Effects of accuracy constraints on reach-to-grasp movements in cerebellar patients

Reach-to-grasp movements of patients with pathology restricted to the cerebellum were compared with those of normal controls. Two types of paradigms with different accuracy constraints were used to examine whether cerebellar impairment disrupts the stereotypic relationship between arm transport and grip aperture and whether the variability of this relationship is altered when greater accuracy is required. The movements were made to either a vertical dowel or to a cross bar of a small cross. All subjects were asked to reach for either target at a fast but comfortable speed, grasp the object between the index finger and thumb, and lift it a short distance off the table. In terms of the relationship between arm transport and grip aperture, the control subjects showed a high consistency in grip aperture and wrist velocity profiles from trial to trial for movements to both the dowel and the cross. The relationship between the maximum velocity of the wrist and the time at which grip aperture was maximal during the reach was highly consistent throughout the experiment. In contrast, the time of maximum grip aperture and maximum wrist velocity of the cerebellar patients was quite variable from trial to trial, and the relationship of these measurements also varied considerably. These abnormalities were present regardless of the accuracy requirement. In addition, the cerebellar patients required a significantly longer time to grasp and lift the objects than the control subjects. Furthermore, the patients exhibited a greater grip aperture during reach than the controls. These data indicate that the cerebellum contributes substantially to the coordination of movements required to perform reach-to-grasp movements. Specifically, the cerebellum is critical for executing this behavior with a consistent, well-timed relationship between the transport and grasp components. This contribution is apparent even when accuracy demands are minimal.     

Deficits in grasp versus reach during acute hemiparesis

We studied how acute hemiparesis affects the ability to perform purposeful movements of proximal versus distal upper extremity segments. Given the gradient of corticospinal input to the spinal motoneuron pools, we postulated that movement performance requiring distal segment control (grasping) should be more impaired than movement performance requiring proximal segment control (reaching) in people with hemiparesis. We tested subjects with acute hemiparesis and control subjects performing reach and reach-to-grasp movements. Three characteristics of movement performance were quantified for each movement: speed, accuracy, and efficiency. For the reach, we calculated peak wrist velocity, endpoint error, and reach path ratio. For the grasp, we calculated peak aperture rate, aperture at touch, and aperture path ratio. To evaluate the relative deficits in reaching versus grasping, performance measures were converted to z-scores using control group means and standard deviations. For both the movements, movement times were longer and performance was more variable in the hemiparetic group compared to the control group. Hemiparetic z-scores indicated that relative deficits in movement speed were small in the two movements, with deficits in grasp being slightly greater than deficits in reach. Relative deficits in accuracy showed a trend for being larger in the reach compared to the grasp, but this difference did not reach statistical significance. In contrast, relative deficits in efficiency were larger in the grasp compared to the reach, with reaching efficiency near the range of normal performance. When considering data across all three movement characteristics, the ability to perform a purposeful movement with the distal segments was not clearly more disrupted than the ability to perform a purposeful movement with the proximal segments in people with acute hemiparesis.     

Grip and load force coupling during discrete vertical arm movements with a grasped object in cerebellar atrophy

Control of isometric grip forces during manipulation of objects is an essential feature of all skilled manual performances. Recent studies suggested that the anticipation of movement-induced loads may be a cerebellar function. We analysed grip force adjustments to fluctuations of inertial loads during discrete vertical movements with a grasped object in five patients with cerebellar atrophy and five healthy control subjects. Normally grip force is precisely adapted to the load fluctuations, in particular to the maximum load force, which occurs early in upward and late in downward movements. Both groups produced similar accelerations of the grasped object and consequently similar maximum loads. However, cerebellar patients established increased static grip forces during stationary holding of the object and increased force ratios between grip and load force at the time of maximum acceleration. These findings are congruent with earlier studies analysing grip and load force coupling in patients with cerebellar lesions. In contrast to earlier studies, we found no significant differences in the timing of grip force onset and grip force maximum relative to the onset of movement and maximum acceleration, respectively, between normal controls and four of five cerebellar patients. However, a regression analysis between grip and load forces during the load increase and decrease phases of the movement suggested deficits in the close temporospatial coupling between the two forces in all cerebellar patients. Our findings give further support to the notion that the cerebellum plays a crucial role in the forward control of grip force magnitude and timing during voluntary object manipulation. Compared to earlier studies, the increase in grip forces may be interpreted as a general control strategy to compensate for motor deficits, whereas impairments of temporal grip force regulation may occur at different degrees of dysfunction during the progression of cerebellar atrophy.     

