in the horizontal plane were associated with EMG activity in a largely different subset of neck muscles including splenius, longissimus capitis and obliquus capitis inferior. The levels of EMG activity during flexion-extension or turning movements were much lower than those observed during other more vigorous behaviors, such as head shaking. Some neck muscles, such as clavotrapezius and sternomastoideus, could only be recruited during forceful or ballistic head movements. Results showed that the patterns of muscular activation were linked not only to the speed and trajectory of the movements of the skull, but also to the kinematics of the motion occurring across different parts of the cervical column.

Animals↗
PubMed1997

Movement-induced gain modulation of somatosensory potentials and soleus H-reflexes evoked from the leg. I. Kinaesthetic task demands.

Movement-related gating of cerebral somatosensory evoked potentials (SEPs) occurs during active and passive movements of both the upper and the lower limbs. The general hypothesis was tested that the brain participates in setting the gain of the ascending path from somatosensory receptors of the human leg to the somatosensory cortex. In experiment 1, SEPs from Cz' and soleus H-reflexes were evoked by electrical stimulation of the tibial nerve in the popliteal fossa during passive movement about the right ankle. Early SEPs and H-reflexes sampled during simple passive movement were significantly attenuated when compared with stationary controls (P<0.05). The additional requirement of tracking the passive ankle movement with the other foot led to a significant relative facilitation of mean SEP, but not H-reflex amplitude, compared with means from passive movement alone (P<0.05). In experiment 2, SEPs were evoked in the active (tracking) leg during a forewarned reaction-time task. Subjects were required to move in a preferred direction or to track the passive movement of their right foot with their left. Significant attenuation of early SEP components occurred 100 ms prior to EMG onset (P<0.05), with no apparent effect due to tracking. In the 3rd experiment, SEPs and H-reflexes were evoked in the passively moved leg (the target for active movement of the left leg) during the same forewarned reaction-time task. During the warning period, SEPs were significantly attenuated compared with stationary controls for non-tracking movements, but not for movements involving tracking (P<0.05). It is concluded that centrifugal factors are important in modulating SEP gain required by the kinaesthetic demands of the task.

Adult↗
PubMed1997

Movement-induced gain modulation of somatosensory potentials and soleus H-reflexes evoked from the leg. II. Correlation with rate of stretch of extensor muscles of the leg.

Attenuation of initial somatosensory evoked potential (SEP) gain becomes more pronounced with increased rates of movement. Manipulation of the range of movement also might alter the SEP gain. It could alter joint receptor discharge; it should alter the discharge of muscle stretch receptors. We hypothesized that: (1) SEP gain reduction correlates with both the range and the rate of movement, and (2) manipulation of range and rate of movement to achieve similar estimated rates of stretch of a leg extensor muscle group (the vasti) results in similar decreases in SEP gain. SEPs from Cz', referenced to Fpz' (2 cm caudal to Cz and Fpz, respectively, according to the International 10-20 System), along with soleus H-reflexes were elicited by electrical stimulation of the tibial nerve at the popliteal fossa. Stable magnitudes of small M-waves indicated stability of stimulation. A modified cycle ergometer with an adjustable pedal crank and electric motor was used to passively rotate the right leg over three ranges (producing estimated vasti stretch of 12, 24 and 48 mm) and four rates (0, 20, 40 and 80 rpm) of movement. Two experiments were conducted. Ranges and rates of pedalling movement were combined to produce two or three equivalent estimated rates of tissue stretch of the vasti muscles at each of 4, 16, 32 and 64 mm/s. Tibial nerve stimuli were delivered when the knee was moved through its most flexed position and the hip was nearing its most flexed position. Means of SEP, H-reflex and M-wave magnitudes were tested for rate and range effects (ANOVA). A priori contrasts compared means produced by equivalent estimated rates of vasti stretch. Increasing the rate of movement significantly increased the attenuation of SEP and H-reflex gain (P<0.05). Increasing the range of movement also significantly increased these gain attenuations (P<0.05). Combining these to achieve equivalent rates of stretch, through different combinations of rate and range, resulted in equivalent depressions of SEP gain. H-reflex gains were similarly conditioned. These results suggest that muscle stretch receptors play a more important role than joint or cutaneous receptors in regulating SEP gain consequent to movement. We note that the present calculation only considers the knee extensors; however, the biomechanical model of stretch applies also to receptors in the hip extensors. This paper and the companion one show that primary factors in the kinaesthetic components of the movement regulate activity-induced gain attenuation of SEPs.

