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Biomedical subjects

M Gentilucci

Publications and source records attributed to M Gentilucci.

At least 37 records · Page 2Linked to original sources
PubMed1997

Tactile input of the hand and the control of reaching to grasp movements.

The role of tactile information of the hand in the control of reaching to grasp movements was investigated. The kinematics of both reaching (or transport) and grasp components were studied in healthy subjects in two experimental conditions. In one condition (control condition) subjects were required to reach and grasp an object that could have two sizes and that could be located at two distances from the viewer. In the other condition (anaesthesia condition) the same movements were executed, but anaesthesia was provided to the subjects' fingertips. In both conditions vision of the hand was prevented during movement. Anaesthesia affected mainly the kinematics of the first phase of grasping, that is, the finger-opening phase. This phase was lengthened and maximal finger aperture increased. In contrast, the duration of the successive phase (finger-closure) was poorly modified. The reaching component was also impaired by anaesthesia. Although the total extent of hand path and the spatial relations between the finger aperture and closure phases did not change between the two conditions, hand path variability increased. This occurred during transport deceleration phase and after the increase in variability of finger path. In addition, the whole movement was slowed down. The results of the present experiment suggest that tactile signals at the beginning and at the end of movement can be used to compute grasp time and to optimise grasp temporal parameters. Alternatively, signals from tactile receptors can be involved in encoding the position sense of the fingers. When this input is lacking, the control of grasp and in particular that of finger-opening phase can be impaired. Finally, the effect of the grasp impairment on the reaching component supports the notion that the coordination between reaching and grasping involves the whole temporal course of the two components.

Adult↗
PubMed1997

Planning an action.

The motor control of a sequence of two motor acts forming an action was studied in the present experiment. The two analysed motor acts were reaching-grasping an object (first target) and placing it on a second target of the same shape and size (experiment 1). The aim was to determine whether extrinsic properties of the second target (i.e. target distance) could selectively influence the kinematics of reaching and grasping. Distance, position and size of both targets were randomly varied across the experimental session. The kinematics of the initial phase of the first motor act, that is, velocity of reaching and hand shaping of grasping, were influenced by distance of the second target. No kinematic difference was found between movements executed with and without visual control of both hand and targets. These results could be due to computation of the general program of an action that takes into account extrinsic properties of the final target. Conversely, they could depend on a visual interference effect produced by the near second target on the control of the first motor act. In order to dissociate the effects due to second target distance from those due to visual interference, two control experiments were carried out. In the first control experiment (experiment 2) subjects executed movements directed towards spatial locations at different distances from the first target, as in experiment 1. However, the near second target was not presented and subjects were required to place the object on an arbitrary near position. Distance of the second (either real or arbitrary) target affected the reaching component of the first motor act, as in experiment 1, but not the grasp component. In the second control experiment (experiment 3), the pure visual interference effect was tested. Subjects were required to reach and grasp the object and to lift it in either presence or absence of a second near stimulus. No effect on the initial phase of the first motor act was observed. The results of the this study suggest a dissociation in the control of reaching and grasping, concerning not only visual analysis of extrinsic properties of the immediate target but also visual analysis of the final target of the action. In other words, the notion of modularity for the motor control can be extended to the construction of an entire action.

Adult↗
PubMed1997

Grasping an illusion.

In the present study we attempted to determine the nature of the visual analysis that is performed on an object in order to grasp it. We required eight healthy subjects to reach and grasp a wooden bar which was superimposed over the shaft of the Müller-Lyer illusion. Vision of both the hand and the bar was allowed. Three different bar lengths were used. Two additional control tasks in which the subjects were required to reproduce the length of the shafts were carried out. The results showed that hand shaping while grasping the bar was influenced by the illusion configurations on which it was superimposed. However, this effect was smaller than that observed in the two tasks of length reproduction. These results support the notion that visual analysis performed on the object of a grasp movement is global and takes into account the object itself, as well as its relationships with surrounding cues. We propose, as suggested previously for reaching movements (Gentilucci, M. et al., Neuropsychologia, 1996, 34, 369-376), two partially independent stages during visuo-motor integration for grasping an object. In the first stage, the object is coded inside an object-centred frame of reference. In the second stage it is transposed in an egocentric frame of reference, in which the spatial relations between object and agent are computed. In this second stage the influence of cues surrounding the target is minimized.

Adult↗
PubMed1996

On orienting the hand to reach and grasp an object.

