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C Prablanc

Publications and source records attributed to C Prablanc.

At least 37 records · Page 2Linked to original sources
PubMed1995

Representation of hand position prior to movement and motor variability.

Pointing accuracy of six human subjects was measured in two blocked conditions where the hand was either never visible (T: target only) or only visible in static position prior to movement onset (H+T: hand+target). It was shown in condition H+T that, viewing the hand prior to movement greatly decreased end-point variability compared with condition T. This effect was associated with a significant modification of the movement kinematics: the H+T condition induced a shortened acceleration phase with a corresponding lengthened deceleration phase, compared with the T condition. These results led us to the hypothesis that viewing the hand prior to movement onset allowed a decrease of pointing variability through a feedback process. This hypothesis was further tested by turning the target off during the deceleration phase of the movement at half peak velocity. It was shown that turning the target off had no effect upon the T condition but induced a significant increase of pointing variability in the H+T condition. This result suggests that vision of the static hand enhances the proprioceptive localization of the limb and allows for a better visual to kinesthesic feedback.

Adult↗
PubMed1995

Vectorial coding of movement: vision, proprioception, or both?

1. Subjects were asked to point toward visual targets without visual reafference from the moving hand in two conditions. In both conditions the pointing fingertip was viewed only before movement onset. 2. In one condition, the pointing fingertip was viewed through prisms that created a visual displacement without altering the view of the target. In another experimental condition, vision of the fingertip was not displaced. Comparison of these two conditions showed that virtually shifting finger position before movement through prisms induced a pointing bias in the direction opposite to the shift. The extent of this pointing bias was about one third of the prismatic shift applied to the fingertip. 3. Analysis of movement initial direction demonstrated that it was also less deviated than predicted from the prismatic shift. In addition, the reaction time and movement time of the reaching movement were increased. 4. This result is interpreted in the framework of the vectorial coding of reaching movement. Proprioception and vision provide two possible sources of information about initial hand position, i.e., the origin of the movement vector. The question remains as to how these two sources of information interact in specifying initial hand position when they are simultaneously available. 5. Our results are thus discussed with respect to a visual-to-visual movement vector hypothesis and a proprioceptive-to-visual vector hypothesis. It is argued that the origin of the putative movement vector is encoded by weighted fusion of the visual and the proprioceptive information about hand initial position.

Adult↗
PubMed1995

Postural and synergic control for three-dimensional movements of reaching and grasping.

1. A fundamental question about motor control is related to the nature of the representations used by the nervous system to program the movement. Theoretically, arm displacement can be encoded either in task (extrinsic) or in joint (intrinsic) space. 2. The present study investigated the organization of complex movements consisting of reaching and grasping a cylindrical object presented along different orientations in space. In some trials, object orientation was suddenly modified at movement onset. 3. At a static level, the final limb angles were highly predictable despite the wide range of possible postures allowed by articular redundancy. Moreover, when object orientation was unexpectedly modified at movement onset, the final angular configuration of the limb was identical to that obtained when the object was initially presented along the orientation reached after the perturbation. 4. At a dynamical level, a generalized synergy was observed, and tight correlations were noted between all joint angles implicated in the movement with the exception of elbow flexion. For this joint angle, which did not vary monotonically, strong partial correlations were however observed before and after movement reversal. 5. These results suggest that natural movements are mostly carried out in joint space by postural transitions.

Arm↗
PubMed1994

The effect of viewing the static hand prior to movement onset on pointing kinematics and variability.

Pointing accuracy and arm movement kinematics of six human subjects were measured in three conditions where the hand was never visible during the ongoing movement: (1) in the dark; (2) the static hand was seen in peripheral vision prior to target presentation, but not during the reaction time (H-T); (3) the static hand was seen in peripheral vision until movement onset (H+T). It was shown that: (1) viewing the hand prior to movement decreased pointing variability as compared to the dark condition. (2) Viewing simultaneously hand and target (H+T) and further decreased pointing variability as compared to the H-T condition. This effect was proportional to the reaction time. (3) A lengthening of the deceleration phase was observed for movements performed in the H+T condition, as compared to the other two conditions. (4) A negative correlation between variability and the first part of the deceleration phase was observed in the H+T condition, but neither in the H-T condition nor in the dark. These results suggest that the decrease in pointing variability observed in the H+T condition is due to a feedback based on kinesthetic reafference. Better encoding of the initial position of the hand relative to the target (as in H+T) would allow a calibration of arm position sense, which is used to drive the hand toward the target during the deceleration phase.