Grip reorganization during wrist transport: the influence of an Altered aperture

Past studies have examined the coupling of reach and grasp components during prehensile movements. Many of these studies have supported the view that these components reflect the output of two parallel, though temporally coupled, motor programs. When the grip aperture is Altered prior to the onset of prehension from its usual, normally flexed position to one of maximal finger extension, our previous work has shown that the grasp component appears to reorganize itself during the reach. This reorganization, consisting of a brief closing and reopening of the grip aperture, only slightly influenced the temporal components of the wrist transport. The present experiment continues this research theme by examining the characteristics of grip aperture reorganization through the comparison of the kinematics of prehension components during movements to two different size objects under normal and Altered grip aperture conditions. It was hypothesized that if the grip reorganization is task dependent it should be related to object size. The experiment found that in the Altered grip condition reorganization did occur, as indicated by a slight closing and reopening of the aperture without influencing the transport of the wrist. The amplitude of and the time to the observed inflection point in the aperture time course were related to object size. The velocity of grip closing for the large object showed double peaks, with the first substantially smaller than the second. Moreover, for the small object, the velocity of grip aperture closing also was double peaked, but the difference between peaks was less pronounced. These changes in grip velocity suggest that the grip reorganization is related to object size. No effect of Altered aperture was observed on the transport component. For both object sizes in the Altered condition, the final peak velocity of grip aperture was statistically significantly correlated with transport time and time to peak deceleration. In contrast, such correlations were not observed for the initial peak velocity of the grip aperture. Furthermore, time to maximum grip aperture was correlated with both time to peak wrist velocity and time peak to wrist deceleration. Thus, as the reach progressed toward the object, the grip and transport components became more interdependent. The results are consistent with the notion that, when a well-practiced, coordinated act such as prehension is confronted with an Altered grip posture at the onset of the reach, the grip can be reorganized during the transport to preserve the relative timing between them. Thus these data add to the growing awareness that not only is there temporal coupling between the reach and grasp components but that these components may be integrated by higher-order control mechanism.     

Adaptation of reach-to-grasp movement in response to force perturbations

This study examined how reach-to-grasp movements are modified during adaptation to external force perturbations applied on the arm during reach. Specifically, we examined whether the organization of these movements was dependent upon the condition under which the perturbation was applied. In response to an auditory signal, all subjects were asked to reach for a vertical dowel, grasp it between the index finger and thumb, and lift it a short distance off the table. The subjects were instructed to do the task as fast as possible. The perturbation was an elastic load acting on the wrist at an angle of 105 deg lateral to the reaching direction. The condition was modified by changing the predictability with which the perturbation was applied in a given trial. After recording unperturbed control trials, perturbations were applied first on successive trials (predictable perturbations) and then were applied randomly (unpredictable perturbations). In the early predictable perturbation trials, reach path length became longer and reaching duration increased. As more predictable perturbations were applied, the reach path length gradually decreased and became similar to that of control trials. Reaching duration also decreased gradually as the subjects adapted by exerting force against the perturbation. In addition, the amplitude of peak grip aperture during arm transport initially increased in response to repeated perturbations. During the course of learning, it reached its maximum and thereafter slightly decreased. However, it did not return to the normal level. The subjects also adapted to the unpredictable perturbations through changes in both arm transport and grasping components, indicating that they can compensate even when the occurrence of the perturbation cannot be predicted during the inter-trial interval. Throughout random perturbation trials, large grip aperture values were observed, suggesting that a conservative aperture level is set regardless of whether the reaching arm is perturbed or not. In addition, the results of the predictable perturbations showed that the time from movement onset to the onset of grip aperture closure changed as adaptation occurred. However, the spatial location where the onset of finger closure occurred showed minimum changes with perturbation. These data suggest that the onset of finger closure is dependent upon distance to target rather than the temporal relationship of the grasp relative to the transport phase of the movement.     

