Biomedical subjects

J L Vercher

Publications and source records attributed to J L Vercher.

At least 19 recordsLinked to original sources

PubMed2006

Vision of the hand prior to movement onset allows full motor adaptation to a multi-force environment.

In everyday life, because of unexpected mechanical perturbation applied to the hand or to the whole body, hand movements may become suddenly inaccurate. With prolonged exposure to the perturbation, trajectories slowly recover their normal accuracy, which is the mark of motor adaptation. However, full development of this adaptive process in complete darkness has been recently challenged in a multi-force environment. Here, we report on the effectiveness of static hand position information as specified through vision prior to movement onset on the adaptative changes, over trials, of pointing movements performed in a gravitoinertial force field. For this, subjects seated off-center on a platform rotating at constant velocity, were either confined to complete darkness (No Vision Session, NV) or provided with vision of the hand resting on the starting position prior to movement onset (Hand Vision Prior to Movement Session, HVPM). Overall, our results showed that adaptation to the centrifugal force was very rapid, and allowed subjects to demonstrate appropriate motor control as early as of the very first trials performed during the rotation period, even in the NV condition. They also showed that the integration by the Central Nervous System (CNS) of visual and proprioceptive information prior to the execution of a reaching movement allows subjects to reach full motor adaptation in a multi-force environment. Furthermore, our data confirm the existence of differentiated motor adaptive mechanisms for centrifugal and Coriolis forces. Adaptation to the former may fully develop on the basis of an a priori coding of the characteristics of the background force level even without visual information, while the latter needs visual cues about hand position prior to movement onset to take place.

Adaptation, Physiological↗

PubMed2002

Galvanic vestibular stimulation in humans produces online arm movement deviations when reaching towards memorized visual targets.

Using galvanic vestibular stimulation (GVS), we tested whether a change in vestibular input at the onset of goal-directed arm movements induces deviations in arm trajectory. Eight head-fixed standing subjects were instructed to reach for memorized visual targets in complete darkness. In half of the trials, randomly-selected, a 3 mA bipolar binaural galvanic stimulation of randomly alternating polarity was triggered by the movement onset. Results revealed significant GVS-induced directional shifts of reaching movements towards the anode side. The earliest significant deviations of hand path occurred 240 ms after stimulation onset. The likely goal of these online deviations of arm trajectory was to compensate for a vestibular-evoked apparent change in the spatial relationship between the target and the hand.

Adult↗

PubMed2001

Supramodal effects of galvanic vestibular stimulation on the subjective vertical.

This study investigated whether the tilt of the subjective vertical induced by galvanic vestibular stimulation, demonstrated by asking subjects to set a rod to the vertical, was specific to the visual modality or could be found in two tasks relying on proprioceptive and somatosensory cues. In all cases, settings were significantly deviated in the direction of the anode, but errors were smaller in the somatosensory tasks than in the visual task. We propose that the effects observed in the somatosensory modality reflects only a modification of the central representation of gravity, whereas visual effects are also in part the consequence of unregistered ocular torsion.

Adult↗

PubMed2001

Visual feedback of the moving arm allows complete adaptation of pointing movements to centrifugal and Coriolis forces in human subjects.

A classical visuo-manual adaptation protocol carried out on a rotating platform was used to test the ability of subjects to adapt to centrifugal and Coriolis forces when visual feedback of the arm is manipulated. Three main results emerge: (a) an early modification of the initial trajectory of the movements takes place even without visual feedback of the arm; (b) despite the change in the initial trajectory, the new external force decreases the accuracy of the pointing movements when vision is precluded; (c) a visual adaptive phase allows complete adaptation of the pointing movements performed in a modified gravitoinertial field. Therefore vision would be essential for subjects to completely adapt to centrifugal and Coriolis forces. However, other sensory signals (i.e. vestibular and proprioceptive) may constitute the basis for early but partial correction of the pointing movements.

Adult↗

PubMed2000

The role of proprioception and attention in a visuomotor adaptation task.