Adult↗
PubMed1994

Eye-head-hand coordination in pointing at visual targets: spatial and temporal analysis.

This study investigated whether the execution of an accurate pointing response depends on a prior saccade orientation towards the target, independent of the vision of the limb. A comparison was made between the accuracy of sequential responses (in which the starting position of the hand is known and the eye centred on the target prior to the onset of the hand pointing movement) and synergetic responses (where both hand and gaze motions are simultaneously initiated on the basis of unique peripheral retinal information). The experiments were conducted in visual closed-loop (hand visible during the pointing movement) and in visual open-loop conditions (vision of hand interrupted as the hand started to move). The latter condition eliminated the possibility of a direct visual evaluation of the error between hand and target during pointing. Three main observations were derived from the present work: (a) the timing of coordinated eye-head-hand pointing at visual targets can be modified, depending on the executed task, without a deterioration in the accuracy of hand pointing; (b) mechanical constraints or instructions such as preventing eye, head or trunk motion, which limit the redundancy of degrees of freedom, lead to a decrease in accuracy; (c) the synergetic movement of eye, head and hand for pointing at a visible target is not trivially the superposition of eye and head shifts added to hand pointing. Indeed, the strategy of such a coordinated action can modify the kinematics of the head in order to make the movements of both head and hand terminate at approximately the same time. The main conclusion is that eye-head coordination is carried out optimally by a parallel processing in which both gaze and hand motor responses are initiated on the basis of a poorly defined retinal signal. The accuracy in hand pointing is not conditioned by head movement per se and does not depend on the relative timing of eye, head and hand movements (synergetic vs sequential responses). However, a decrease in the accuracy of hand pointing was observed in the synergetic condition, when target fixation was not stabilised before the target was extinguished. This suggests that when the orienting saccade reaches the target before hand movement onset, visual updating of the hand motor control signal may occur. A rapid processing of this final input allows a sharper redefinition of the hand landing point.

Adult↗
PubMed1994

Is there an optimal arm posture? Deterioration of finger localization precision and comfort sensation in extreme arm-joint postures.

Processing of joint redundancy is one of the most important problems in motor control. For instance, gaze orientation can be obtained with an infinite number of eye and head combinations. It has been proposed that a solution to this problem might be the minimization of eye and head position-signal errors. For arm movements, where the excess of degrees of freedom is even higher, cost function was proposed as a criterion for movement selection, reflecting some comfort variable evoked from the peripheral inputs, e.g. optimal muscular energy cost or glucose consumption. However, no biological implication of comfort on motor control has yet been demonstrated. We have further investigated this approach by hypothesizing that arm posture choice also relies on a minimization of position-signal errors arising from individual joints. The prediction is that accuracy of fingertip localization by pointing made by the contralateral hand would be enhanced for comfortable postures of the target arm and degraded for uncomfortable postures using extreme joint positions. Results show an increase in pointing variability when extreme joint postures are used (wrist flexion, shoulder elevation, or both). This increase in pointing variability is proportional to the increase in subjective discomfort rating. Individual joint effects can be added arithmetically into a whole arm value for both discomfort rating and pointing variable and constant error. These results suggest that the choice of comfortable postures for the arm corresponds to an optimization of arm position-signal reliability. This new constraint might be a useful tool for further investigation on posture or trajectory formation.

Arm↗
PubMed1994

Optimal contributions of head and eye positions to spatial accuracy in man tested by visually directed pointing.