Adult↗
PubMed2001

Simulating mechanical consequences of voluntary movement upon whole-body equilibrium: the arm-raising paradigm revisited.

Voluntary arm-raising movement performed during the upright human stance position imposes a perturbation to an already unstable bipedal posture characterised by a high body centre of mass (CoM). Inertial forces due to arm acceleration and displacement of the CoM of the arm which alters the CoM position of the whole body represent the two sources of disequilibrium. A current model of postural control explains equilibrium maintenance through the action of anticipatory postural adjustments (APAs) that would offset any destabilising effect of the voluntary movement. The purpose of this paper was to quantify, using computer simulation, the postural perturbation due to arm raising movement. The model incorporated four links, with shoulder, hip, knee and ankle joints constrained by linear viscoelastic elements. The input of the model was a torque applied at the shoulder joint. The simulation described mechanical consequences of the arm-raising movement for different initial conditions. The variables tested were arm inertia, the presence or not of gravity field, the initial standing position and arm movement direction. Simulations showed that the mechanical effect of arm-raising movement was mainly local, that is to say at the level of trunk and lower limbs and produced a slight forward displacement of the CoM (1.5 mm). Backward arm-raising movement had the same effect on the CoM displacement as the forward arm-raising movement. When the mass of the arm was increased, trunk rotation increased producing a CoM displacement in the opposite direction when compared to arm movement performed without load. Postural disturbance was minimised for an initial standing posture with the CoM vertical projection corresponding to the ankle joint axis of rotation. When the model was reduced to two degrees of freedom (ankle and shoulder joints only) the postural perturbation due to arm-raising movement increased compared to the four-joints model. On the basis of these results the classical assumption that APAs stabilise the CoM is challenged.

Arm↗
PubMed2002

The preferred sensory direction of muscle spindle primary endings influences the velocity coding of two-dimensional limb movements in humans.

The present study compares how accurately two different but close velocities of movement are discriminated by populations of muscle spindle primary afferents whether or not one takes into account the direction of the movement and the preferred sensory directions of the units (i.e., the direction of movement to which the afferents are the most sensitive). The activities of 26 muscle spindle primary endings originating from the tibialis anterior, the extensor digitorum longus, the extensor hallucis longus, and the peroneus lateralis muscles were recorded in the lateral peroneal nerve. Their responses to movements imposed at two velocities (12.5 and 18 mm/s) were analyzed. These movements were straight-line movements imposed in eight directions and circular movements in both clockwise and anticlockwise directions. The encoding of the movement velocity was analyzed in two ways. First, the mean frequencies of discharge of the muscle spindle afferents were compared for the two velocities. Second, the data were analyzed using a "neuronal population vector model." This model is based on the idea that such neuronal coding can be analyzed in terms of a series of population vectors (i.e., mean contribution of all the muscle spindle afferents within one directionally tuned muscle) and by finally calculating a sum vector. The results showed no clear and consistent difference in the response frequency of the muscle spindle afferents for the two velocities of movement compared. Rather, the most consistently significant differences between the two velocities were in the lengths of the sum vectors. It is concluded that the encoding of two-dimensional movement velocity relies on populations of muscle spindle afferents coming from the whole set of muscles surrounding a particular joint, each muscle making an instantaneous, oriented, and weighted contribution to the sensory coding of the kinematics parameters.

Action Potentials↗
PubMed2003

Differential contributions of vision and proprioception to movement accuracy.

