Biomedical subjects

H Gréa

Publications and source records attributed to H Gréa.

5 recordsLinked to original sources

PubMed2001

Functional anatomy of nonvisual feedback loops during reaching: a positron emission tomography study.

Reaching movements performed without vision of the moving limb are continuously monitored, during their execution, by feedback loops (designated nonvisual). In this study, we investigated the functional anatomy of these nonvisual loops using positron emission tomography (PET). Seven subjects had to "look at" (eye) or "look and point to" (eye-arm) visual targets whose location either remained stationary or changed undetectably during the ocular saccade (when vision is suppressed). Slightly changing the target location during gaze shift causes an increase in the amount of correction to be generated. Functional anatomy of nonvisual feedback loops was identified by comparing the reaching condition involving large corrections (jump) with the reaching condition involving small corrections (stationary), after subtracting the activations associated with saccadic movements and hand movement planning [(eye-arm-jumping minus eye-jumping) minus (eye-arm-stationary minus eye-stationary)]. Behavioral data confirmed that the subjects were both accurate at reaching to the stationary targets and able to update their movement smoothly and early in response to the target jump. PET difference images showed that these corrections were mediated by a restricted network involving the left posterior parietal cortex, the right anterior intermediate cerebellum, and the left primary motor cortex. These results are consistent with our knowledge of the functional properties of these areas and more generally with models emphasizing parietal-cerebellar circuits for processing a dynamic motor error signal.

Adult↗

PubMed2000

Postural invariance in three-dimensional reaching and grasping movements.

The question of whether the final arm posture to be reached is determined in advance during prehension movements remains widely debated. To address this issue, we designed a psychophysical experiment in which human subjects were instructed to reach and grasp, with their right arm, a small sphere presented at various locations. In some trials the sphere remained stationary, while in others (the perturbed trials) it suddenly jumped, at movement onset, to a new unpredictable position. Our data indicate that the final configuration of the upper limb is highly predictable for a given location of the sphere. For movements directed at stationary objects, the variability of the final arm posture was very small in relation to the variability allowed by joint redundancy. For movements directed at "jumping" objects, the initial motor response was quickly amended, allowing an accurate grasp. The final arm posture reached at the end of the perturbed trials was neither different from nor more variable than the final arm posture reached at the end of the corresponding stationary trials (i.e. the trials sharing the same final object location). This latter result is not trivial, considering both joint redundancy and the motor reorganization imposed by the change in sphere location. In contrast to earlier observations, our data cannot be accounted for by biomechanical or functional factors. Indeed, the spherical object used in the present study did not constrain the final arm configuration or the hand trajectory. When considered together, our data support the idea that the final posture to be reached is planned in advance and used as a control variable by the central nervous system.

Adult↗

PubMed2000

Proprioception does not quickly drift during visual occlusion.

Several perceptual studies have shown that the ability to estimate the location of the arm degrades quickly during visual occlusion. To account for this effect, it has been suggested that proprioception drifts when not continuously calibrated by vision. In the present study, we re-evaluated this hypothesis by isolating the proprioceptive component of position sense (i.e., the subjects were forced to rely exclusively on proprioception to locate their hand, which was not the case in earlier studies). Three experiments were conducted. In experiment 1, subjects were required to estimate the location of their unseen right hand, at rest, using a visual spot controlled by the left hand through a joystick. Results showed that the mean accuracy was identical whether the localization task was performed immediately after the positioning of the hand or after a 10-s delay. In experiments 2 and 3, subjects were required to point, without vision of their limb, to visual targets. These two experiments relied on the demonstration that biases in the perception of the initial hand location induced systematic variations of the movement characteristics (initial direction, final accuracy, end-point variability). For these motor tasks, the subjects did not pay attention to the initial hand location, which removed the possible occurrence of confounding cognitive strategies. Results indicated that movement characteristics were, on average, not affected when a 15-s or 20-s delay was introduced between the positioning of the arm at the starting point and the presentation of the target. When considered together, our results suggest that proprioception does not quickly drift in the absence of visual information. The potential origin of the discrepancy between our results and earlier studies is discussed.

Adult↗

PubMed2000

An 'automatic pilot' for the hand in human posterior parietal cortex: toward reinterpreting optic ataxia.

We designed a protocol distinguishing between automatic and intentional motor reactions to changes in target location triggered at movement onset. In response to target jumps, but not to a similar change cued by a color switch, normal subjects often could not avoid automatically correcting fast aiming movements. This suggests that an 'automatic pilot' relying on spatial vision drives fast corrective arm movements that can escape intentional control. In a patient with a bilateral posterior parietal cortex (PPC) lesion, motor corrections could only be slow and deliberate. We propose that 'on-line' control is the most specific function of the PPC and that optic ataxia could result from a disruption of automatic hand guidance.

Adult↗

PubMed1998

Final posture of the upper limb depends on the initial position of the hand during prehension movements.

The question of knowing how the nervous system transforms a desired position and orientation of the hand into a set of arm and forearm angles has been widely addressed during the last few decades. Despite this fact, it still remains unclear as to whether a unique posture of the arm is associated with every location and orientation of the hand in space. The main objective of the present study a was to address this question. To this end, we studied a prehension task requiring human subjects to reach and grasp a cylindrical object presented at different locations, along variable orientations. In contrast to previous investigations, we considered the influence of the initial position of the hand. Results showed that the posture of the arm: (1) varied systematically as a function of the movement starting point; (2) was stereotyped for a particular subject given a configuration of the object and a movement starting location; (3) was altered at both the distal and proximal levels when the orientation of the object was changed; (4) was similarly influenced by the experimental factors in all the subjects, except one. When considered together, the previous results support three main conclusions: First, the nervous system solves the joint redundancy problem using fixed strategies. Second, these fixed strategies do not provide a single correspondence between hand configuration and arm posture. Third, the position and orientation of the hand in space are unlikely to be controlled through separate independent neural pathways.

Adult↗