Coordination of multi-joint arm movements in cerebellar ataxia: Analysis of hand and angular kinematics

Kinematic abnormalities of fast multijoint movements in cerebellar ataxia include abnormally increased curvature of hand trajectories and an increased hand path and are thought to originate from an impairment in generating appropriate levels of muscle torques to support normal coordination between shoulder and elbow joints. Such a mechanism predicts that kinematic abnormalities are pronounced when fast movements are performed and large muscular torques are required. Experimental evidence that systematically explores the effects of increasing movement velocities on movement kinematics in cerebellar multijoint movements is limited and to some extent contradictory. We, therefore, investigated angular and hand kinematics of natural multijoint pointing movements in patients with cerebellar degenerative disorders and healthy controls. Subjects performed self-paced vertical pointing movements with their right arms at three different target velocities. Limb movements were recorded in three-dimensional space using a two-camera infrared tracking system. Differences between patients and healthy subjects were most prominent when the subjects performed fast movements. Peak hand acceleration and deceleration were similar to normals during slow and moderate velocity movements but were smaller for fast movements. While altering movement velocities had little or no effect on the length of the hand path and angular motion of elbow and shoulder joints in normal subjects, the patients exhibited overshooting motions (hypermetria) of the hand and at both joints as movement velocity increased. Hypermetria at one joint always accompanied hypermetria at the neighboring joint. Peak elbow angular deceleration was markedly delayed in patients compared with normals. Other temporal movement variables such as the relative timing of shoulder and elbow joint motion onsets were normal in patients. Kinematic abnormalities of multijoint arm movements in cerebellar ataxia include hypermetria at both the elbow and the shoulder joint and, as a consequence, irregular and enlarged paths of the hand, and they are marked with fast but not with slow movements. Our findings suggest that kinematic movement abnormalities that characterize cerebellar limb ataxia are related to an impairment in scaling movement variables such as joint acceleration and deceleration normally with movement speed. Most likely, increased hand paths and decomposition of movement during slow movements, as described earlier, result from compensatory mechanisms the patients may employ if maximum movement accuracy is required.     

The reach to grasp movement of blind subjects

The importance of vision for the processing and coordination of the transport and manipulation components of a reach to grasp movement was assessed. Four blind volunteers (two men, two women; aged 25–40) were compared with matched control groups: (1) blindfolded and (2) full vision. Subjects reached 20 or 30 cm for a large or small diameter (6 cm or 0.7 cm, respectively) cylinder. For condition 1 trials they were given no instruction as to the type of grasp to adopt; for condition 2 they were instructed to consistently use a precision grip; while for condition 3 they were required to use whole hand prehension. Blind subjects demonstrated a double grip pattern and either a low-velocity phase (20 cm) or a double transport movement (30 cm). However, their pattern of prehension with respect to intrinsic (size) and extrinsic (distance) cylinder properties was similar to that of the control groups. Grip aperture was appropriately scaled and, when greater precision was required, deceleration time was prolonged. Temporal coupling was evident between the two components. It was concluded that experience of vision is not necessary for the coordination or patterning of the basic reach to grasp movement. It does allow, however, for a movement consisting of only one opening and closing of the hand.     