We examined the relative roles of visual and proprioceptive information about initial hand position on movement accuracy. A virtual reality environment was employed to dissociate visual information about hand position from the actual hand position. Previous studies examining the effects of such dissociations on perception of hand location have indicated a bias toward the visually displayed position. However, an earlier study, which employed optical prisms to dissociate visual and proprioceptive information prior to targeted movements, suggested a bias in movement direction toward that defined by the actual hand position. This implies that visual and proprioceptive information about hand position may be differentially employed for perceptual judgments and movement planning, respectively. We now employ a virtual reality environment to systematically manipulate the visual display of the hand start position from the actual hand position during movements made to a variety of directions. We asked whether subjects would adjust their movements in accord with the virtual or the actual hand location. Subjects performed a series of baseline movements toward one of three targets in each of three blocks of trials. Interspersed among these trials were "probe" trials in which the cursor location, but not the hand location, was displaced relative to the baseline start position. In all cases, cursor feedback was blanked at movement onset. Our findings indicated that subjects systematically adjusted the direction of movement in accord with the virtual, not the actual, start location of the hand. These findings support the hypothesis that visual information about hand position predominates in specifying movement direction.

Adolescent↗
PubMed2003

Elbow impedance during goal-directed movements.

The mechanical properties and reflex actions of muscles crossing the elbow joint were examined during a 60-deg voluntary elbow extension movement. Brief unexpected torque pulses of identical magnitude and time-course (20-Nm extension switching to 20-Nm flexion within 30 ms) were introduced at various points of a movement in randomly selected trials. Single pulses were injected in different trials, some before movement onset and some either during early, mid, late or ending stages of the movement. Changes in movement trajectory induced by a torque pulse were determined over the first 50 ms by a nearest-neighbor prediction algorithm, and then a modified K-B-I (stiffness-damping-inertia) model was fit to the responses. The stiffness and damping coefficients estimated during voluntary movements were compared to values recorded during trials in which subjects were instructed to strongly co-contract while maintaining a static posture. This latter protocol was designed to help determine the maximum impedance a subject could generate. We determined that co-contraction increased joint stiffness greatly, well beyond that recorded under control conditions. In contrast, the stiffness magnitudes were quite small during routine voluntary movements, or when the subjects relaxed their limb. Furthermore, the damping coefficients were always significant and increased measurably at the end of movement. Reflex activity, as measured by EMG responses in biceps and triceps brachii, showed highly variable responses at latencies of 160 ms or greater. These reflexes tended to activate both elbow flexors and extensors simultaneously. These findings suggest that very low intrinsic muscle stiffness values recorded during point-to-point motion render an equilibrium point or impedance control approach implausible as a means to regulate movement trajectories. In particular, muscle that is shortening against inertial loads seems to exhibit much smaller stiffness than similarly active isometric muscle, although some degree of damping is always present and does not simply co-vary with stiffness. Although the limb muscles can be co-contracted statically or during movement with an observable increase in stiffness and even task performance, this control strategy is rarely utilized, presumably due to the greater energetic cost.

Confidence Intervals↗
PubMed2003

Contribution of the cerebellum to self-initiated synchronized movements: a PET study.

Positron emission tomography (PET) was used to examine the neural substrate underlying self-initiated versus externally triggered synchronized movements. Seven healthy subjects performed synchronized right index finger and foot movements in two conditions: either by setting them going at their own pace (self-initiated condition) or by reacting to randomly dispensed auditory signals (externally triggered condition). In addition, subjects either self-initiated or performed in reaction to an audible tone a sequence of finger and foot movements. We hypothesized that cerebellar activity would reflect the behavioural difference observed when hand and foot are self-initiated synchronously compared to when these movements are externally triggered. Consistent with early observations by one of us (Paillard 1948, Année Psychologique, pp 28-47), subjects exhibited a precession of finger initiation over foot dorsi-flexion in the externally triggered condition, and a precession of foot dorsi-flexion over finger onset in the self-initiated condition. In addition to the cortical areas already described in the literature as differently activated in self-initiated and externally triggered movements, we found, according to the research hypothesis, a prominent activation of the left postero-lateral hemi-cerebellum in self-initiated synchronized movements when compared to the externally triggered movements. No cerebellar activity was found for self-initiated sequence of hand-foot movements when compared to externally triggered sequence of hand and foot movements. We suggest that this cerebellar activity could be related to some motor timing processes specifically required by the self-initiated synchronized movements.

Adult↗
PubMed2003

Postural muscle activity during bilateral and unilateral arm movements at different speeds.