Encoding of visual target location in extrapersonal space requires convergence of at least three types of information: retinal signals, information about orbital eye positions, and the position of the head on the body. Since the position of gaze is the sum of the head position and the eye position, inaccuracy of spatial localization of the target may result from the sum of the corresponding three levels of errors: retina, ocular and head. In order to evaluate the possible errors evoked at each level, accuracy of target encoding was assessed through a motor response requiring subjects to point with the hand towards a target seen under foveal vision, eliminating the retinal source of error. Subjects had first to orient their head to one of three positions to the right (0, 40, 80 degrees) and maintain this head position while orienting gaze and pointing to one of five target positions (0, 20, 40, 60, 80 degrees). This resulted in 11 combinations of static head and eye positions, and corresponded to five different gaze eccentricities. The accuracy of target pointing was tested without vision of the moving hand. Six subjects were tested. No systematic bias in finger pointing was observed for eye positions ranging from 0 to 40 degrees to the right or left within the orbit. However, the variability (as measured by a surface error) given by the scatter of hand pointing increased quadratically with eye eccentricity. A similar observation was made with the eye centered and the head position ranging from 0 to 80 degrees, although the surface error increased less steeply with eccentricity. Some interaction between eye and head eccentricity also contributed to the pointing error. These results suggest that pointing should be most accurate with a head displacement corresponding to 90% of the gaze eccentricity. These results explain the systematic hypometry of head orienting towards targets observed under natural conditions: thus the respective contribution of head and eye to gaze orientation might be determined in order to optimize accuracy of target encoding.

Adult↗
PubMed1993

Error processing in pointing at randomly feedback-induced double-step stimuli.

Two experiments were conducted to determine the spatial and temporal organization of the arm trajectory in human subjects as they pointed to single- and double-step target displacements. Subjects pointed either without (Experiment 1) or with (Experiment 2) vision of their moving hand throughout the trial. In both experiments, target perturbation occurring in double-step trials was clearly perceived by the subjects and was randomly introduced either at the onset or at peak velocity of hand movement. Regardless of whether or not visual reafference from the pointing hand was available, subjects corrected the trajectory of their moving hand to accommodate the double-step. Moreover, asymmetrical velocity profiles were observed for responses to both types of target, with or without vision of the moving hand. The acceleration phase was a fixed pattern independent of the type of step stimulation. However, a clear dissociation, both in the deceleration phase and accuracy of responses to double-step targets, emerged according to the timing of target perturbation. When targets were perturbed at the onset of hand movement, subjects modulated the deceleration phase of their response to compensate for 88 to 100% of the second target displacement. In contrast, when targets were perturbed at peak velocity of hand movement, subjects were unable to modulate the deceleration phase adequately and compensated for only 20 to 40% of the perturbation. These results suggest that motor error is dynamically evaluated during the acceleration phase of a movement toward a perturbed target, allowing amendments to the trajectory to be performed during the deceleration phase. This main corrective process appears to be basically independent of visual reafference from the moving hand.

Journal Article↗
PubMed1992

Automatic control during hand reaching at undetected two-dimensional target displacements.

1. The aim of this study was to demonstrate that goal-directed pointing movements, executed at normal speed to a small visual target, but without vision of the movement, do not rely on preprogrammed commands (open-loop process); by contrast these responses are under the control of a feedback loop, which compares the ongoing response and the goal (or its internal representation). When the location of this goal is changed at the onset of the movement, an automatic correction of the path occurs. Modification of the goal was obtained by presenting a target in the peripheral visual field that the subject had to look at and point at as quickly and accurately as possible. When the orienting ocular saccade reached its peak velocity, statistically corresponding to the hand movement onset, the target was suddenly shifted 10 degrees in a random direction. This perturbation was undetected by the subject because of the absence of perception during the saccade. For the compensation to occur, the initial orientation of the movement and also its extent had to be modified. The results revealed 1) a nearly complete compensation of the movement path and a 66- to 80-ms duration lengthening; 2) relatively short reaction times to the perturbations (from 145 to 174 ms, with effective reaction times even 40 ms shorter); 3) nearly identical spatiotemporal movement characteristics to the perturbations, regardless of whether vision of the hand was allowed, suggesting that corrections were subserved by the same mechanisms. 2. The spatiotemporal characteristics of these unconscious corrections were similar to those observed in the classical double-step experiments investigating the intentional modifications of ongoing movements and suggest that they might share some common low-level mechanisms. That is, they could rely on visuokinesthetic feedback loops, which compare the updated information provided by the eye at the end of the saccade and the proprioceptive information of the end point effector (the fingertip here); they could also rely on feed-forward processes detecting the discrepancy between an efference copy of the movement and the new goal; or they could rely on a combination of those two main processes.