Subjects were required to reach and grasp a parallelepiped, the position, orientation and size of which were varied. The kinematics of reaching and grasping movements was studied in full vision and in no vision conditions. Both direction and movement amplitude of reaching were affected by object orientation. Conversely, both the time course of finger axis orientation and the angular displacement of the hand at wrist were influenced by object position. These results were not modified by the absence of visual control. Finger aperture during grasping was affected by both object size and orientation. This latter result was not due to a distorted size perception, as shown by a control matching experiment. Taken together, the results of the present study suggest the integration between distal and proximal components during reaching and grasping.

Adult↗
PubMed1996

Differential influence of the visual framework on end point accuracy and trajectory specification of arm movements.

In this study the influence of visual scene on both arm end point accuracy and spatial path kinematics was evaluated. Eight subjects, immersed in a virtual environment, were required to point to one of ten targets located at two distances and in five directions. Targets were presented in frameworks of different complexity. The simplest framework was constituted by a uniform background, the most complex framework was constituted by a perspective-arranged grid. In the other two conditions it consisted of lines having a direction parallel to either the subject's sagittal or frontal body axis. Movements were executed without vision of both target and framework. The results showed that pointing movements were hypometric in all conditions. No difference in end point localization was observed between movements executed after presentation of the simplest and the most complex scenes. However, hypometria significantly increased when the scene was formed by lines parallel to the subject's sagittal axis. Visual information on the scene was also used to specify hand path parameters. Trajectory curvature increased with decreasing complexity of the framework. Correspondingly, the pointing kinematics varied. Taken together, these results suggest that visual analysis of cues surrounding the target can influence both target localisation and hand path planning. However, scene complexity is directly related only to determining trajectory curvature. We conclude that planning an arm movement consists of at least two processes: target localisation and hand path specification. Environmental visual cues forming the scene are taken into account differently during the two processes.

Adult↗
PubMed1996

Mechanisms for distance reproduction in perceptual and motor tasks.

Two experiments were carried out: the control experiment and the doubling-distance experiment. In the control experiment subjects were presented with two visual stimuli whose distance was randomly varied. Subjects were required to reproduce the interstimulus remembered distance in two conditions. In one condition (reproduction by pointing) they pointed to a virtual position in space. In the other condition (visual reproduction) they matched the distance by using two other visual stimuli. In the doubling-distance experiment, distances between the two randomly presented stimuli were half of the distances used in the control experiment. Subjects were required to reproduce the double of the presented distance. As in the control experiment, reproduction was executed in two conditions: reproduction by pointing and visual reproduction. In both experiments variable and constant errors were measured. Pointing kinematics were also analysed. The results of the control experiment showed that subjects underestimated distance in reproduction by pointing, whereas they overestimated distance in visual reproduction. Variable errors increased with increasing distance, whereas they were not influenced by the type of reproduction. In the doubling-distance experiment, subjects generally overestimated distance by the same amount in both conditions. However, overestimation decreased with distance during reproduction by pointing. Pointing kinematics varied between the two experiments. The results of the control experiment confirm the hypothesis that perceptual judgement and visuo-motor transformation are two separate processes during which the same object attributes are independently analysed. However, the results of the doubling-distance experiment suggest that perceptual judgement and visuo-motor transformation use the same mechanisms when object attributes are deduced by mental elaboration.

Acceleration↗
PubMed1996

Visual illusion and action.

The role of allocentric cues on movement control was investigated in the present study. Pointing movements directed to the more distant vertex of closed and open configurations of the Muller-Lyer illusion, as well as to the vertex of control lines, were studied in four experimental conditions. In the first (full-vision condition) subjects saw both stimulus and their hand before and during movement, in the second (non-visual feedback condition) they saw the stimulus, but not their hand during movement. In the two remaining conditions (no-vision conditions) vision of the scene and the hand was precluded. Pointing was executed 0 sec (no vision 0 sec delay condition) or 5 sec (no-vision 5 sec delay condition) after the light was switched off. The Muller-Lyer illusion affected pointing kinematics with respect to the control lines. Subjects undershot and overshot the vertex location, respectively, of the closed and open configuration. Correspondingly, the entire kinematics were changed. The main result was, however, a gradually increasing effect of the perceptual illusion when pointing was executed from memory compared to the full-vision condition. These data are discussed according to the hypothesis that the system underlying visual perception in the allocentric frame of reference and that involved in motor action can functionally interact. The strength of this interaction depends upon the efficiency of the egocentric frame of reference by which motor actions are constructed.

Adolescent↗
PubMed1995

Unconscious updating of grasp motor program.

