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PubMed2006

Head movements evoked by electrical stimulation in the frontal eye field of the monkey: evidence for independent eye and head control.

When the head is free to move, electrical stimulation in the frontal eye field (FEF) evokes eye and head movements. However, it is unclear whether FEF stimulation-evoked head movements contribute to shifting the line of sight, like visually guided coordinated eye-head gaze shifts. Here we investigated this issue by systematically varying initial eye (IEP) and head (IHP) positions at stimulation onset. Despite the large variability of IEP and IHP and the extent of stimulation-evoked gaze amplitudes, gaze displacement was entirely accounted for by eye (re head) displacement. Overall, the majority (3/4) of stimulation-evoked gaze shifts consisted of eye-alone movements, in which head movements were below the detection threshold. When head movements did occur, they often started late (re gaze shift onset) and coincided with rapid eye deceleration, resulting in little change in the ensuing gaze amplitudes. These head movements often reached their peak velocities over 100 ms after the end of gaze shifts, indicating that the head velocity profile was temporally dissociated from the gaze drive. Interestingly, head movements were sometimes evoked by FEF stimulation in the absence of gaze shifts, particularly when IEP was deviated contralaterally (re the stimulated side) at stimulation onset. Furthermore, head movements evoked by FEF stimulation resembled a subset of head movements occurring during visually guided gaze shifts. These unique head movements minimized the eye deviation from the center of the orbit and contributed little to gaze shifts. The results suggest that head motor control may be independent from eye control in the FEF.

Animals↗
PubMed1990

Gaze control in the cat: studies and modeling of the coupling between orienting eye and head movements in different behavioral tasks.

1. Orienting movements, consisting of coordinated eye and head displacements, direct the visual axis to the source of a sensory stimulus. A recent hypothesis suggests that the CNS may control gaze position (gaze = eye-relative-to-space = eye-relative-to-head + head-relative-to-space) by the use of a feedback circuit wherein an internally derived representation of gaze motor error drives both eye and head premotor circuits. In this paper we examine the effect of behavioral task on the individual and summed trajectories of horizontal eye- and head-orienting movements to gain more insight into how the eyes and head are coupled and controlled in different behavioral situations. 2. Cats whose heads were either restrained (head-fixed) or unrestrained (head-free) were trained to make orienting movements of any desired amplitude in a simple cat-and-mouse game we call the barrier paradigm. A rectangular opaque barrier was placed in front of the hungry animal who either oriented to a food target that was visible to one side of the barrier or oriented to a location on an edge of the barrier where it predicted the target would reappear from behind the barrier. 3. The dynamics (e.g., maximum velocity) and duration of eye- and head-orienting movements were affected by the task. Saccadic eye movements (head-fixed) elicited by the visible target attained greater velocity and had shorter durations than comparable amplitude saccades directed toward the predicted target. A similar observation has been made in human and monkey. In addition, when the head was unrestrained both the eye and head movements (and therefore gaze movements) were faster and shorter in the visible- compared with the predicted-target conditions. Nevertheless, the relative contributions of the eye and head to the overall gaze displacement remained task independent: i.e., the distance traveled by the eye and head movements was determined by the size of the gaze shift only. This relationship was maintained because the velocities of the eye and head movements covaried in the different behavioral situations. Gaze-velocity profiles also had characteristic shapes that were dependent on task. In the predicted-target condition these profiles tended to have flattened peaks, whereas when the target was visible the peaks were sharper. 4. Presentation of a visual cue (e.g., reappearance of food target) immediately before (less than 50 ms) the onset of a gaze shift to a predicted target triggered a midflight increase in first the eye- and, after approximately 20 ms, the head-movement velocity.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1995

Activity of neurons in the medial pontomedullary reticular formation during orienting movements in alert head-free cats.