Acceleration↗
PubMed1991

Saccade metrics in multiple sclerosis: versional velocity disconjugacy as the best clue?

We studied the parameters (latency, amplitude, peak velocity) of horizontal saccades in 32 patients with multiple sclerosis (MS) and 20 healthy, age matched control subjects. Saccades were recorded by direct-current electro-oculography technique (EOG). The patients were divided in 2 groups according to the absence or the presence of clinical internuclear ophthalmoplegia (INO). In both groups, we found increased latency, hypometria and reduced velocity. The disconjugacy of saccades was measured by calculating the ratio of abduction and adduction peak velocities (the versional disconjugacy index: VDI). Though the absolute value of this index might be dependent on the recording technique, its variation is not. Interestingly, the VDI was significantly increased in the groups of MS patients without clinical INO, indicating a more severe slowing in adduction. We concluded that VDI may be a very useful index in detecting subtle disorders in saccades conjugacy.

Electrooculography↗
PubMed1989

The timing of mentally represented actions.

The performance of subjects walking blindly to previously inspected visual targets (located at 5, 10 or 15 m from the subjects) was studied in 2 experiments. In Expt. 1, subjects selected as good visual imagers were instructed to build up a mental representation of the target. Then they had to either actually walk or imagine themselves walking to the target. Walking time was measured in both the actual and the mental performance. It was found that subjects took almost exactly the same time in the two conditions. Accuracy of these subjects was also measured in the actual walking task. They were found to make no direction errors and to slightly overshoot target location. Subjects from another, control, group, who received no instructions about visual imagery made much larger errors. In Expt. 2, actual and mental walking times were measured in the same subjects as in Expt. 1, while they carried a 25-kg weight on their shoulders. In this condition, actual walking time was the same as in Expt. 1, although mental walking time was found to increase systematically by about 30%. These results are discussed in terms of the neural parameters encoded in the motor program for actually executing or mentally performing an action.

Adult↗
PubMed1988

Vestibuloocular reflex inhibition and gaze saccade control characteristics during eye-head orientation in humans.

1. In natural conditions, gaze (i.e., eye + head) orientation is a complex behavior involving simultaneously the eye and head motor systems. Thus one of the key problems of gaze control is whether or not the vestibuloocular reflex (VOR) elicited by head rotation and saccadic eye movement linearly add. 2. Kinematics of human gaze saccades within the oculomotor range (OMR) were quantified under different conditions of head motion. Saccades were visually triggered while the head was fixed or passively moving at a constant velocity (200 deg/s) either in the same direction as, or opposite to, the saccade. Active eye-head coordination was also studied in a session in which subjects were trained to actively rotate their head at a nearly constant velocity during the saccade and, in another session, during natural gaze responses. 3. When the head was passively rotated toward the visual target, both maximum and mean gaze velocities increased with respect to control responses with the head fixed; these effects increased with gaze saccade amplitude. In addition, saccade duration was reduced so that corresponding gaze accuracy, although poorer than for control responses, was not dramatically affected by head motion. 4. The same effects on gaze velocity were present during active head motion when a constant head velocity was maintained throughout saccade duration, and gaze saccades were as accurate as with the head fixed. 5. During natural gaze responses, an increased gaze velocity and a decreased saccade duration with respect to control responses became significant only for gaze displacement larger than 30 degrees, due to the negligible contribution of head motion for smaller responses. 6. When the head was passively rotated in the opposite direction to target step, gaze saccades were slower than those obtained with the head fixed; but their average accuracy was still maintained. 7. These results confirm a VOR inhibition during saccadic eye movements within the OMR. This inhibition, present in all 16 subjects studied, ranged from 40 to 96% (for a 40 degree target step) between subjects and increased almost linearly with target step amplitude. Furthermore, the systematic difference between instantaneous VOR gain estimated at the time of maximum gaze velocity and mean VOR gain estimated over the whole saccadic duration indicates a decay of VOR inhibition during the ongoing saccade. 8. A simplified model is proposed with a varying VOR inhibition during the saccade. It suggests that VOR inhibition is not directly controlled by the saccadic pulse generator.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1988