The role of proprioception in the control and adaptation of visuomotor relationships is still unclear. We have studied a deafferented subject, IW, and control subjects in a task in which they used single joint elbow extension to move to a visual target, with visual feedback of the terminal position provided by a cursor displayed in the plane of their movements. We report the differences in movement accuracy between the deafferented subject and controls in the normal task and when challenged with a cognitive load, counting backwards. All subjects were less accurate when counting; this was a small effect for the controls (<10% change) but much greater for the deafferented subject (>60% change). We also examined changes in movement kinematics when the instructed amplitude was altered via a changed gain between final arm position and presentation of the feedback cursor. The deafferented subject maintained temporal movement parameters stable and altered amplitude by scaling force (i.e. changed peak velocity), whereas the controls scaled both movement velocity and duration. Finally, we compared the subjects' adaptation of movement amplitude after a period of exposure to the changed visuomotor gain. The deafferented subject was able to adapt, but his adaptation was severely impaired by the counting task. These results suggest that proprioception is not an absolute requirement for adaptation to occur. Instead, proprioception has a more subtle role to play in the adjustment to visuomotor perturbations. It has an important role in the control of reaching movements, while in the absence of proprioception, attention appears necessary to monitor movements.

Adaptation, Physiological↗

PubMed2000

Surgical modification of the AC/A ratio and the binocular alignment ("Phoria") at distance; its influence on accommodative esotropia: a study of 21 cases.

PURPOSE: To study these changes in patients and their relation to manifestation of accommodative esotropia and response to surgery. METHODS: Patient-subjects: 21 patients who had difficulty adjusting to spectacle correction and underwent bilateral medial rectus loop suspension-recession surgery to alleviate their accommodative esotropia. Measurements of AC/A, uncorrected distance phoria, and related parameters were taken before and after surgical intervention was performed and were analyzed using analysis of variance (ANOVA). RESULTS: Both AC/A ratios and the distance phorias are "statistically significantly" (p< or =0.05), and independently, reduced by this surgical procedure. The manifestation of accommodative esotropia and response to surgery can be fairly accurately predicted from the values of AC/A ratio and distance phoria. CONCLUSION: Surgical treatment of accommodative esotropia creates well defined, long-lasting reductions in the AC/A ratio and the distance phoria.

Accommodation, Ocular↗

PubMed1999

Does the oculo-manual co-ordination control system use an internal model of the arm dynamics?

The hypothesis that during self-moved target tracking, the eye-arm co-ordination control system uses an internal model of the arm dynamics was tested. The contribution of arm proprioception to this model was also assessed. Subjects (nine healthy adults and one deafferented subject) were requested to make forearm movements and visually track an arm-driven target. Unexpected changes in mechanical properties of the manipulandum were used to modify the dynamical conditions of arm movement. The smooth pursuit gain (SPG) was computed before and during the perturbation. Results showed a decrease of SPG during perturbation in control subjects only. We propose that an internal model of the arm dynamics may be used to co-ordinate eye and arm movements, and arm proprioception may contribute to this internal model.

Adult↗

PubMed1999

The oculomanual coordination control center takes into account the mechanical properties of the arm.

When the eyes and arm are involved in a tracking task, the characteristics of each system differ from those observed when they act alone: smooth pursuit (SP) latency decreases from 130 ms in external target tracking tasks to 0 ms in self-moved target tracking tasks. Two models have been proposed to explain this coordination. The common command model suggests that the same command be addressed to the two sensorimotor systems, which are otherwise organized in parallel, while the coordination control model proposes that coordination is due to a mutual exchange of information between the motor systems. In both cases, the interaction should take into account the dynamic differences between the two systems. However, the nature of the adaptation depends on the model. During self-moved target tracking a perturbation was applied to the arm through the use of an electromagnetic brake. A randomized perturbation of the arm increased the arm motor reaction time without affecting SP. In contrast, a constant perturbation produced an adaptation of the coordination control characterized by a decrease in arm latency and an increase in SP latency relative to motor command. This brought the arm-to-SP latency back to 0 ms. These results support the coordination control model.