1. Single unit activities of 236 neurons were recorded in the medial pontomedullary reticular formation during visually triggered orienting gaze shifts in 10 alert cats under head-free conditions using movable tungsten-needle electrodes attached to the skull. The activities were analyzed mainly in relation to the head movement that was triggered by presentation of a light-emitting diode (LED) in one of eight directions separated radially by 45 deg after fixation of the center LED. Of these, 120 neurons were recorded in the pontine reticular formation, chiefly in the nucleus reticularis pontis caudalis, and the remaining 116 were in the medullary reticular formation, chiefly in the nucleus reticularis gigantocellularis. Activities of 65 pontine and 65 medullary neurons were modulated in relation to the dynamic phase of orienting movements ("orienting-related neurons"). Activities of the remaining neurons were modulated either irregularly or not at all during orienting movement ("irregular or no-response neurons"). Input from the contralateral superior colliculus and cerebral cortex and projections to the spinal cord were also investigated. 2. Among the orienting-related neurons, 62 pontine and 55 medullary neurons showed increases in activity preceding the onset of eye and head movement by 0-155 ms ("pretype"). Three pontine and 10 medullary neurons showed increases in activity only after the onset of movement ("posttype"). Of the pretype neurons, 61 pontine and 51 medullary neurons showed directional preference of activity ("directional" neurons). One pontine and four medullary neurons were classified as "omnidirectional" because these neurons increased activity preceding movements in all directions tested, and no directional preference was apparent. 3. In the pretype-directional cells, the average firing frequency during bursts was correlated with amplitude and angular velocity of head movements. Activities of the directional neurons during movements in the eight different directions could be well fitted with cosine functions in the majority of cases. The preferred directions of most pontine neurons and of about half the medullary neurons, as determined by first-degree sinusoidal regression analysis, were distributed around the ipsiversive horizontal axis. However, there were also a considerable number of neurons whose preferred directions were upward, downward, contraversive, or oblique in the medulla. 4. Among the directional cells preferring ipsiversive horizontal movements, 11 pontine neurons showed activity, the onset of which was locked to visual stimuli with latencies of 40-70 ms, in addition to phasic discharges locked to the onset of movement. This "stimulus-locked activity" was sometimes modulated depending on the attentional state of the animal.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2003

Torsional and vertical eye movements during head tilt dynamic characteristics.

PURPOSE: As the first response to a Bielschowsky head tilt test (BHTT), a fast transient torsional eye movement in the same direction as the head tilt has been shown with the three-dimensional (3D)-video oculography (3D-VOG) technique, and this movement is paralleled by a transient vertical vergence shift inducing a physiological skew deviation. The purpose of the present study was to investigate these dynamic eye movements further in response to a BHTT paradigm with the magnetic search coil technique. METHODS: Ten healthy subjects performed a BHTT (15 degrees, 30 degrees, and 45 degrees) toward each shoulder while (search coil) the monocular eye and head positions were recorded. The same head tilt paradigm was repeated in a second test while (3D-VOG) the binocular eye position was recorded. RESULTS: Subsequent to the initiation of the head tilt (latency, approximately 160 ms) a rapid torsional eye movement (mean peak velocity: 40 deg/s; mean amplitude: 4 degrees) was seen in the same direction as the head movement, followed by a somewhat slower return movement. This torsion was synchronous with a vertical vergence eye movement (mean amplitude 3 degrees). The vertical vergence was always with left eye over right eye in the rightward head tilt and in head straightening from the left shoulder. In the left head tilt and in the head straightening from the right shoulder, this movement was always with the right eye over the left eye. CONCLUSIONS: A torsional and vertical vergence back-and-forth eye movement induced by a BHTT was confirmed with the search coil technique. Utricular inertia due to an interaural head translation, combined with a stimulation of the vertical semicircular canals, seems to be a plausible explanation for these eye movements.

Adult↗
PubMed2001

Autobiographical memories become less vivid and emotional after eye movements.

