Spatio-temporal and kinematic analysis of pointing movements performed by cerebellar patients with limb ataxia

Three patients with cerebellar limb ataxia and three age-matched controls performed arm-pointing movements towards a visual stimulus during an experimental procedure using a double-step paradigm in a three-dimensional space. Four types of trajectories were defined: P1, single-step pointing movement towards the visual stimulus in the initial position S1; P2, double-step pointing movement towards S1; P3, double-step straight pointing movement towards the second position S2; and P4, double-step pointing movement towards S2 with an initial direction towards S1. We found that the cerebellar patients, as well as the controls, were able to modify their motor programs, but with impaired timing, severe anomalies in the direction and amplitude of the changed movement trajectories and alteration of the precision of the pointing movements. Received: 26 February 1997 / Accepted: 13 October 1997     

The gap effect for eye and hand movements in double-step pointing

The existence of a temporal gap between the offset of a fixation target and the onset of a peripheral target generally reduces the saccadic and manual reaction time in response to the peripheral target. Using a double-step paradigm, the present experiment investigated whether a temporal gap between the extinction of the first target and the presentation of the second target can help in reducing the time to trigger the corrective eye movements and to correct the arm trajectory towards the final target position. A gap was introduced between the presentation of the initial target and a new unexpected goal-target during the movement. The results replicated the gap effect for the corrective saccade to the second target, but revealed an opposite effect for the correction of the reaching movements as the arm correction occurred later in the Gap than in the No-Gap conditions. These results suggest that the information available for the arm motor system to correct the trajectory in relation to the second target was different in the Gap and No-Gap conditions. In the No-Gap condition, the correction of reaching movements would be based on retinal errors between the first and the second targets whereas, in the Gap condition, the correction would be based on information derived from the corrective saccade-related signals to the second target.     

Multi-joint limbs permit a flexible response to unpredictable events

The human arm is kinematically redundant, which may allow flexibility in the execution of reaching movements. We have compared reaching movements with and without kinematic redundancy to unpredictable double-step targets. Subjects sat in front of a digitising tablet and were able to view an arc of four targets reflected in the mirror as virtual images in the plane of the tablet. They were instructed to move, from a central starting point, in as straight a line as possible to a target. In one-third of trials, the target light switched to one of its neighbours during the movement. Subjects made 60 movements using shoulder, elbow and wrist and then another 60 movements in which only shoulder and elbow movement were allowed. By restraining the wrist, the limb was made non-redundant. The path length was calculated for each movement. In single-step trials, there was no significant difference between path lengths performed with and without wrist restraint. As expected there was a significant increase in path length during double-step trials. Moreover this increase was significantly greater when the wrist was restrained. The variability across both single- and double-step movements was significantly less while the wrist was restrained. Importantly the performance time of the movements did not alter significantly for single-step, double-step or restrained movements. These results suggest that the nervous system exploits the intrinsic redundancy of the limb when controlling voluntary movements and is therefore more effective at reprogramming movements to double-step targets. Received: 24 March 1997 / Accepted: 7 July 1997     

The timing of color and location processing in the motor context

In this study, the use of color and location as stimulus attributes manipulated during a simple action was aimed at comparing how dorsal (location) and ventral (color) features are integrated in action and the timing of their processing. Eighteen subjects were presented with a green dot on a computer screen, which they were required to point at and touch. In 20% of the trials, the location or the color of the target was altered at the onset of movement to this stimulus, requiring the participant to modify the initially programmed response according to specific motor instructions. In the ’location-go’ group, the target changed in location and participants were instructed to reach the displaced stimulus by correcting their ongoing movement. In the ’location-stop’ and ’color-stop’ groups, subjects were instructed to interrupt their movement when the target changed location or color, respectively. Results showed that the latency of the first responses to the perturbation clearly depended on the stimulus attribute and not on the motor instruction tested: the response to color change was obtained about 80 ms later than both conditions involving location change. It is concluded that: (1) color processing is slower than location processing, and (2) the first reactions to the location change occur after the same delay irrespective of the response required from the subject. Received: 1 September 1997 / Accepted: 5 February 1998     