Adaptation, Physiological↗

PubMed1998

Updating visual space during passive and voluntary head-in-space movements.

The accuracy of our spatially oriented behaviors largely depends on the precision of monitoring the change in body position with respect to space during self-motion. We investigated observers' capacity to determine, before and after head rotations about the yaw axis, the position of a memorized earth-fixed visual target positioned 21 degrees laterally. The subjects (n=6) showed small errors (mean=-0.6 degrees) and little variability (mean=0.9 degrees) in determining the position of an extinguished visual-target position when the head (and gaze) remained in a straight-ahead position. This accuracy was preserved when subjects voluntary rotated the head by various magnitudes in the direction of the memorized visual target (head rotations ranged between 5 degrees and 60 degrees). However, when the chair on which the subjects were seated was unexpectedly rotated about the yaw axis in the direction of the target (chair rotations ranged between 6 degrees and 36 degrees ) during the head-on-trunk rotations, the performance was markedly decreased, both in terms of spatial precision (mean error=5.6 degrees ) and variability (mean=5.7 degrees). A control experiment showed that the prior knowledge of chair rotation occurrence had no effect on the perceived target position after head-trunk movements. Updating an earth-fixed target position during head-on-trunk rotations could be achieved through both cervical and vestibular signals processing, but, in the present experiment, the vestibular output was the only signal that had the potentiality to contribute to accurate coding of the target position after simultaneous head and trunk movements. Our results therefore suggest that the vestibular output is a noisy signal for the central nervous signal to update the visual space during head-in-space motion.

Adult↗

PubMed1997

Role of arm proprioception in calibrating the arm-eye temporal coordination.

When subjects track with the eyes an arm-attached target, eye latency is shorter than when tracking an external target. This improved synchrony could result from either a common command addressed to the two systems or from an influence of the arm command on eye motion initiation. According to the first hypothesis, the eyes should start moving long before the arm, because of the difference in dynamics. We recorded arm and eye motion together with biceps muscle activity in controls and a deafferented subject. Data support the second hypothesis. Moreover, the deafferented subject showed a lesser correlation between arm and eye motions than controls, suggesting a role for arm proprioception in the calibration of the temporal relationship between arm and eye movements.

Adult↗

PubMed1997

Manuo-ocular coordination in target tracking. I. A model simulating human performance.

During eye tracking of a self-moved target, human subjects' performance differs from eye-alone tracking of an external target. Typical latency between target and eye motion onsets is shorter, ocular smooth pursuit (SP) saturation velocity increases and the maximum target motion frequency at which the SP system functions correctly is higher. Based on a previous qualitative model, a quantitative model of the coordination control between the arm motor system and the SP system is presented and evaluated here. The model structure maintains a high level of parallelism with the physiological system. It contains three main parts: the eye motor control (containing a SP branch and a saccadic branch), the arm motor control and the coordination control. The coordination control is achieved via an exchange of information between the arm and the eye sensorimotor systems, mediated by sensory signals (vision, proprioception) and motor command copy. This cross-talk results in improved SP system performance. The model has been computer simulated and the results have been compared with human subjects' behavior observed during previous experiments. The model performance is seen to quantitatively fit data on human subjects.

Hand↗

PubMed1997

Manuo-ocular coordination in target tracking. II. Comparing the model with human behavior.