OBJECTIVES: To test (1) whether eye movements during retrieval of emotional memories are followed by less vividness and less emotionality of future recollections, (2) whether this effect, if present, is stronger than the effects of a control activity (finger tapping), (3) whether the alleged effects of tapping and eye movements are stronger than a no-movement, control condition (mere imagery), (4) whether reductions in vividness and emotionality after eye movements (and finger tapping) are specific to negative memories or also occur in the case of positive memories. METHOD: Sixty healthy volunteers recalled either positive or negative memories and scored the vividness and emotionality of the recollections. Next, memories were recalled whilst the participant was performing rapid eye movements, finger tapping, or not performing a dual task. Then participants were asked to recall the event again and to rate its vividness and emotionality. RESULTS: Compared to finger tapping and the no-dual-task condition, recollections after eye movements made future recollections less vivid. After eye movements, but not after the other interventions, negative memories became less negative, and positive memories became less positive. CONCLUSION: The findings show that eye movements not only reduce vividness and emotionality of memories during the eye moving, but also affect future recollections, during which no eye movements are made. Some theoretical explanations are discussed. As to clinical implications, it is suggested that if there is a role for eye-movement-based treatments, it is very limited.

Adolescent↗
PubMed1994

Movement- and task-related activations of motor cortical areas: a positron emission tomographic study.

Using repeated measurements of regional cerebral blood flow with positron emission tomography, we investigated the regional cortical activations induced in 10 normal subjects, by two different finger motor tasks, i.e., a repeated flexion-extension of all fingers and a repeated flexion-extension of the middle finger. The all-finger movement only activated the primary sensorimotor cortex (SM) and the supplementary motor area (SMA) contralateral to the movement. However, the activation of the SMA was clearly task related during this motor task, because it was only observed when the movement was triggered by an auditory cue but not when it was self-paced. The middle finger movement was performed during self-paced conditions. It induced a much more complex pattern of activation than the all-finger movement, characterized by a high degree of SM and SMA activation contralateral to the side of the movement, as well as a slight ipsilateral activation of these areas. We suggest that this pattern of cortical activation may reflect the process of individuating finger movement or the early stages of motor learning of this unusual and technically difficult movement. Our data also confirm that the SM activation is closely linked to the intrinsic parameters of the movement; while the SMA may be activated by different aspects of the movement realization and preparation.

Adult↗
PubMed1987

Plasticity of cortical function related to voluntary movement motor learning and compensation following brain dysfunction.

Processes of motor learning and of compensation after localized brain dysfunction were studied in the monkey tasked with conditioned (visually-initiated, reaction-time) hand movements. Field potentials in various cortical areas of the cerebral hemisphere were recorded successively for many months with electrodes implanted on the surface and in the depth of the cortex. The potentials associated with the conditioned movement were found to change during processes of learning the movement and during courses of degradation as well as recovery after the brain dysfunction. Motor learning of the reaction-time movement can be categorized into "recognition" and "skill" learnings. The former is the process for associating the visual stimulus with the movement and accompanied mainly by increasing activities of prefrontal, premotor and prestriate cortices. The latter is attaining better performances in executing the movement, particularly with shorter and more fixed reaction time, and is accompanied by recruitment of the cerebro-cerebellar interaction. The conditioned movements which had been well established were experimentally disturbed by transient, local cooling of different cortical areas or by cerebellar hemispherectomy. Three possible mechanisms of compensation are proposed as follow: 1. Substitution: Compensation occurring immediately through substitutional neuronal circuits. On transient cooling of the forelimb motor cortex, the somatosensory cortex became predominant in motor function and replaced the disabled motor cortex in executing the reaction-time movement, although activity was weak and slow (paretic but not paralytic). Resection of the cerebellar hemisphere also induced the compensatory motor function of the somatosensory cortex so that the movement could be performed although it was weak, slow and clumsy. 2. Relearning: Compensation by relearning through normally unused neuronal circuits. Prolonged and variable reaction times after cerebellar hemispherectomy persisted when the operation included both dentate and interpositus nuclei but recovered within about three weeks when the interpositus nucleus was preserved. It is suggested that the information processing for the well accomplished reaction-time movement is mainly mediated by the cerebro-cerebellar neuronal circuit including the dentate nucleus but is gradually relearned through the normally unused circuits involving the interpositus nucleus after the dentate nucleus lesion. 3. Rebuilding: Compensation by rebuilt neuronal circuits, e.g., by sprouting and/or regeneration (see S. Kawaguchi in this book).