Multiple frames of reference for pointing to a remembered target

Pointing with an unseen hand to a visual target that disappears prior to movement requires maintaining a memory representation about the target location. The target location can be transformed either into a hand-centered frame of reference during target presentation and remembered under that form, or remembered in terms of retinal and extra-retinal cues and transformed into a body-centered frame of reference before movement initiation. The main goal of the present study was to investigate whether the target is stored in memory in an eye-centered frame, a hand-centered frame or in both frames of reference concomitantly. The task was to locate, memorize, and point to a target in a dark environment. Hand movement was not visible. During the recall delay, participants were asked to move their hand or their eyes in order to disrupt the memory representation of the target. Movement of the eyes during the recall delay was expected to disrupt an eye-centered memory representation whereas movement of the hand was expected to disrupt a hand-centered memory representation by increasing movement variability to the target. Variability of movement amplitude and direction was examined. Results showed that participants were more variable on the directional component of the movement when required to move their hand during recall delay. On the contrary, moving the eyes caused an increase in variability only in the amplitude component of the pointing movement. Taken together, these results suggest that the direction of the movement is coded and remembered in a frame of reference linked to the arm, whereas the amplitude of the movement is remembered in an eye-centered frame of reference.     

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     

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°, 26° and 52° on each side) were studied for five different head positions (0°, 45° and 80° 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. Finally, a decrease in the directional bias took place within blocks of trials. These results indicate that head position signals are used during the process of transforming motor coordinates from the visual to the pro-prioceptive system of reference.     

Horizontal vestibulo-ocular reflex and head stability in response to torso perturbations during visual search

 Eye, head, and torso movements were recorded using magnetic search coils while six normal human subjects made unconstrained eye and head movements as they searched for targets in a panoramic visual environment. Torso movements were imposed by pseudorandom rotations of a servomotor-driver chair in which subjects were seated; body motion was partially transmitted to the head as a perturbation. Horizontal vestibulo-ocular reflex (VOR) gain (eye velocity divided by head velocity) and head gain (head velocity divided by torso velocity) were determined. Measurements were performed with unaided vision and while subjects wore ×4 binocular telescopic spectacles. Since the head was free to move during the experiment, much of the perturbation delivered to the torso was compensated by head rotation on the neck. During the 50 ms immediately following chair rotation, the head corrected 98% of the torso motion. For the interval 50–80 ms after the perturbation 81–85% of the perturbation was corrected by head movement. The degree of head compensation did not significantly depend on magnification or type of visual target. The density distribution for VOR gain was calculated over the entire course of each trial and was found to be sharply centered between 0.9 and 1.0 for trials with unmagnified vision. The gain density distribution with ×4 telescopes was broader and centered around 1.5, reflecting visual enhancement. Gain of the VOR was also determined during four discrete epochs covering the period from 50 ms before to 130 ms after the onset of each imposed torso rotation. The first, second, and fourth epochs were 50 ms each, while the third epoch was 30 ms. The torso began to rotate in the second epoch (0–50 ms), and the onset of head rotation was in the third epoch (50–80 ms). Gains of the VOR determined during the first three epochs were in response to self-generated head rotation and were not significantly different from each other, averaging 1.0±0.4 (n=1604, mean±SD) with unaided vision and increased significantly (P<0.05) to 1.4±0.6 (n=2464) with telescopic spectacles. Gain of the VOR during the fourth (80–130 ms) epoch was in response to the imposed perturbation; this averaged 0.9±0.3 (n=1380) with unaided vision and increased significantly to 1.1±0.4 (n=2185) with telescopic spectacles. The wearing of telescopic spectacles thus induced an enhancement of VOR gain, which was dependent on the context of the associated head movement. The greater enhancement of VOR gain during self-generated head movement suggests that the large enhancement may be at least partially mediated by the motor program itself. However, the smaller, but still significant gain enhancement with telescopic spectacles observed during unpredictable, externally imposed head motion had a latency too short to be mediated by visual pursuit. We propose that the smaller gain enhancement during passive rotation is due to a small, context-dependent, parametric increase in the gain of canal or proprioceptive mediated eye movements. Received: 27 February 1998 / Accepted: 11 November 1998     

Rapid adaptation of torso pointing movements to perturbations of the base of support