Several studies have shown that humans track a moving visual target with their eyes better if the movement of this target is directly controlled by the observer's hand. The improvement in performance has been attributed to coordination control between the arm motor system and the smooth pursuit (SP) system. In such a task, the SP system shows characteristics that differ from those observed during eye-alone tracking: latency (between the target-arm and the eye motion onsets) is shorter, maximum SP velocity is higher and the maximum target motion frequency at which the SP can function effectively is also higher. The aim of this article is to qualitatively evaluate the behavior of a dynamical model simulating the oculomotor system and the arm motor system when both are involved in tracking visual targets. The evaluation is essentially based on a comparison of the behavior of the model with the behavior of human subjects tracking visual targets under different conditions. The model has been introduced and quantitatively evaluated in a companion paper. The model is based on an exchange of internal information between the two sensorimotor systems, mediated by sensory signals (vision, arm muscle proprioception) and motor signals (arm motor command copy). The exchange is achieved by a specialized structure of the central nervous system, previously identified as a part of the cerebellum. Computer simulation of the model yielded results that fit the behavior of human subjects observed during previously reported experiments, both qualitatively and quantitatively. The parallelism between physiology and human behavior on the one hand, and structure and simulation of the model on the other hand, is discussed.

Behavior↗

PubMed1997

The role of ocular muscle proprioception during modifications in smooth pursuit output.

The output of the smooth pursuit (SP) system can be increased by adding a portion of the recorded eye motion onto target motion, producing a situation analogous to that occurring with weakened ocular muscles. This change is most likely the result of alterations in the signals that code eye and target motion. We have assessed the contribution of one such signal, that arising from ocular proprioception, to the modification process during monocular SP by preventing the motion of the non-viewing eye with a suction scleral lens. The large increases normally observed for SP velocity following the modification period were substantially reduced under these conditions. Similar alterations were also observed in a manual tracking task. These results demonstrate that ocular proprioceptive signals serve to stabilize the output of the SP system following perturbations, via the recoding of eye and target motion.

Humans↗

PubMed1996

The relative contribution of retinal and extraretinal signals in determining the accuracy of reaching movements in normal subjects and a deafferented patient.

This experiment investigated the relative extent to which different signals from the visuo-oculomotor system are used to improve accuracy of arm movements. Different visuo-oculomotor conditions were used to produce various retinal and extraretinal signals leading to a similar target amplitude: (a) fixating a central target while pointing to a peripheral visual target, (b) tracking a target through smooth pursuit movement and then pointing to the target when its excursion ceased, and (c) pointing to a target reached previously by a saccadic eye movement. The experiment was performed with a deafferented subject and control subjects. For the deafferented patient, the absence of proprioception prevented any comparison between internal representations of target and limb (through proprioception) positions during the arm movement. The deafferented patient's endpoint therefore provided a good estimate of the accuracy of the target coordinates used by the arm motor system. The deafferented subject showed relatively good accuracy by producing a saccade prior to the pointing, but large overshooting in the fixation condition and undershooting in the pursuit condition. The results suggest that the deafferented subject does use oculomotor signals to program arm movement and that signals associated with fast movements of the eyes are better for pointing accuracy than slow ramp movements. The inaccuracy of the deafferented subject when no eye movement is allowed (the condition in which the controls were the most accurate) suggests that, in this condition, a proprioceptive map is involved in which both the target and the arm are represented.

Adult↗

PubMed1996

Self-moved target eye tracking in control and deafferented subjects: roles of arm motor command and proprioception in arm-eye coordination.