Action Potentials↗
PubMed1993

Torques generated at the human elbow joint in response to constant position errors imposed during voluntary movements.

The stiffness of the human elbow joint was investigated during targeted, 1.0-rad voluntary flexion movements at speeds ranging from slow (1.5 rad/s) to very fast (6.0 rad/s). A torque motor produced controlled step position errors in the execution of the movements. The steps began at the onset of movement, rose to an amplitude of 0.15 rad in 100 ms, and had a duration equal to movement duration. The net joint torque (muscle torque) resisting the step perturbation was computed from the applied torque, the joint acceleration, and the limb inertia. Subjects resisted the imposed step changes with approximately step changes in the net muscle torque. The mean resistance torque divided by the step amplitude was computed and is referred to as the stiffness. The stiffness increased with the voluntary movement speed, over the range of speeds (1.5-6 rad/s). The stiffness increased linearly with the magnitude of the net muscle torque on the unperturbed trials (referred to as "background torque"). The stiffness changed by only 20% when the step amplitude ranged from 0.05 to 0.15 rad. The mechanical resonant frequency (fr), estimated from the average stiffness estimates, ranged from 0.8 to 3.0 Hz. The resonant frequency approximately equaled the principal frequency component of the movement fm. On average: fr = 0.96 fm +0.46. During the fixed, 100-ms rise time of the step, the resistance was not linearly related to the background torque. At slower speeds the resistance was relatively greater during this rise time. However, when the imposed step perturbation was modified so that its rise time occurred in a time proportional to the movement duration (rather than in the fixed, 100-ms, time), the muscle torque resisting the motor during this rise time was proportional to the background torque. When these modified step responses were plotted on a time scale normalized to the movement duration, they all had approximately the same shape. Apparently the muscle viscosity scaled with the stiffness so as to maintain the constant response shape (constant damping ratio). The observed "tuning" of the mechanical properties to the movement speed is suggested to be important in the robust generation of smooth stereotyped voluntary movements.

Adult↗
PubMed1994

The role of putamen and pallidum in motor initiation in the cat. I. Timing of movement-related single-unit activity.

The participation of basal ganglia in motor initiation was studied in six cats operantly trained to perform a ballistic flexion movement, triggered after a brief sound in a simple reaction time condition or delayed after the same sound in the presence of a tone cue. The activity of 356 neurons was recorded in the putamen and in the pallidum (globus pallidus and entopeduncular nucleus). A total of 19.4% of the neurons were not related to the conditioned flexion movement: they were either unrelated to the task (10.1%) or related to other periods of the motor performance such as trial beginning or reward delivery (9.3%). About 60% of the remaining neurons--defined as task-related--exhibited changes in firing rate that occurred, in the reaction time condition, less than 100 ms after the go signal and therefore began prior to movement onset. For most neurons, in the delayed condition, these early changes were absent, suggesting that their occurrence in the reaction time condition was not a sensory response but rather was related to the movement initiation. In addition, for many neurons these changes shifted in time, remaining time-locked to the movement. Correlations between these early changes in activity and motor parameters were demonstrated, suggesting that these changes were movement-related. For most neurons the firing levels observed during intertrial intervals and during foreperiod were similar. The mean discharge rate during the foreperiod was 19.2 impulses/s. Three patterns of activity were observed before movement: increases in discharge rate (61% of task-related neurons), transient decreases followed by increases (11%), or prolonged decreases (28%). Only minor differences were found between the characteristics of the populations of neurons recorded in the three sites under study: on average the neurons recorded in the globus pallidus were more active than the neurons recorded in the putamen or in the entopeduncular nucleus. The fact that, for certain neurons, the changes of activity prior to movement were different in reaction time condition and in delayed condition showed that the pattern of activity preceding movement might depend on the temporal requirements for motor initiation. Taken together with the motor effects obtained in the same task following GABA-receptor activation with muscimol microinjections in these structures, the present results suggest that putamen and pallidal neurons participate in the initiation of the conditioned movement under study.