Kinematics of centrifugal and centripetal saccadic eye movements in man.

The kinematics of centrifugal and centripetal saccadic eye movements were quantified in human subjects. The maximum velocity of centripetal saccades increased with the eccentricity of the orbital starting point and was systematically higher than that of centrifugal saccades starting from primary orbital position. The slope of this linear increase was related to target step amplitude (2.6 and 3.9 deg/sec/deg for 20 and 30 deg, respectively). Despite these velocity changes, saccade amplitude was maintained by corresponding variations of the duration of deceleration. These findings, which are relevant with respect to saccadic control theories, indicate that initial eye position must be considered before comparing saccades based on their kinematic properties.

Adaptation, Ocular↗
PubMed1986

Vestibulo-ocular reflex (VOR) induced by passive head rotation and goal-directed saccadic eye movements do not simply add in man.

Additivity between vestibulo-ocular reflex (VOR) and saccadic eye movements was quantified in man by passively rotating the subject's head as he tracked a stepping target. A systematically increased gaze (i.e. eye + head) saccadic velocity was observed when the head was rotated toward the target, as compared to a head-fixed condition, indicating that VOR and saccades do not fully add. Moreover, although VOR was assumed to be inhibited during ocular saccades, mean gaze saccadic amplitude remained unchanged. This would suggest an on-line computation of gaze position to be fed back to the saccadic system in order to stop the saccade once gaze reaches its goal.

Eye Movements↗
PubMed1986

Visual control of reaching movements without vision of the limb. I. Role of retinal feedback of target position in guiding the hand.

The spatial and temporal organization of hand and eye movements were studied in normal human subjects as they pointed toward small visual targets. The experiment was designed to assess the role of information about target position in correcting the trajectory of the hand when view of the hand was not available. To accomplish this, the duration of target presentation was systematically varied across blocks of trials. The results of this experiment showed that pointing movements were about 3 times more accurate when the target was present throughout the entire pointing movement, than when the target disappeared shortly after the hand movement had begun. These data indicate that pointing movements made without view of the limb are not purely preprogrammed but instead, are corrected during their execution. These modifications to the motor program are smoothly integrated into the ongoing movement and must depend upon comparing visual information about the position of the target with nonvisual information about the position of the limb. The source of this non-visual information was not directly established in the present experiment but presumably must be derived from kinesthetic reafferences and/or efference copy.

Biofeedback, Psychology↗
PubMed1986

Visual control of reaching movements without vision of the limb. II. Evidence of fast unconscious processes correcting the trajectory of the hand to the final position of a double-step stimulus.

In this study, a visual target was localized by both limb and eye. The experimental procedure provided an opportunity to analyze the limb movement trajectories to the target whose location was displaced during saccades. Absence of visual information about position of the moving limb did not interfere with correction of the trajectory of pointing movements. These corrections reflect the new information about target position that becomes available at the end of the first saccade. Mean localization errors to stationary and to displaced targets were not significantly different. This result suggests that subjects were able to compare visual (retinal + eye position) information about the position of the target with information about the position of their moving limb derived from kinesthesis and/or efference copies of the motor commands. An analysis of velocity profiles indicates that the observed corrections of hand movement to target displacement could not be identified by an inflexion point in the trajectory. None of the subjects reported seeing the target change location. In other words, the motor command was adjustable despite the failure of changes in visual locus to reach consciousness.

Adult↗