The muscle activation patterns in anterior and posterior leg muscles were investigated with two types of perturbations to standing balance. Subjects stood with each foot on adjacent force platforms and performed arm flexion movements to shoulder height. Nine subjects performed ten repetitions unilaterally and bilaterally at 100, 75, 50, 25, and 12.5% of maximal acceleration as measured by an accelerometer placed on the dominant hand. Four subjects also performed the fastest movements while leaning forwards and backwards. The area and latency of the EMG activity from the quadriceps (QUAD), hamstrings (BF), soleus (SOL), and tibialis anterior (TA) were measured bilaterally, along with the excursions of the center of pressure (COP) during each movement. In both unilateral and bilateral tasks, subjects showed a scaling of EMG area and COP excursion with the acceleration of the arm movement. Prior to movement onset, significant scaling of EMG area with movement speed occurred in both unilateral and bilateral tasks in most muscles. Following movement onset, EMG areas scaled significantly to movement speed in only the anterior musculature, with the exception of the left BF. The latency of BF was consistent for the four fastest movements. Only the slowest movements resulted in a significant rightward shift of the BF EMG latency. During the unilateral task, the ipsilateral hamstrings were activated significantly earlier than in the bilateral task and the contralateral hamstrings were activated significantly later. It was also observed that subjects utilized one of two different strategies to maintain balance. Five individuals displayed simultaneous anterior/posterior muscle activation while the other four displayed a reciprocal pattern of activation. Regardless of the initial standing position (leaning forwards or backwards), subjects used the same simultaneous or reciprocal activation strategy. The results indicate that muscle activation patterns change with different tasks, but remain the same during variations of the same task.

Adult↗
PubMed2004

Target-dependent differences between free and constrained arm movements in chronic hemiparesis.

This study compares the kinematic and kinetic characteristics of constrained and free upper limb movements in eight subjects with chronic hemiparesis. Movements of the dominant and nondominant limbs were also examined in five control subjects. Rapid movements were performed in the horizontal plane from a central starting point to five targets located to require various combinations of flexion/extension rotations at the elbow and shoulder. Support of the upper limb against gravity loading was provided either by a low-friction air-bearing apparatus (constrained condition) or by voluntary generation of abduction and external rotation torques at the shoulder (free condition). Data analysis focused on the peak joint torques generated during the acceleratory phase of movement, and on the net change in joint angles at the elbow and shoulder. We found that movement parameters were broadly invariant with support condition for either limb of control subjects, as well as for the nonparetic limb of hemiparetic subjects. In contrast, support condition had a target-dependent effect on movements of the paretic limb. Relative to the constrained condition, peak torques for free arm movements were significantly reduced for distal targets requiring elbow extension and/or shoulder flexion torques. However, peak elbow flexion and shoulder extension joint torques for proximal targets were relatively unaffected by support condition. Of perhaps more functional importance, free movements were characterized by a target-dependent restriction in the hand's work area that reflected a reduced range of active elbow extension, relative to constrained movements. The target-dependent effects of support condition on movements of the paretic limb are consistent with the existence of abnormal constraints on muscle activation patterns in subjects with chronic hemiparesis, namely an abnormal linkage between activation of the elbow flexors and shoulder extensors, abductors, and external rotators.

Acceleration↗
PubMed2004

Expected and unexpected head yaw movements result in different modifications of gait and whole body coordination strategies.