We investigated whether pointing movements made with the torso would adapt to movement-contingent augmentation or attenuation of their spatial amplitude. The pointing task required subjects standing on a platform in the dark to orient the mid-sagittal plane of their torso to the remembered locations of just extinguished platform-fixed visual targets without moving their feet. Subjects alternated pointing at two chest-high targets, 60° apart, (1) in a baseline period with the stance platform stationary, (2) during exposure to concomitant contra or ipsiversive platform rotations that grew incrementally to 50% of the velocity of torso rotation, and (3) after return in one step to stationary platform conditions. The velocity and amplitude of torso movements relative to space decreased 25–50% during exposure to contraversive platform rotations and increased 20–50% during ipsiversive rotations. Torso rotation kinematics relative to the platform (as well as the platform-fixed targets and feet) remained virtually constant throughout the incremental exposure period. Subjects were unaware of the altered motion of their body in space imposed by the platform and did not perceive their motor adjustments. Upon return to stationary conditions, torso rotation movements were smaller and slower following adaptation to contraversive rotations and larger and faster after ipsiversive platform rotations. These results indicate a rapid sensory-motor recalibration to the altered relationship between spatial (inertial) torso motion and intended torso motion relative to the feet, and rapid re-adaptation to normal conditions. The adaptive system producing such robust torso regulation provides a critical basis for control of arm, head, and eye movements.     

Dorsal and ventral visual stream contributions to perception-action interactions during pointing

The Ebbinghaus illusion, in which a central circle surrounded by large circles appears to be smaller than a central circle surrounded by small circles, affects the speed of pointing movements. When the central circle appears to be big, pointing movements directed towards it are faster than when the central circle appears to be small. This effect could be due to an interaction between ventral stream processing associated with determining relative object size and dorsal stream processing associated with sensorimotor output. Alternatively, the dorsal stream alone could mediate the effect via the transformation of object shape representations into motor output within the parietal lobe. Finally, ventral stream processing could be integrated into motor output through projections to the prefrontal cortex and subsequently to the motor areas of the cortex, thus bypassing the dorsal stream. These three alternatives were tested by disrupting either the ventral or dorsal stream processing using transcranial magnetic stimulation (TMS) while subjects made pointing movements as quickly and accurately as possible to the central target circles within the Ebbinghaus illusion display. The relative changes in reaction time, movement speed, and movement accuracy for small versus large appearing target circles were compared when TMS was delivered over each site as well as at a control site (SMA). The results showed that TMS over either the dorsal or ventral stream but not the SMA reduced the influence of the illusion on the pointing movement speed but did not affect reaction time or movement accuracy. A second control experiment was completed in which TMS was delivered during pointing movements to target circles of physically different sizes that were not surrounded by either large or small circles. This allowed us to determined whether the effect we observed in the main experiment was due specifically to the relative size information contained within the illusory display and the effect this has on the preparation of pointing responses or to an influence on basic perceptual and sensorimotor processes occurring within the ventral and dorsal streams, respectively. The results showed that the affect on pointing movement speed was still present with dorsal but not ventral stream stimulation. Taken together, this evidence suggests that the ventral stream contributes to pointing movements based on relative object size information via its projections to the prefrontal areas and not necessarily through interactions with the dorsal stream.     

Effects of spatial attention on directional manual and ocular responses

 The aim of the present study was to investigate how spatial attention influences directional manual and saccadic reaction times. Two experiments were carried out. In experiment 1 subjects were instructed to perform pointing responses toward targets that were located either in the same or the opposite hemifield with respect to the hemifield in which an imperative stimulus was presented. In experiment 2, they were instructed to make saccadic or pointing responses. The direction of the responses was indicated by the shape of the imperative stimulus. Reaction time (RT), movement time, and, in experiment 2, saccadic trajectory were measured. The imperative stimulus location was either cued (endogenous attention) or uncued. In the latter case the imperative stimulus presentation attracted attention (exogenous attention). The main results of the experiments were the following: First, exogenous attention markedly decreased the RTs when the required movement was directed toward the imperative stimulus location. This directional effect was much stronger for pointing than for ocular responses. Second, endogenously allocated attention did not influence differentially RTs of pointing responses directed toward or away the attended hemifield. In contrast, endogenous attention markedly favored the saccadic responses when made away from the cued hemifield. Third, regardless of cueing, the direction of movement affected both pointing and saccadic reaction times. Saccadic reaction times were faster when the required movement was directed upward, while manual reaction times were faster when the movement was directed downward. Fourth, lateralized spatial attention deviated the trajectory of the saccades contralateral to the attention location. This pattern of results supports the notion that spatial attention depends on the activation of the same sensorimotor circuits that program actions in space. Received: 11 June 1996 / Accepted: 26 October 1996     