Animals↗
PubMed1994

The movement-induced modulation in discriminability between cutaneous nonpainful stimuli depends on test stimulus intensity.

The purpose of this study was to find out whether the finger movement-induced modulation of cutaneous discrimination thresholds varies with the intensity level of the test stimulation in various movement conditions. The effect of active and passive finger movement on cutaneous sensitivity to nonpainful electrical stimulation of threshold and suprathreshold intensity was studied in human subjects. The detection threshold and the just-noticeable amplitude difference (discrimination threshold) at two suprathreshold intensities (3 x and 10 x detection threshold) were determined using a forced-choice paradigm before and after (controls) or during finger movement. In one condition the stimuli were applied just prior to the movement. The finger was actively or passively moved at the frequency of 1.5 Hz or 3 Hz, and the test stimuli were applied to the moving finger, except in one condition to the contralateral finger. The contralateral condition was used to rule out vigilance- and attention-related mechanisms as a cause of sensitivity changes. Active as well as passive movement of the finger produced a significant increase in the detection threshold during the movement. Also just prior to the movement the detection threshold was increased. Suprathreshold discrimination thresholds in the moving finger were not significantly changed during or just prior to the active movement, whereas during passive movement the discrimination threshold to suprathreshold level stimulation was significantly decreased. When test stimuli were applied to the finger contralateral to the actively moving finger, neither the detection threshold nor the discrimination between stimuli of suprathreshold intensities were significantly changed.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

Effects of weak antagonist on fast elbow flexion movements in man.

By using a mathematical model and experiments involving electrical simulation of antagonistic muscles, we have formed the hypothesis (Wierzbicka et al. 1986) that in one-joint movements the antagonist muscle not only provides braking torque but also controls movement time. To get additional experimental support for this hypothesis, we studied elbow flexion movements performed by patients with spinal cord injury at the C5-6 level who had relatively normal strength in their biceps muscle and little or no voluntary control of the triceps. Seven quadriplegic patients and six control subjects performed elbow flexion movements of 10 degrees, 20 degrees, and 30 degrees "as fast and accurately as possible". Despite the lack of antagonist, patients used the same "pulse height" strategy as control subjects to scale their responses with movement amplitude. However, patients' movement time was on average twice that of control subjects, and durations of both accelerative and decelerative phases of movement were increased. Movement speed and acceleration were reduced to 20-50% of the corresponding values of control subjects. Patients tended to overshoot the target to a larger extent than control subjects, particularly 10 degrees targets, with nearly twice the error. We performed the same experiments using an external torque motor to assist the weak triceps. When a constant extensor torque of 2.5 or 5 Nm was provided by the motor, patients were able to move faster, and movement accuracy improved to within the normal range. These results provide direct evidence that the lack of an antagonist has an important effect on completion time and accuracy of fast goal-directed movements.

Adult↗
PubMed1991

Scaling of the metrics of visually-guided arm movements during motor learning in primates.

Hand trajectory, tangential velocity and acceleration, time and distance until peak velocity and reaction time were analyzed during the process of learning a skilled, visually-guided arm movement. Primates were trained to move a cursor with a manipulandum from a start box to target boxes displayed on a horizontal video screen during control conditions and when the relationship (gain) between the cursor and manipulandum was altered. The animals adapted to the altered feedback over 100-200 trials. A subsequent testing phase with randomly interspersed trials using the control gain demonstrated that the animals had modified their movements appropriately for the novel gain. Examination of the kinematics revealed that in adapting to a novel gain, primates scaled movement amplitude, tangential velocity, acceleration, and duration appropriately for the distance the hand needed to travel. Yet time to peak velocity was kept constant. Reaction time also remained unchanged for three of the four animals. Movements were performed in two phases, the first from movement onset to peak velocity and the second from peak velocity until the end of the movement. During the first phase the shape of the trajectory and velocity profile were stereotypic and without evidence of any corrections, consistent with this phase being essentially open loop. However, corrections occurred in the second phase and we propose visual feedback was used to correct for the difference in hand/cursor position. Learning appeared to involve utilizing the errors from previous trials to modify the early feedforward phase of subsequent trials. Peak tangential velocity, total movement duration and distance reached at peak tangential velocity all scaled linearly with the total movement distance required at each gain. Based on regression analyses, for none of these variables were the changes in learning completely adequate to compensate for total distance required. However, distance to peak velocity scaled with peak velocity in relation to the control gain. The results show that non-human primates adopt a consistent strategy when learning to scale a multi-joint movement. The metrics of the movement scaled yet the time to peak velocity remained constant, suggesting independent control of time and amplitude. Keeping time to peak velocity constant as well as the scaling of peak velocity with distance to peak velocity are viewed as ways to simplify the learning process.