The effect of a pictorial illusion on closed-loop and open-loop prehension

It has been proposed that movements to visible and remembered targets are sensitive to qualitatively different types of visual information. When the target is continuously visible, prehensile movements are thought to reflect veridical object size, whereas memory-dependent prehension is sensitive to the perceived size of the object. This hypothesis was explored by assessing the influence of illusory target width on prehension kinematics in three visual conditions: closed-loop (CL; full vision during the response), open-loop brief-delay (OL; visual occlusion coincident with the movement initiation cue) and open-loop 3-s delay (OL3; visual occlusion 3 s prior to movement initiation). To modulate illusory target width, objects were placed on backgrounds consisting of three forms of the Müller-Lyer (ML) figure. Peak grip aperture was sensitive to the ML figure in the OL and OL3, but not CL conditions, suggesting that perceptual information is used to modulate this grasping parameter when the movement is programmed and executed on the basis of visual memory. Peak-aperture velocity was affected by the ML illusion in all three visual conditions, suggesting that perceived object size might be important for modulating this aspect of prehension, independent of memory requirements. The different sensitivity of grip aperture and aperture velocity to illusory target width in the CL condition suggests that grasp preshaping might reflect multiple visuomotor processes. The results of this study are consistent with the tenets of the two-stream model of visual processing.     

Control of aperture closure initiation during trunk-assisted reach-to-grasp movements

The present study investigated how the involvement and direction of trunk movement during reach-to-grasp movements affect the coordination between the transport and grasping components. Seated young adults made prehensile movements in which the involvement of the trunk was varied; the trunk was not involved, moved forward (flexion), or moved backward (extension) in the sagittal plane during the reach to the object. Each of the trunk movements was combined with an extension or flexion motion of the arm during the reach. Regarding the relationship between the trunk and arm motion for arm transport, the onset of wrist motion relative to that of the trunk was delayed to a greater extent for the trunk extension than for the trunk flexion. The variability of the time period from the peak of wrist velocity to the peak of trunk velocity was also significantly greater for trunk extension compared to trunk flexion. These findings indicate that trunk flexion was better integrated into the control of wrist transport than trunk extension. In terms of the temporal relationship between wrist transport and grip aperture, the relationship between the time of peak wrist velocity and the time of peak grip aperture did not change or become less steady across conditions. Therefore, the stability of temporal coordination between wrist transport and grip aperture was maintained despite the variation of the pattern of intersegmental coordination between the arm and the trunk during arm transport. The transport–aperture coordination was further assessed in terms of the control law according to which the initiation of aperture closure during the reach occurs when the hand crosses a hand-to-target distance threshold for grasp initiation, which is a function of peak aperture, wrist velocity and acceleration, trunk velocity and acceleration, and trunk-to-target distance at the time of aperture closure initiation. The participants increased the hand-to-target distance threshold for grasp initiation in the conditions where the trunk was involved compared to the conditions where the trunk was not involved. An increase also occurred when the trunk was extended compared to when it was flexed. The increased distance threshold implies an increase in the hand-to-target distance-related safety margin for grasping when the trunk is involved, especially when it is extended. These results suggest that the CNS significantly utilizes the parameters of trunk movement together with movement parameters related to the arm and the hand for controlling grasp initiation.     

Grasping with the left and right hand: a kinematic study

The goal of the present study was to compare prehension movements of the dominant and the non-dominant hand. Twenty right-handed volunteers (age 20–30 years) reached forward to grasp a cylindrical object, which was lifted and then placed into a target position in a retraction–insertion movement. The movements were performed at three different velocities (normal, deliberately fast, or slowly) both, under visual control, and in a no-vision condition. Analysis of the kinematic data revealed that the speed of hand transport influenced pre-shaping of both hands in a similar way. In the visual condition, the grip aperture increased about linearly with peak transport velocity, while it increased non-linearly with shorter movement duration. Comparison of the regression parameters showed that these relationships were nearly identical for both hands. The dominant hand was faster in inserting the object into the target position. Otherwise, no significant inter-manual differences were found. During prehension without visual control, the fingers opened more and movement duration was prolonged. Except for a larger grip aperture of the dominant hand at the end of the acceleration phase, the kinematic data of both hands were again comparable. This invariance was in contrast to performance in fine motor skills such as a pegboard test and drawing movements, where there was a clear advantage of the dominant hand. The similar pre-shaping of both hands during prehension is discussed with regard to a common motor representation of grasping.     