Grasp modification during prehension movements was studied in response to slight variations of somesthetic information about object size. Three experiments were carried out. In experiment 1 eight subjects were required to reach and grasp an object whose size could either increase or decrease, whereas its visual image remained unmodified. The object size was changed during the experiment with uninformed subjects after a block of trials during which visual and somesthetic information were congruent. At the end of the experiment subjects were required to reproduce the size of the object with their fingers (matching test). Results showed that maximal grip aperture during prehension as well as finger aperture in the matching test were modified according to variation in object size, although no subject realized that the object had changed during the experiment. Grasp time was also altered by object size change. Greater and earlier adaptation in maximal grip aperture, as well as perturbation of grasp time, were observed for decrease than for increase in object size. However, complete compensation was never reached for both parameters. Constant confidence in vision could have prevented both complete compensation and conscious detection of object change. This was investigated in two additional experiments. In experiment 2 visual information was made unreliable by informing subjects about variation in grasped object size. This led to greater and earlier modification in maximal grip aperture than in experiment 1. Grasp time was kept almost constant regardless of size variation. In experiment 3 vision of the stimulus was prevented and no information on change in object size was given to subjects. The results of experiment 3 were similar to those of experiment 1, although modification in maximal grip aperture was larger for increase in object size. Correspondingly, grasp time was more affected by increase than by decrease in object size. The results of the three experiments suggest that kinematic parameters usually considered as dependent on object properties, such as maximal grip aperture, were modified in order to compensate perturbation of temporal parameters. This modification induced a "pragmatic" knowledge of object size (as showed by the results of the matching test), although awareness was not reached.

Adult↗
PubMed1994

The fronto-parietal cortex of the prosimian Galago: patterns of cytochrome oxidase activity and motor maps.

We mapped the motor areas of the prosimian Galago crassicaudatus using intracortical electrical microstimulation and morphological and histochemical (cytochrome oxidase) techniques. Stimulation data showed that on the brain convexity there is an area (area Frontalis posterior, F post.) from which movements could be evoked at low threshold (< 10 microA). This area is somatotopically organized, with the leg represented medially, the arm centrally and the face and mouth laterally. Proximal and distal movements are not segregated. Most of the evoked movements, even at threshold, consist of movements involving two or more joints. F post. is characterized by a three-band cytochrome oxidase activity pattern. It has an agranular structure, but it lacks pyramidal cells that are larger than those observed in other areas. In front of F post. there is an area histochemically similar to it, Frontalis intermedialis (F int.). This area consists of two cytoarchitectonic divisions: an agranular division (F int. pars caudalis) and a disgranular division (F int. pars rostralis). The excitability threshold of F int. is relatively high (10 to 30 microA). Eye, ear and neck movements are elicited from its lateral part, whereas trunk movements associated with limb movements are elicited from its medial part. Caudal to F post., there is another region from which movements can be evoked with currents between 10 to 30 microA. This region has the same medio-lateral somatotopic arrangement of F post. Typically, single joint movements are elicited from it. Proximal and distal movements are not segregated. In spite of its homogeneity in terms of motor response, the posterior excitable region is formed by two anatomically separate areas: anterior somatic area (S ant.) and posterior somatic area (S post.). S ant. has a typical koniocortex structure, whereas S post, resembles the parakoniocortex as defined by Sanides (J. Hirnforsch., 9 (1967) 225-252). Histochemically both areas are made up of four longitudinal stripes differing for enzymatic activity. The three superficial stripes tend to merge together and are sharply separated from a deeply located, light stripe. This stripe is homogeneous in S ant., whilst its central part shows an increase in activity in S post. The possible homologies between the motor and somatic areas of the galago and monkey as well as their role in movement control are discussed.

Animals↗
PubMed1994

Dissociating visual and kinesthetic coordinates during pointing movements.

Goal-directed movements imply that the visual coordinates in which the localisation of the goal is coded are transformed into proprioceptive coordinates in which the arm movement is coded. The two systems of coordinates are normally superimposed. Using a virtual reality device attached to the subject's head, we have created a situation where these systems were dissociated from each other. The virtual environment involved virtual visual targets and an image of the subject's hand reconstructed from the output of a data glove wore by the subject's right hand. When the subject's head was rotated, the visual targets and the image of the hand rotated by the same amount. Movements of the real hand were thus in conflict with those of the reconstructed hand, which appeared to err in the direction of head rotation. Pointing movements directed at five targets (0 degree, 26 degrees and 52 degrees on each side) were studied for five different head positions (0 degree, 45 degrees and 80 degrees to the right and to the left). The results showed a significant pointing bias towards head position, except for the left-most targets in the right head rotations. Constant errors in azimuth were proportional to the amount of head rotation. When the head was rotated to the right, constant errors in azimuth were greater during pointing towards right than left targets. Similarly, they were greater for left than for right stimuli when the head was rotated to the left. Errors in amplitude were not influenced by the direction nor the amount of head rotation.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

The role of proprioception in the control of prehension movements: a kinematic study in a peripherally deafferented patient and in normal subjects.