Effects of large-scale limb deafferentation on the morphological and physiological organization of the forepaw barrel subfield (FBS) in somatosensory cortex (SI) in adult and neonatal rats

The physiological representation of the shoulder and surrounding body was examined in layer IV of somatosensory cortex (SI) in rats that had underground removal of the forelimb, either as newborns on postnatal day three (PND-3) or as adults (at least 8 weeks of age). Electrophysiological recordings were used to map the shoulder and body representations (physiological map), and the mitochondria marker, cytochrome oxidase (CO), was used to visualize recording sites in barrel and barrel-like structures (morphological map) in layer IV of deafferents and intact controls. The SI shoulder representation lies in a nebulously stained region that lies posterior to the forearm, wrist, and forepaw representations; the latter region is associated with the well-defined forepaw barrel subfield (FBS). The major findings are: (1) the shoulder is represented as a single zone located at the posterior extent of the SI body map in intact rats; (2) limb deafferentation in adult or neonatal rats that were physiologically mapped 6–16 weeks post-amputation resulted in two or more islets of ”new” representation of the shoulder in the FBS in addition to the representation of the ”original” shoulder in the posterior part of the body map; (3) deafferentations made in neonatal rats, physiologically mapped as adults, had a significantly greater (Mann-Whitney U) amount of ”new” cortical representation within the FBS than did rats deafferented as adults; (4) fewer unresponsive sites in the FBS were found for neonate deafferents than for adult deafferents; (5) evoked response latencies following electrical stimulation of the shoulder were shortest for cortical sites within the ”original” shoulder representation in intact controls, and latencies recorded at the ”original” shoulder representation in deafferents were also shorter than latencies recorded in ”new” shoulder representations in both groups of deafferents; and (6) morphological maps of the FBS were altered in neonate deafferents to the extent that the barrel structure was poorly formed, as exemplified by the absence of the four mediolateral running bands; however, the overall ovoid shape of the FBS was still apparent, but not as sharply defined as for intact controls or adult deafferents. Possible mechanisms for reorganization following large-scale deafferentation are discussed. Received: 13 August 1998 / Accepted: 19 April 1999     

PET study of visually and non-visually guided finger movements in patients with severe pan-sensory neuropathies and healthy controls

Although patients with sensory neuropathies and normal muscle power are rare, they have been extensively studied because they are a model for dissociating the sensory and motor components of movement. We have examined these patients to determine the cerebral functional anatomy of movement in the absence of proprioceptive input. In addition, the disabling symptoms of these patients can be substantially improved by visually monitoring their movements. We hypothesized that, during visually guided movements, these patients would show overactivity of regions specialized for visuomotor control with the possible additional involvement of areas that normally process somatosensory information. We used positron emission tomography (PET) and the tracer H₂ ¹⁵O to determine the functional anatomy of visually and non-visually guided finger movements in three patients with long-standing pan-sensory neuropathies and normal muscle power and six healthy controls. Five conditions were performed with the right hand: a sequential finger movement task under visual guidance, the same motor task without observation of the hand, monitoring a video of the same sequential finger movement, a passive visual task observing a reversing checkerboard, and an unconstrained rest condition. Data were analyzed using conventional subtraction techniques with a statistical threshold of z>2.33 with corrections for multiple comparisons. When compared with the control group, activation was not deficient in any brain areas of the patient cohort in any of the contrasts tested. In particular, in the non-visually guided movement task, in which meaningful visual and proprioceptive input was absent, the patient group activated primary motor, premotor, and cerebellar regions. This suggests that these areas are involved in motor processing independent of sensory input. In all conditions involving visual observation of hand movements, there was highly significant overactivity of the left parietal operculum (SII) and right parieto-occipital cortex (PO) in the patient group. Recent non-human primate studies have suggested that the PO region contains a visual representation of hand movements. Overactivity of this area and the activation of SII by visual input appear to indicate that compensatory overactivity of visual areas and cross-modal plasticity of somatosensory areas occur in deafferented patients. These processes may underlie their ability to compensate for their proprioceptive deficits. Received: 18 May 1998 / Accepted: 18 January 1999     