The coupling of arm and finger movements during prehension

The experiments reported here were aimed at testing the degree of coupling of motor components during the act of prehension. Hand movements were recorded bidimensionnally by a Selspot system which monitored the displacement of IREDS placed at the thumb and index finger tips, at the metacarpophalangeal joint of the index and at the radial styloid. Targets were three-dimensional trnaslucent dowels placed concentrically at 30 cm from the subject. The dowels were 10° apart from each other. In blocked and control trials, one dowel was illuminated and served as a target for the movement. In the perturbed trials (20% of cases) one dowel was illuminated first and the light was unexpectedly shifted to another dowel at the onset of the subject's movements. Kinematic analysis of the movement revealed the following: 1. In blocked and control trials, the wrist moved with a single acceleration to the target dowel. Meanwhile, the finger grip (computed as the distance between thumb and index IREDS) increased up to a maximum size, located in time at about 60% of movement time and then decreased until contact with the dowel. 2. In perturbed trials the initial wrist acceration was aborted. A new acceleration started about 180 ms after the first, in order to reorient the hand to the new target. Similarly, the initial grip aperture also aborted and reincreased in synchrony with the second wrist acceleration. 3. Perturbations increased movement time by only 95 ms on average. The first peak in acceleration indicating abortion of the initial movement occured 100 ms after the movement onset, i.e., 30 ms earlier than in non perturbed trials. These data revealed very fast alterations in movements kinematics in response to perturbations at the visual input level, which preserved accuracy of the movements. In addition, they showed temporary coupling of the finger grip with acceleration of the wrist.     

The speed-accuracy trade-off in manual prehension: effects of movement amplitude,object size and object width on kinematic characteristics

Earlier studies have suggested that the size of an object to be grasped influences the time taken to complete a prehensile movement. However, the use of cylindrical objects in those studies confounded the effects of object size — extent orthogonal to the reach axis — and object width — extent along the reach axis. In separating these effects, the present study demonstrates that movement time is not affected by manipulation of object size, as long as the latter does not approach the maximal object size that can be grasped. Object width, on the other hand, is shown to exert a systematic influence on movement time: Smaller object widths give rise to longer movement times through a lengthening of the deceleration phase of the movement, thus reproducing the effect of target width on the kinematics of aiming movements. As in aiming, movement amplitude also affects the movement time in prehension, influencing primarily the acceleration phase (i.e. peak velocity attained). The effects of object width and movement amplitude were found to combine in a way predicted by Fitts' law, allowing a generalisation of the latter to the transport component in prehensile actions. With respect to the grasp component, both object size and object width are shown to affect peak hand aperture. Increasing object width thus lowers the spatial accuracy demands on the transport component, permitting a faster movement to emerge. At the same time, the hand opens to a larger grip in order to compensate for eventual directional errors that result. Finally, with respect to the control mode of the grasp component, it was found that peak finger closing velocity scales to distance to be covered, defined as the peak hand aperture minus object size.     

Reprogramming of grip aperture in a double-step virtual grasping paradigm

 The present study investigated the control of manual prehension movements in humans. Subjects grasped luminous virtual discs with the thumb and index finger, and we recorded the instantaneous grip aperture, defined as the 3-D distance between the thumb and index finger. Target size could remain constant (single-step trials) or unexpectedly change shortly after target appearance (double-step trials). In single-step responses, grip aperture varied throughout the movement in a consistent fashion. Double-step responses exhibited distinct corrective modifications, which followed the target change with a latency similar to the normal reaction time. This suggests that visual size information has a fast and continuous access to the processes involved in grip formation. The grip-aperture profiles of single-step responses had a different shape when the target called for an increase than when it called for a decrease in the initial finger distance. The same asymmetry was observed for aperture corrections in double-step trials. These findings indicate that increases and decreases of grip aperture are controlled through separate processes, engaged equally by the appearance and by the size change of a target. Corrections of grip aperture in double-step trials had a higher peak velocity and reached their maximum as well as their final value earlier than the aperture profiles of single-step trials. Nevertheless, the total duration of double-step trials was prolonged. These response characteristics did not fit with either of the three corrective strategies previously proposed for double-step pointing movements, which could indicate that grasping and pointing movements are controlled by different mechanisms. However, more data are needed to substantiate this view. Received: 20 April 1998 / Accepted: 28 October 1998     