In this study we investigated the role of proprioception in the control of prehension movements, with particular reference to the grasp component. Grasp and transport kinematics were studied in a peripherally deafferented patient and in five healthy subjects. Two experiments were carried out: the prehension experiment and the grasp perturbation experiment. In the prehension experiment both the patient and the control subjects were required to reach and grasp three objects of different size, located at three different distances, both with and without visual feedback. In the grasp perturbation experiment a mechanical perturbation was applied to the fingers during prehension movements, again executed with and without visual feedback. In the prehension experiment temporal parameters of the patient's movements were generally slowed, with greater variability on some measures. However, over the first phase of the movement the pattern of the patient's hand opening and transport acceleration, scaled to object size and distance, was the same as that of controls, both with and without visual feedback. On the contrary, during the final phase of the movement (the finger closure phase and deceleration) the patient's performance differed significantly from the controls. These phases were abnormally lengthened and frequent movement adjustments were observed. In the grasp perturbation experiment the patient was not able to compensate for the perturbations applied to the fingers, even with visual feedback. The data allowed us to investigate also the respective contribution of proprioception and of vision of the hand in the control of prehension. We compared prehension kinematics in two conditions: (a) with visual but no proprioceptive feedback (in the patient) and (b) with proprioceptive but no visual feedback (in the controls). In both experiments proprioceptive control was more efficient than visual control. The results of this study are interpreted in favour of the strict dependence of prehension control on proprioception. The first phase of the movement, however, can be appropriately planned and executed without the necessity of either proprioceptive or visual information about the hand.

Adult↗
PubMed1994

Dissociation between perception and visuomotor transformation during reproduction of remembered distances.

1. In complete darkness subjects were presented with two visual stimuli whose distance was randomly varied. The subjects were required to reproduce the interstimulus remembered distance in two conditions. In one condition (reproduction by pointing) they pointed to a virtual position in space. In the other condition (visual reproduction) they used two other visual stimuli. One of them was fixed, and the other could be manually moved. Constant and variable errors were measured in the two conditions. 2. Constant error varied between the two conditions. In the pointing task subjects slightly overestimated the shorter distances, and underestimated the longer ones. During visual reproduction, they consistently overestimated all distances, and the error of overestimation tended to increase with distance. Statistical comparison between the errors in the two conditions was significant. Variable error increased with distance in both conditions, but did not show any significant difference between the two tasks. 3. The results of the present experiment support the hypothesis that perception and visuo-motor transformation are two separate processes in which the same object attributes are independently analysed. However, the finding that variable error did not change between the two tasks suggests that some stages are common to the two processes.

Adult↗
PubMed1993

Coordination between the transport and the grasp components during prehension movements.

In this study, the possible influence of the transport on the grasp component of prehension movements was investigated. The first phase of the transport (acceleration phase) and of the grasp (finger aperture phase) kinematics were studied under conditions of visual and non-visual object presentation (prehension experiment). In the non-visual condition, object size was estimated by haptics and object position was estimated by proprioception. Eight subjects were required to reach and grasp three objects of different size located at two distances. An additional experiment (matching experiment) was carried out to control the scaling of object size in two conditions. The results showed that in the matching experiment size estimation for large objects was similar in the two conditions, whereas small stimuli were underestimated in the haptic condition. In the prehension experiment, maximal finger aperture and velocity of finger aperture were greater in the non-visual than in the visual condition, and the difference was greater for small than for large stimuli. Moreover, in both conditions, finger opening was larger for prehension movements directed to the far than to the near objects, but only for smaller stimuli. Hand trajectory variability increased in the non-visual condition and with the distance, whereas finger opening variability was only affected by the non-visual condition. For smaller stimuli, increased finger opening with distance was positively correlated with the increase in wrist variability in the visual condition, but not in the non-visual condition. Furthermore, increased finger opening between visual and non-visual conditions was correlated with the increase in wrist variability, for smaller objects at the near object location.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1993

Study of selective reaching and grasping in a patient with unilateral parietal lesion. Dissociated effects of residual spatial neglect.