Animals↗
PubMed1993

Motor cortical activity preceding a memorized movement trajectory with an orthogonal bend.

Two monkeys were trained to make an arm movement with an orthogonal bend, first up and then to the left ([symbol: see text]), following a waiting period. They held a two-dimensional manipulandum over a spot of light at the center of a planar working surface. When this light went off, the animals were required to hold the manipulandum there for 600-700 ms and then move the handle up and to the left to receive a liquid reward. There were no external signals concerning the "go" time or the trajectory of the movement. It was hypothesized that during that period signs of directional processing relating to the upcoming movement would be identified in the motor cortex. Following 20 trials of the memorized movement trajectory, 40 trials of visually triggered movements in radially arranged directions were performed. The activity of 137 single cells in the motor cortex was recorded extracellularly during performance of the task. It was found that 62.8% of the cells changed activity during the memorized waiting period. During the waiting period, the population vector (Georgopoulos et al. 1983, 1984) began to grow approximately 130 ms after the center light was turned off; it pointed first in the direction of the second part of the memorized movement (<--) and then rotated clockwise towards the direction of the initial part of the movement (increases). These findings indicate processing of directional information during the waiting period preceding the memorized movement. This conclusion was supported by the results of experiments in ten human subjects, who performed the same memorized movement ([symbol: see text]). In 10% of the trials a visual stimulus was shown in radially arranged directions in which the subjects had to move; this stimulus was shown at 0, 200, and 400 ms from the time the center light was turned off. We found that as the interval increased the reaction time shortened for the visual stimulus that was in the same direction as the upward component of the memorized movement.

Animals↗
PubMed1991

Control of asymmetrical bimanual movements.

When movements are performed together in the upper-limbs, a strong tendency emerges to synchronize the patterns of motor output. This is most apparent when trying to do different things at the same time. The present experiment explored the simultaneous organization and control of spatiotemporally different movements. There were two practice conditions: symmetrical and asymmetrical. In the symmetrical condition, subjects performed a series of unidirectional elbow flexion movements, followed by a series of elbow flexion-extension-flexion (reversal) movements in both limbs simultaneously. In the asymmetrical practice condition, subjects performed the unidirectional movement in the left limb together with the reversal movement in the right limb. Findings revealed a tendency for each limb movement to assimilate the features of its counterpart under the latter condition. This effect was "asymmetrical" in that the unidirectional movement was more attracted to the reversal movement than vice versa. Nevertheless, subjects were able to partly suppress this synchronization tendency as was evident from the moderate cross correlations between the angular acceleration patterns of both limb movements and from an increasingly successful differentiation of the activity levels in the right and left limb muscles. All together, these findings provide evidence for some degree of parallel control of spatiotemporally different actions. The data are discussed in view of the possible suppression of a bilaterally distributed motor control system, that is mainly held responsible for activiting proximal limb musculature.

Arm↗
PubMed1990

Orbital position and eye movement influences on visual responses in the pulvinar nuclei of the behaving macaque.