Integration of visual and auditory information for hand actions: preliminary evidence for the contribution of natural sounds to grasping

When we reach out to grasp objects, vision plays a major role in the control of our movements. Nevertheless, other sensory modalities contribute to the fine-tuning of our actions. Even olfaction has been shown to play a role in the scaling of movements directed at objects. Much less is known about how auditory information might be used to program grasping movements. The aim of our study was to investigate how the sound of a target object affects the planning of grasping movements in normal right-handed subjects. We performed an experiment in which auditory information could be used to infer size of targets when the availability of visual information was varied from trial to trial. Classical kinematic parameters (such as grip aperture) were measured to evaluate the influence of auditory information. In addition, an optimal inference modeling was applied to the data. The scaling of grip aperture indicated that the introduction of sound allowed subjects to infer the size of the object when vision was not available. Moreover, auditory information affected grip aperture even when vision was available. Our findings suggest that the differences in the natural impact sounds of objects of different sizes being placed on a surface can be used to plan grasping movements.     

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.     

Impaired prehension is associated with lesions of the superior and inferior hand representation within the human cerebellum

Impairment of patients with cerebellar disease in prehension is well recognized. So far specific localizations within the human cerebellum associated with the impairment have rarely been assessed. To address this question we performed voxel-based lesion symptom mapping (VLSM) in patients with chronic focal cerebellar lesions in relation to specific deficits in prehensile movements. Patients with stroke within the posterior inferior cerebellar artery territory (n = 13) or the superior cerebellar artery (SCA) territory (n = 7) and corresponding control subjects were included in the study. Participants reached out, grasped, and lifted an object with either the left or right hand and with fast or normal movement speed. Both kinematic and grip-force parameters were recorded. Magnetic resonance imaging anatomical scans of the cerebellum were acquired, and lesions were marked as regions of interest. For VLSM analysis, a nonparametric test (Brunner-Munzel) was applied. Cerebellar patients showed clear abnormalities in hand transport (impaired movement speed and straightness) and, to a lesser degree, in hand shaping (increased finger touch latencies) while grip function was preserved. Deficits were most prominent in patients with SCA lesions and for ipsilesional, fast movements. Disorders in hand transport may be more difficult to compensate than deficits in hand shaping and grip-force control in chronic focal lesions of the cerebellum because of higher demands on predictive control of interaction torques. Lesions of the superior cerebellar cortex (lobules IV, V, VI) were associated with slower hand transport, whereas lesions of both superior (lobules VI, V, VI) and inferior cerebellar cortex (lobules VII, VIII) were associated with impaired movement straightness. These findings show that both the superior and inferior hand representations within the cerebellum contribute to hand transport during prehensile movements; however, they may have a different functional role.     

Impaired anticipatory finger grip-force adjustments in a case of cerebellar degeneration

We describe adjustments in grip force as a consequence of fluctuations in inertial load force during vertical movements of the upper limb in a patient with cerebellar degeneration. Normally grip force is adapted to load-force fluctuations, in particular to the maximum load force, which occurs early in upward movements and late in downward movements. Increased grip force during movement was observed in the patient, but the timing of maximum grip force was not different between upward and downward movements. This suggests impaired cerebellar prediction of the dynamic consequences of voluntary movement. Received: 25 August 1998 / Accepted: 23 March 1999     

An analysis of spatiotemporal variability during prehension movements: effects of object size and distance