In the present study we investigated the possibility of a dissociation between the visual control of reaching and the visual control of grasping in a prehension task. To this purpose we studied the kinematics of prehension movements in a patient with a right parietal lesion and in six right-handed healthy control subjects. The task we used was one in which the subjects had to reach and grasp target objects in the presence or absence of a simultaneously presented distractor object. All stimuli were presented in the space ipsilateral to the lesion. The distractor could be either of the same or different size to the target object and was presented either to the right or to the left of the target. The following parameters of the prehension 'transport' component were analysed: wrist trajectory, transport time, tangential peak velocity, acceleration. Maximal finger aperture, time to maximal finger aperture, peak acceleration and time to peak acceleration of grip aperture were the parameters of the 'grasping' component analysed. The results showed that, although the patient had no misreaching, her hand trajectory deviated abnormally towards the distractor position when the distractor was to the right (ipsilateral) side of the target. In contrast, the grasp kinematics was not affected by the distractors, even when the size of the right distractor was different from the target. It appears, therefore, that the attentional shift towards the ipsilesional side, typical of neglect patients, determines a surprising dissociation in motor control. In the presence of a right distractor, the patient plans and partially executes a reaching movement towards that object and simultaneously performs a grasping movement towards a second object, i.e. the centrally located target. The presentation of distractors had no effects on the prehension kinematics of the control subjects.

Attention↗
PubMed1992

Temporal coupling between transport and grasp components during prehension movements: effects of visual perturbation.

The temporal coupling between the transport and grasp components of prehension movements was investigated through two experiments. In Experiment 1, six normal subjects were required to reach and grasp each of three spheres located at three different distances (Blocked trials). In Experiment 2, a visual perturbation paradigm was used in which the location of the object to be reached and grasped could change at the beginning of arm movement (Perturbed trials). The same subjects participated in both experiments. Kinematics of wrist trajectory (transport component) and of distance between thumb and index finger (grasp component) were analyzed. The results of Experiment 1 showed that the two components could be temporally coupled during their time course. In Experiment 2, the visual perturbation affected both the components, but different times were required by each component to reorganize the movement towards the new target. These different times caused the decoupling of those events that appeared synchronized in Experiment 1. Finally, evidence was found to suggest that planning of grip formation takes into account not only the perceived characteristics of the object, but also the time planned by the transport component to reach the object.

Acceleration↗
PubMed1992

Space coding by premotor cortex.

Many neurons in inferior area 6, a cortical premotor area, respond to visual stimuli presented in the space around the animal. We were interested to learn whether the receptive fields of these neurons are coded in retinotopic or in body-centered coordinates. To this purpose we recorded single neurons from inferior area 6 (F4 sector) in a monkey trained to fixate a light and detect its dimming. During fixation visual stimuli were moved towards the monkey both within and outside the neuron's receptive field. The fixation point was then moved and the neuron retested with the monkey's gaze deviated to the new location. The results showed that most inferior area 6 visual neurons code the stimulus position in spatial and not in retinal coordinates. It is proposed that these visual neurons are involved in generating the stable body-centered frame of reference necessary for programming visually guided movements.

Animals↗
PubMed1992

Motor control of voluntary arm movements. Kinematic and modelling study.

The motor control of pointing and reaching-to-grasp movements was investigated using two different approaches (kinematic and modelling) in order to establish whether the type of control varies according to modifications of arm kinematics. Kinematic analysis of arm movements was performed on subjects' hand trajectories directed to large and small stimuli located at two different distances. The subjects were required either to grasp and to point to each stimulus. The kinematics of the subsequent movement, during which subject's hand came back to the starting position, were also studied. For both movements, kinematic analysis was performed on hand linear trajectories as well as on joint angular trajectories of shoulder and elbow. The second approach consisted in the parametric identification of the black box (ARMAX) model of the controller driving the arm movement. Such controller is hypothesized to work for the correct execution of the motor act. The order of the controller ARMAX model was analyzed with respect to the different experimental conditions (distal task, stimulus size and distance). Results from kinematic analysis showed that target distance and size influenced kinematic parameters both of angular and linear displacements. Nevertheless, the structure of the motor program was found to remain constant with distance and distal task, while it varied with precision requirements due to stimulus size. The estimated model order of the controller confirmed the invariance of the control law with regard to movement amplitude, whereas it was sensitive to target size.

Adult↗