1. When a visual target is moved by the subject's hand (self-moved target tracking), smooth pursuit (SP) characteristics differ from eye-alone tracking: SP latency is shorter and maximal eye velocity is higher in self-moved target tracking than in eye-alone tracking. The aim of this study was to determine which signals (motor command and/or proprioception) generated during arm motion are responsible for the decreased time interval between arm and eye motion onsets in self-moved target tracking. 2. Six control subjects tracked a visual target whose motion was generated by active or passive movements of the observer's arm in order to determine the role played by arm proprioception in the arm-eye coordination. In a second experiment, the participation of two subjects suffering complete loss of proprioception allowed us to assess the contribution of arm motor command signals. 3. In control subjects, passive movement of the arm led to eye latencies significantly longer (130 ms) than when the arm was actively self-moved (-5 ms:negative values meaning that the eyes actually started to move before the target) but slightly shorter than in eye-alone tracking (150 ms). These observations indicate that active movement of the arm is necessary to trigger short-latency SP of self-moved targets. 4. Despite the lack of proprioceptive information about arm motion, the two deafferented subjects produced early SP (-8 ms on average) when they actively moved their arms. In this respect they did not differ from control subjects. Active control of the arm is thus sufficient to trigger short-latency SP. However, in contrast with control subjects, in deafferented subjects SP gain declined with increasing target motion frequency more rapidly in self-moved target tracking than in eye-alone tracking. 5. The deafferented subjects also tracked a self-moved target while the relationship between arm and target motions was altered either by introducing a delay between arm motion and target motion or by reversing target motion relative to arm motion. As with control subjects, delayed target motion did not affect SP latency. Furthermore, the deafferented subjects adapted to the reversed arm-target relationship faster than control subjects. 6. The results suggest that arm motor command is necessary for the eye-to-arm motion onset synchronization, because eye tracking of the passively moved arm was performed by control subjects with a latency comparable with that of eye-alone tracking of an external target. On the other hand, as evidenced by the data from the deafferented subjects, afferent information does not appear to be necessary for reducing the time between arm motion and SP onsets. However, afferent information appears to contribute to the parametric adjustment between arm motor command and visual information about arm motion.

Adult↗

PubMed1995

Encoding the position of a flashed visual target after passive body rotations.

The capacity of the central nervous system (CNS) for processing vestibular signals during passive whole-body rotations to update the internal representation of a visual target position in relation to the body was assessed. Results showed that subjects mislocalized previously presented visual targets after body rotations in complete darkness. Detailed analysis of the results suggested that the large target mislocalization stemmed not only from a systematic underestimation of rotation magnitude but also from the incapacity of the CNS to use the vestibular signals to accurately update the internal representation of the target position in relation to the body after passive rotations.

Adult↗

PubMed1995

Internal representation of gaze direction with and without retinal inputs in man.

The contribution of retinal and extraretinal signals to the coding of eye position in the head was studied in human subjects (Ss). Horizontal saccades were produced in darkness. For some trials, before returning gaze direction to the starting position, a visual signal briefly stimulated the foveal retina. Results showed that this retinal input helped Ss to perceive gaze orientation more accurately after the saccade suggesting that the internal representation of eye position was improved when both extraretinal and retinal signals were available.

Acoustic Stimulation↗

PubMed1995

Failure to update the egocentric representation of the visual space through labyrinthine signal.

The present study evaluated the capacity to compare retinal and vestibular signals in a heterosensorial matching task. In the first experiment, subjects evaluated the magnitude of passive whole-body rotations in relation to the eccentricity of a visual target briefly presented before rotation. Such a task elicits multimodal sensory stimulations experienced by a subject during normal goal-oriented head movements, i.e., retinal and vestibular stimulations. A good capacity to evaluate vestibular signals in relation with retinal inputs might suggest that the labyrinthine output is part of a cognitive feedback-loop controlling active head movements oriented toward a visual target and/or that the labyrinthine signal might make a major contribution to judging the position of the target in space after goal-directed head movements. Results showed that body rotation magnitudes had to exceed the amplitude of the visual target by about 120 and 89% to be perceived as having a similar magnitude to a 10 degree and an 18 degree visual target, respectively. A second experiment was designed to test whether this major discrepancy originated either from (a) an overestimation of the peripheral visual target locations, (b) an underestimation of the labyrinthine signal, (c) a deficiency in matching sensory signals from different modalities, or (d) any combination of (a), (b), and (c). In the second experiment, the actual perception of retinal and labyrinthine signals, as indicated by verbal responses, was quantified. Results from this experiment showed that most of the large underestimation of the vestibular stimulation found in Experiment I ought to emerge from a poor capacity to integrate heterogeneous sensory signals by the perceptual system rather than from pure misperception of the retinal and/or labyrinthine signals. Overall, results from Experiments I and II argue for a deficiency of the CNS to integrate labyrinthine signals for updating the egocentric representation of the peripheral visual target during passive body (head) rotations.

Adult↗