 Human prehension movements have been studied with regard to the parallel processing of motor control and sensorimotor coordination. Temporal aspects of the movement (e.g., onset time and duration) have been studied extensively, while spatial aspects have not been studied systematically. Thus, the purpose of this study was to examine spatiotemporal variability of the transport (wrist trajectory) and grasp (grip aperture between the index finger and the thumb) components. In this experiment, the extrinsic (e.g., distance) and intrinsic object properties (e.g., object size) were manipulated. Subjects were required to pick up an aluminum cylinder as quickly and accurately as possible using the index finger and the thumb. It was found that object size significantly affected both transport and grasp components. Distance mainly affected the transport component. These kinematic results were consistent with the findings of earlier studies. Furthermore, the distribution of mean within-subject variability across normalized movement time for the transport component was not the same as that of the grasp component, suggesting that the different motor control processes exist. The peak amplitudes in variability of the wrist trajectory and the grip aperture were obtained at similar points throughout movement time. Furthermore, the peak of wrist variability depended on distance not object size, while that of aperture variability depended on both distance and object size. These results strongly support the hypothesis that the grasp component is adjusted using dynamic information provided from the transport component as the wrist moves toward the object. We also found that wrist variability converged to the target point, while aperture variability was biphasic: it converged, at least, around the point of maximum aperture in the first phase and then remained constant in the second phase. This result suggests that the two components are under different control processes. We hypothesize that the transport component can be modeled as a single feedforward system, while the grasp component can be divided into two separate mechanisms. Received: 4 March 1996 / Accepted: 29 January 1997     

Control of aperture closure initiation during reach-to-grasp movements under manipulations of visual feedback and trunk involvement in Parkinson’s disease

The present project was aimed at investigating how two distinct and important difficulties (coordination difficulty and pronounced dependency on visual feedback) in Parkinson’s disease (PD) affect each other for the coordination between hand transport toward an object and the initiation of finger closure during reach-to-grasp movement. Subjects with PD and age-matched healthy subjects made reach-to-grasp movements to a dowel under conditions in which the target object and/or the hand were either visible or not visible. The involvement of the trunk in task performance was manipulated by positioning the target object within or beyond the participant’s outstretched arm to evaluate the effects of increasing the complexity of intersegmental coordination under different conditions related to the availability of visual feedback in subjects with PD. General kinematic characteristics of the reach-to-grasp movements of the subjects with PD were altered substantially by the removal of target object visibility. Compared with the controls, the subjects with PD considerably lengthened transport time, especially during the aperture closure period, and decreased peak velocity of wrist and trunk movement without target object visibility. Most of these differences were accentuated when the trunk was involved. In contrast, these kinematic parameters did not change depending on the visibility of the hand for both groups. The transport-aperture coordination was assessed in terms of the control law according to which the initiation of aperture closure during the reach occurred when the hand distance-to-target crossed a hand-target distance threshold for grasp initiation that is a function of peak aperture, hand velocity and acceleration, trunk velocity and acceleration, and trunk-target distance at the time of aperture closure initiation. When the hand or the target object was not visible, both groups increased the hand-target distance threshold for grasp initiation compared to its value under full visibility, implying an increase in the hand-target distance-related safety margin for grasping. The increase in the safety margin due to the absence of target object vision or the absence of hand vision was accentuated in the subjects with PD compared to that in the controls. The pronounced increase in the safety margin due to absence of target object vision for the subjects with PD was further accentuated when the trunk was involved compared to when it was not involved. The results imply that individuals with PD have significant limitations regarding neural computations required for efficient utilization of internal representations of target object location and hand motion as well as proprioceptive information about the hand to compensate for the lack of visual information during the performance of complex multisegment movements.     

Spatial and temporal control of trunk-assisted prehensile actions

The present study utilized a trunk-assisted prehension task to examine the hypothesis that there is spatial regularity between the grasp and transport components. To test this hypothesis, we varied movement amplitude, reach speed, and object size. When examining the opening and closure phases of aperture formation, it was found that the distance to peak aperture increased systematically with hand-path trajectory length, while the distance from peak aperture to the object remained constant, which supports the notion of state-space control. Regarding the relationship among the body segments involved, temporal measures such as relative time to peak aperture, and peak velocity of the arm and trunk were altered by the changes in both object size and reach speed. It was also found that the time to peak trunk velocity was coupled with the time to peak arm velocity as well as with the time to peak aperture. Based on these results, it appears that the trunk is closely linked not only to the arm motion, but also to the aperture formation. Collectively, these findings suggest that, during trunk-assisted prehension, the arm and the trunk are coordinated by neuromotor synergies that appear to position grip aperture for a stable closure to grasp the object.