Age-related differences in corrected and inhibited pointing movements

It has been widely reported that aging is accompanied by a decline in motor skill performance and in particular, it has been shown that older subjects take longer to adapt their ongoing reach in response to a target location shift. In the present experiment, we investigated the influence of aging on the ability to perform trajectory corrections in response to a target jump, but also assessed inhibition by asking a younger and an older group of participants to either adapt or stop their ongoing movement in response to a target location change. Results showed that although older subjects took longer to initiate, execute, correct and inhibit an ongoing reach, they performed both tasks with the same level of accuracy as the younger sample. Moreover, the slowing was also observed when older subjects were asked to point to stationary targets. Our findings thus indicate that aging does not specifically influence the ability to perform or inhibit fast online corrections to target location changes, but rather produces a general slowing and increased variability of movement planning, initiation and execution to both perturbed and stationary targets. For the first time, we demonstrate that aging is not accompanied by a decrease in the inhibition of motor control.

Online corrections can produce illusory bias during closed-loop pointing

This experiment examined whether the impact of pictorial illusions during the execution of goal-directed reaching movements is attributable to ocular motor signaling. We analyzed eye and hand movements directed toward both the vertex of the Müller–Lyer (ML) figure in a closed-loop procedure. Participants pointed to the right vertex of a visual stimulus in two conditions: a control condition wherein the figure (in-ML, neutral, out-ML) presented at response planning remained unchanged throughout the movement, and an experimental condition wherein a neutral figure presented at response planning was perturbed to an illusory figure (in-ML, out-ML) at movement onset. Consistent with previous work from our group (Heath et al. in Exp Brain Res 158:378–384, 2004; Heath et al. in J Mot Behav 37:179–185, 2005b), action-bias present in both conditions; thus illusory bias was introduced into during online control. Although primary saccades were influenced by illusory configurations (control conditions; see Binsted and Elliott in Hum Mov Sci 18:103–117, 1999a), illusory bias developed within the secondary “corrective” saccades during experimental trials (i.e., following a veridical primary saccade). These results support the position that a unitary spatial representation underlies both action and perception and this representation is common to both the manual and oculomotor systems.     

Pointing movements are affected by size-contrast illusions

 The influence that the perceived size of visual targets has on the characteristics of pointing movements was investigated in the present study. A size-contrast illusion, known as the Ebbinghaus or Tichener circles, was employed. In this illusion, a target circle surrounded by several smaller circles is perceived to be larger than a target circle of the same physical size surrounded by several larger circles. Movement times of open-loop pointing responses directed to the perceptually smaller target circle were significantly longer than the movement times of pointing responses directed to the perceptually larger target circle. The extent of this difference was similar to that observed when pointing responses were directed at physically different-sized target circles that were not surrounded by other circles. In addition, when the perceptually smaller circle was enlarged so that it appeared to be the same size as the perceptually larger circle, the movement times became equivalent. This evidence supports the contention that the relative rather than the absolute size of the target has a major impact on the control and execution of pointing movements. Such a conclusion contradicts those made previously concerning grasping movements made under similar conditions and implies that pointing responses are more directly influenced by visual perceptual processing than grasping responses. Received: 3 September 1998 / Accepted: 15 December 1998     

Postural adjustments accompanying fast pointing movements in standing, sitting and lying adults