During locomotion we routinely make voluntary head movements, similar to those made during steering tasks, in order to scan our environment and obtain information about objects in the environment and our proximity to these objects. Given the importance that head segment orientation during locomotion has received in the recent literature, two studies were designed to investigate responses following a voluntarily generated and an unexpected, externally applied head turn. During a voluntary head turn, an efferent copy of the head movement could cancel the sensory effects of the head turn, effectively isolating the movement response to that segment. Alternatively, if the steering synergy is a part of our motor repertoire, as has been suggested, movement of the head could automatically release a steering "synergy" of segmental control and coordination. A unique head mounted air-jet apparatus, designed and developed at the University of Waterloo, was used for both studies to ensure that auditory stimuli and the physical presence of the apparatus on the head were similar for participants of the two experiments. During certain points in the gait cycle, this device was triggered and a short burst of compressed air (350 ms) was released to cue participants to make a voluntary head turn (Experiment 1). The same device was triggered in Experiment 2; however, in this experiment compressed air was released for a longer duration (1,500 ms) which resulted in an unexpected and quick turn of the participants' head to either the left or right. In these experiments, vision was also manipulated in certain trials with liquid crystal display glasses that occluded vision for the duration of the head turn. Data from the first experiment indicates that a subset of the steering synergy previously observed is released following the voluntary head movement; however, the travel trajectory path is preserved, suggesting that sensory input resulting from the head movement is partially nullified by the central nervous system. Overall safety is ensured by maintaining the same travel path. In the second experiment, an unexpected perturbation was applied to the head during locomotion to determine how the absence of an efferent copy of the movement pattern influences the level of control over body segments during locomotion. Whole body responses similar to those observed during steering tasks were observed following application of this unexpected head perturbation. It is proposed that the CNS interprets an unexpected yaw movement of the head as a change in the frame of reference, and global modifications of the walking trajectory, similar to that observed during steering tasks, are made in the perceived new direction of travel. Collectively this work extends our understanding of how the CNS establishes a head based orientation frame for locomotion. The CNS interprets and integrates anticipated and unexpected changes in sensory information from the head segment and subsequently modifies locomotion patterns according to the perceived whole body orientation in space. The sequence of control following these head movements appears to be part of a movement repertoire that is not immutable; maintaining whole body stability during locomotion is paramount.

Acoustic Stimulation↗
PubMed2004

On-line vs. off-line utilization of peripheral visual afferent information to ensure spatial accuracy of goal-directed movements.

Manual aiming movements are often initiated when one gazes at the target, while the hand is seen in peripheral vision. The objective of the present study was to determine whether vision of one's hand in peripheral vision and/or central vision as it progresses towards the target can be used to modulate the direction and the extent components of the initial movement impulse. Participants performed video aiming movements while vision of the cursor they were moving was permitted for its whole trajectory, 40 degrees to 15 degrees of visual angle, 15 degrees to 0 degrees of visual angle, or not visible at all. Movements were to be completed within prescribed movement times varying between 300 ms and 900 ms. The results did not reveal endpoint accuracy or variability differences between the 40 degrees -15 degrees and the 15 degrees -0 degrees visual feedback conditions. Both conditions yielded lower endpoint bias and variability than the no-vision condition from early on after movement initiation. This indicates that the visual afferent information available in the 40 degrees -15 degrees and the 15 degrees -0 degrees visual feedback conditions could be used to better plan upcoming movements than the no vision condition. From these data, it appears very unlikely that different portions of the retina are specialized for processing different movement attributes as has been suggested in the past (Paillard 1980; Paillard and Amblard 1985). Both the peripheral and central retina are apt at detecting on-line extent and direction errors in one's movement. In addition, the data cast serious doubts on the widely accepted proposition that the movement initial impulse is essentially ballistic.

Adult↗
PubMed2004

Development of visuomotor representations for hand movement in young children.

The stability and adaptability of visuomotor representations for hand movement in young children was investigated using a visuomotor adaptation paradigm in which the real-time visual feedback of pen movement was rotated 45 degrees clockwise during exposure trials. Four, six, and eight-year-old children performed line drawings to visual targets, from a common centered position ("center-out task"), in the horizontal plane under normal (pre-, and post-exposure), and rotated (exposure) visual feedback conditions. Analysis of pre-exposure trials indicated that older children performed faster, straighter, smoother, and showed more patterned movements than the younger children. Initial direction of movement, computed at 80 ms after movement onset, showed a progressive tuning of movement direction with increasing age. On introduction of the screen cursor rotation, all age group children showed improvement in their planning (initial directional error) and execution (movement time, movement length, root mean square error, and normalized jerk) error scores from early to late-exposure trials, but the 4-year-olds were less affected than older age children by the distortion during the early exposure period. Moreover, only the oldest group of children showed significant after-effects during post-exposure trials indicating that only this age group learned the internal model of the distorted environment. The absence of after-effects for initial movement direction observed in the two younger age groups suggest that these children might have less developed (i.e. more broad) internal visuomotor representations for hand movements, and that their internal representations are sharpened (i.e. tuned) with visuomotor experience.

Age Factors↗