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Biomedical subjects

C Urquizar

Publications and source records attributed to C Urquizar.

7 recordsLinked to original sources
PubMed2000

Functional adaptation of reactive saccades in humans: a PET study.

It is known that the saccadic system shows adaptive changes when the command sent to the extraocular muscles is inappropriate. Despite an abundance of supportive psychophysical investigations, the neurophysiological substrate of this process is still debated. The present study addresses this issue using H2(15)O positron emission tomography (PET). We contrasted three conditions in which healthy human subjects were required to perform saccadic eye movements toward peripheral visual targets. Two conditions involved a modification of the target location during the course of the initial saccade, when there is suppression of visual perception. In the RAND condition, intra-saccadic target displacement was random from trial-to-trial, precluding any systematic modification of the primary saccade amplitude. In the ADAPT condition, intra-saccadic target displacement was uniform, causing adaptive modification of the primary saccade amplitude. In the third condition (stationary, STAT), the target remained at the same location during the entire trial. Difference images reflecting regional cerebral-blood-flow changes attributable to the process of saccadic adaptation (ADAPT minus RAND; ADAPT minus STAT) showed a selective activation in the oculomotor cerebellar vermis (OCV; lobules VI and VII). This finding is consistent with neurophysiological studies in monkeys. Additional analyses indicated that the cerebellar activation was not related to kinematic factors, and that the absence of significant activation within the frontal eye fields (FEF) or the superior colliculus (SC) did not represent a false negative inference. Besides the contribution of the OCV to saccadic adaptation, we also observed, in the RAND condition, that the saccade amplitude was significantly larger when the previous trial involved a forward jump than when the previous trial involved a backward jump. This observation indicates that saccade accuracy is constantly monitored on a trial-to-trial basis. Behavioral measurements and PET observations (RAND minus STAT) suggest that this single-trial control of saccade amplitude may be functionally distinct from the process of saccadic adaptation.

Adaptation, Physiological↗
PubMed1998

Functional anatomy of saccadic adaptation in humans.

Positron emission tomography (PET) was used to investigate the neurophysiological substrate of human saccadic adaptation. Subjects made saccadic eye movements toward a visual target that was displaced during the course of the initial saccade, a time when visual perception is suppressed. In one condition, displacement was random from trial to trial, precluding any systematic modification of the initial saccade amplitude. In the second condition, the direction and magnitude of displacement were consistent, causing adaptative modification of the initial saccade amplitude. PET difference images reflecting metabolic changes attributable to the process of saccadic adaptation showed selective activation of the medioposterior cerebellar cortex. This localization is consistent with neurophysiological findings in monkeys and brain-lesioned humans.

Adaptation, Physiological↗
PubMed1996

Integrated control of hand transport and orientation during prehension movements.

At a descriptive level, prehension movements can be partitioned into three components ensuring, respectively, the transport of the arm to the vicinity of the target, the orientation of the hand according to object tilt, and the grasp itself. Several authors have suggested that this analytic description may be an operational principle for the organization of the motor system. This hypothesis, called "visuomotor channels hypothesis," is in particular supported by experiments showing a parallelism between the reach and grasp components of prehension movements. The purpose of the present study was to determine whether or not the generalization of the visuomotor channels hypothesis, from its initial form, restricted to the grasp and transport components, to its actual form, including the reach orientation and grasp components, may be well founded. Six subjects were required to reach and grasp cylindrical objects presented at a given location, with different orientations. During the movements, object orientation was either kept constant (unperturbed trials) or modified at movement onset (perturbed trials). Results showed that both wrist path (sequence of positions that the hand follows in space), and wrist trajectory (time sequence of the successive positions of the hand) were strongly affected by object orientation and by the occurrence of perturbations. These observations suggested strongly that arm transport and hand orientation were neither planned nor controlled independently. The significant linear regressions observed, with respect to the time, between arm displacement (integral of the magnitude of the velocity vector) and forearm rotation also supported this view. Interestingly, hand orientation was not implemented at only the distal level, demonstrating that all the redundant degrees of freedom available were used by the motor system to achieve the task. The final configuration reached by the arm was very stable for a given final orientation of the object to grasp. In particular, when object tilt was suddenly modified at movement onset, the correction brought the upper limb into the same posture as that obtained when the object was initially presented along the final orientation reached after perturbation. Taken together, the results described in the present study suggest that arm transport and hand orientation do not constitute independent visuomotor channels. They also further suggest that prehension movements are programmed, from an initial configuration, to reach smoothly a final posture that corresponds to a given "location and orientation" as a whole.

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↗
PubMed1992

A non-contact system for 2-dimensional trajectory recording.

A new non-contact, real-time, 2-dimensional recording technique is described. This technique used in a cat training procedure, permits on-line monitoring of the position of a hand-held food target displaced at variable speeds, directions and amplitudes in front of the animal. Due to the optical constraints imposed by this training procedure, 2 orthogonally located 1-dimensional position-sensitive detectors (PSDs) are used in the near infrared bandwidth in association with a pair of pulsing infrared emitting diodes (IREDs) orthogonally mounted on the monitored object. This geometrical configuration and the wide infrared emission angle insure the visibility of each IRED from its associated PSD, whatever the position of the IRED pair in the working area. A computer program controls the sequence of both IREDs pulses and sampling of the corresponding PSDs outputs and provides the x and y coordinates of the IREDs pair at a 80-Hz rate. This non-contact linear technique can be duplicated at a very low cost for use in a variety of contexts, even under extreme luminance conditions.

Animals↗
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↗