The present study evaluated the effect of different positions, which varied in the amount of bodily support, on postural control during fast pointing movements. Fourteen adult subjects were studied in standing, various sitting and lying positions. Multiple surface electromyograms (EMGs) of arm, neck, trunk and upper leg muscles and kinematics were recorded during a standard series of unilateral arm movements. Two additional series, consisting of bilateral arm movements and unilateral arm movements with an additional weight, were performed to assess whether additional task-load affected postural adjustments differently in a sitting and standing position. Two pointing strategies were used – despite identical instructions. Seven subjects showed an elbow extension throughout the movements. They used the deltoid (DE) as the prime mover (DE group). The other seven subjects performed the movement with a slight elbow flexion and used the biceps brachii (BB) as the prime mover (BB group). The two strategies had a differential effect on the postural adjustments: postural activity was less and substantially later in the BB-group than in the DE group. Anticipatory postural muscle activity was only present in the DE group during stance. In all positions and task-load conditions the dorsal postural muscles were activated before their ventral antagonists. The activation rate, the timing and – to a lesser extent – the amplitude of the dorsal muscle activity was position dependent. The position dependency was mainly found in the caudally located lumbar extensor (LE) and hamstrings (HAM) muscles. The EMG amplitude of LE and HAM was also affected by body geometry (trunk and pelvis position). Position and body geometry had only a minor effect on the activity of the neck and thoracic extensor muscles. This difference in behaviour of lower and upper postural muscles suggests that they could serve different postural tasks: the lower muscles being more involved in keeping the centre of mass within the limits of the support surface, and the upper ones in counteracting the reaction forces generated by movement onset. Increasing task-load by performing bilateral movements and – to a minor extent – during loaded unilateral movements affected the temporal and quantitative characteristics of the postural adjustments during standing and sitting in a similar way. The effect was present mainly during the early part of the response (within 100 ms after prime mover onset). This suggests that feedforward or anticipatory mechanisms play a major role in the task-specific modulation of postural adjustments. Received: 9 April 1997 / Accepted: 9 October 1997     

Target viewing time and velocity effects on prehension

 The goal of the present study was to understand which characteristics (movement time or velocity) of target motion are important in the control and coordination of the transport and grasp-preshape components of prehensile movements during an interception task. Subjects were required to reach toward, grasp and lift an object as it entered a target area. Targets approached along a track at four velocities (500, 750, 1000 and 1250 mm/s) which were presented in two conditions. In the distance-controlled condition, targets moving at all velocities traveled the same distance. In the viewing-time-controlled condition, combinations of velocity and starting distances were performed such that the moving target was visible for 1000 ms for all trials. Analyses of kinematic data revealed that when, target distance was controlled, velocity affected all transport-dependent measures; however, when viewing time was controlled, these dependent measures were no longer affected by target velocity. Thus, the use of velocity information was limited in the viewing-time-controlled condition, and subjects used other information, such as target movement time, when generating the transport component of the prehensile movement. For the grasp-preshape component, both peak aperture and peak-aperture velocity increased as target velocity increased, regardless of condition, indicating that target velocity was used to control the spatial aspects of aperture formation. However, the timing of peak aperture was affected by target velocity in the distance-controlled condition, but not in the viewing-time-controlled condition. These results provide evidence for the autonomous generation of the spatial and temporal aspects of grasp preshape. Thus, an independence between the transport and grasp-preshape phases was found, whereby the use of target velocity as a source of information for generating the transport component was limited; however, target velocity was an important source of information in the grasp-preshape phase. Received: 16 March 1998 / Accepted: 2 February 1999     

A comparison of curvatures of left and right hand movements in a simple pointing task

Human arm movements towards visual targets are remarkably reproducible in several tasks and conditions. Various authors have reported that trajectories of unconstrained point-to-point movements are slightly curved, smooth and have bell-shaped velocity profiles. The hand paths of such movements show small - but significant – curvatures throughout the workspace. The cause of these curvatures is still obscure. Traditionally this curvature is explained as the result of an optimisation process or is ascribed to mechanical or dynamic properties of the effector system. Recently, however, it has been suggested that these curvatures are due at least partly, to the visual misperception of straight lines. To evaluate the latter hypothesis, we compared unconstrained, self-paced point-to-point movements that subjects made with their right and left hand. We assume that the visual misperception may depend on the position in the workspace, subject, etc. but not on the hand used to make the movement. Therefore we argue that if curvature is caused by a visual misperception of straight lines, curvatures should be the same for movements made with the left and right hand. Our experiments cast strong doubt on the hypothesis that curvatures are the result of a visual distortion, because curvatures of the left hand trajectories, mirrored in the mid-sagittal plane, are found to be accurately described by trajectories of the right hand. Estimates of the effect of visual distortion on movement curvature show that, if present, this effect is very small compared with other sources that contribute to movement curvature. We found that curvatures depend strongly on the subject and on the direction and distance of the movement. Curvatures do not seem to be caused purely by the dynamic properties of the arm, since curvatures do not change significantly with increasing movement velocity. Therefore, we conclude that curvatures reflect an inherent property of the control of multi-joint arm movements. Reveived: 29 October 1996 / Accepted: 1 October 1997