We studied the influences of eye movements on the visual responses of neurons in two retinotopically organized areas of the pulvinar of the macaque. Cells were recorded from awake, trained monkeys, and visual responses were characterized immediately before and after the animals made saccadic eye movements. A significant proportion of the cells were more responsive to stimuli around the time of eye movements than they were at other intervals. Other cells had response reduction. For some neurons, the change in excitability was associated with orbital position and not the eye movement. For other cells the change was present with eye movements of similar amplitude and direction but with different starting and ending positions. Here it appears that the eye movement is the important parameter. Other cells had effects related to both eye position and eye movements. In all cells tested, the changes in excitability were present when the experiments were conducted in the dark as well as in the light. This suggests that the mechanism of the effect is related to the eye position or eye movement and not visual-visual interactions. For about half of the neurons with modulations, the response showed facilitation for stimuli presented in the most responsive region of the receptive field but not for those at the edge of the field. For the other cells there was facilitation throughout the field. Thus, a gradient of modulation in the receptive field may vary among cells. These experiments demonstrate modulations of visual responses in the pulvinar by eye movements. Such effects may be part of the visual-behavioral improvements at the end of eye movements and/or contribute to spatial constancy.

Animals↗
PubMed1994

Effects of transcranial magnetic stimulation on single and sequential arm movements.

We studied in humans the effects of transcranial stimulation of cortical motor areas on the execution of single and sequential rapid arm movements. In a reaction time paradigm with an auditory "go" signal, stimulation given after an auditory tone and before the start of movements delayed the onset but did not affect the subsequent performance of single or sequential movements; high intensities of cortical stimulation determined a long-lasting inhibition of movements. Cortical stimulation given during the execution of a sequential movement temporarily interrupted the movements. Reaction time was not prolonged and movements were not inhibited when cortical stimulation was delivered before the auditory tone and the start of movement. Neither electrical stimulation of the corticospinal tracts at the cervico medullary junction nor magnetic stimulation of the cervical roots delayed the onset or interrupted the execution of movements. Transcranial stimulation affects the performance of both single and sequential movements, through cortical mechanisms that interfere with the transfer of the motor program from other cortical structures to the motor cortex.

Adult↗
PubMed1994

The speed-accuracy trade-off in manual prehension: effects of movement amplitude, object size and object width on kinematic characteristics.

Earlier studies have suggested that the size of an object to be grasped influences the time taken to complete a prehensile movement. However, the use of cylindrical objects in those studies confounded the effects of object size-extent orthogonal to the reach axis-and object width-extent along the reach axis. In separating these effects, the present study demonstrates that movement time is not affected by manipulation of object size, as long as the latter does not approach the maximal object size that can be grasped. Object width, on the other hand, is shown to exert a systematic influence on movement time: Smaller object widths give rise to longer movement times through a lengthening of the deceleration phase of the movement, thus reproducing the effect of target width on the kinematics of aiming movements. As in aiming, movement amplitude also affects the movement time in prehension, influencing primarily the acceleration phase (i.e. peak velocity attained). The effects of object width and movement amplitude were found to combine in a way predicted by Fitts' law, allowing a generalisation of the latter to the transport component in prehensile actions. With respect to the grasp component, both object size and object width are shown to affect peak hand aperture. Increasing object width thus lowers the spatial accuracy demands on the transport component, permitting a faster movement to emerge. At the same time, the hand opens to a larger grip in order to compensate for eventual directional errors that result.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1984

Initial agonist burst duration depends on movement amplitude.

The initial burst of EMG activity associated with arm movements made by normal human subjects was studied. Subjects made visually guided, steptracking movements of different amplitudes and speeds. The duration of the initial agonist burst was greater for large than for small amplitude movements. The burst duration was not continuously graded but was either short (70 ms) for small amplitude movements (less than 20 deg) or long (140 ms) for large amplitude ones (greater than 50 deg). Movements of intermediate amplitudes (30-40 deg) were made with both short and long duration bursts. The increase in the duration of the initial agonist burst for large movements was produced by the appearance of a second component in the burst. Both components were of the same duration and occurred before movement peak velocity was reached. Intramuscular recording showed that both components originate from the same muscle. Similar observations were made in both fast and slow movements and in both the biceps and triceps muscles when they were being used as agonists. The data show that the central nervous system has two mechanisms for generation of large amplitude movements: modulation of the magnitude of the initial agonist burst and generation of a second component or pulse of agonist activity at the start of movement.

Arm↗