The effect of target modality on visual and proprioceptive contributions to the control of movement distance

The goal of this study was to determine whether the sensory nature of a target influences the roles of vision and proprioception in the planning of movement distance. Two groups of subjects made rapid, elbow extension movements, either toward a visual target or toward the index fingertip of the unseen opposite hand. Visual feedback of the reaching index fingertip was only available before movement onset. Using a virtual reality display, we randomly introduced a discrepancy between actual and virtual (cursor) fingertip location. When subjects reached toward the visual target, movement distance varied with changes in visual information about initial hand position. For the proprioceptive target, movement distance varied mostly with changes in proprioceptive information about initial position. The effect of target modality was already present at the time of peak acceleration, indicating that this effect include feedforward processes. Our results suggest that the relative contributions of vision and proprioception to motor planning can change, depending on the modality in which task relevant information is represented.     

Target and hand position information in the online control of goal-directed arm movements

The present study compared the contribution of visual information of hand and target position to the online control of goal-directed arm movements. Their respective contributions were assessed by examining how human subjects reacted to a change of the position of either their seen hand or the visual target near the onset of the reaching movement. Subjects, seated head-fixed in a dark room, were instructed to look at and reach with a pointer towards visual targets located in the fronto-parallel plane at different distances to the right of the starting position. LEDs mounted on the tip of the pointer were used to provide true or erroneous visual feedback about hand position. In some trials, either the target or the pointer LED that signalled the actual hand position was shifted 4.5 cm to the left or to the right during the ocular saccade towards the target. Because of saccadic suppression, subjects did not perceive these displacements, which occurred near arm movement onset. The results showed that modifications of arm movement amplitude appeared, on average, 150 ms earlier and reached a greater extent (mean difference=2.7 cm) when there was a change of target position than when a change of the seen hand position occurred. These findings highlight the weight of target position information to the online control of arm movements. Visual information relative to hand position may be less contributive because proprioception also provides information about limb position.     

A motor signal and “visual” size perception

Recent models of the visual system in primates suggest that the mechanisms underlying visual perception and visuomotor control are implemented in separate functional streams in the cerebral cortex. However, a little-studied perceptual illusion demonstrates that a motor-related signal representing arm position can contribute to the visual perception of size. The illusion consists of an illusory size change in an afterimage of the hand when the hand is moved towards or away from the subject. The motor signal necessary for the illusion could be specified by feedforward and/or feedback sources (i.e. efference copy and/or proprioception/kinesthesis). We investigated the nature of this signal by measuring the illusion's magnitude when subjects moved their own arm (active condition, feedforward and feedback information available), and when arm movement was under the control of the experimenter (passive condition, feedback information available). Active and passive movements produced equivalent illusory size changes in the afterimages. However, the illusion was not obtained when an afterimage of subject's hand was obtained prior to movement of the other hand from a very similar location in space. This evidence shows that proprioceptive/kinesthetic feedback was sufficient to drive the illusion and suggests that a specific three-dimensional registration of proprioceptive input and the initial afterimage is necessary for the illusion to occur.     

Differential contributions of vision and proprioception to movement accuracy

We examined the relative roles of visual and proprioceptive information about initial hand position on movement accuracy. A virtual reality environment was employed to dissociate visual information about hand position from the actual hand position. Previous studies examining the effects of such dissociations on perception of hand location have indicated a bias toward the visually displayed position. However, an earlier study, which employed optical prisms to dissociate visual and proprioceptive information prior to targeted movements, suggested a bias in movement direction toward that defined by the actual hand position. This implies that visual and proprioceptive information about hand position may be differentially employed for perceptual judgments and movement planning, respectively. We now employ a virtual reality environment to systematically manipulate the visual display of the hand start position from the actual hand position during movements made to a variety of directions. We asked whether subjects would adjust their movements in accord with the virtual or the actual hand location. Subjects performed a series of baseline movements toward one of three targets in each of three blocks of trials. Interspersed among these trials were "probe" trials in which the cursor location, but not the hand location, was displaced relative to the baseline start position. In all cases, cursor feedback was blanked at movement onset. Our findings indicated that subjects systematically adjusted the direction of movement in accord with the virtual, not the actual, start location of the hand. These findings support the hypothesis that visual information about hand position predominates in specifying movement direction.     

Kinematics of fast hemiparetic aiming movements toward stationary and moving targets

The aim of the present study was to gain insight into the control that hemiparetic subjects have over fast, unimanual aiming movements. Twelve hemiparetic subjects with cerebral palsy and twelve healthy subjects were asked to hit, as quickly as possible, stationary and moving targets projected onto a frontoparallel screen. The task was performed with the nonpreferred (spastic/nondominant) and preferred (nonspastic/dominant) arm. Although the pattern of kinematics of hemiparetic subjects generally corresponded with that reported in earlier reaching and grasping studies, the commonly observed prolonged movement time of the nonpreferred arm as compared to the preferred arm was absent. The spatial variability of the lateral hand displacements toward stationary targets was highest in the spastic arm of the hemiparetic subjects, indicating diminished motion stability. Even though hemiparetic subjects were expected to be unable to adjust their movements flexibly to the position and the velocity of a moving target, they used an initial estimate of where moving targets would be hit in the same way as the healthy subjects did, i.e., they started aiming toward a position in front of the target. In both subject groups, this spatial estimate and the movement time (MT) varied as a function of target velocity, suggesting that the use of target-velocity information in hitting moving targets is unaffected in spastic hemiparetic subjects. The results are related to possible deficits in the regulation of cocontraction underlying movement stability.     

Effects of object shape and visual feedback on hand configuration during grasping

Normal subjects gradually preshape their hands during a grasping movement in order to conform the hand to the shape of a target object. The evolution of hand preshaping may depend on visual feedback about arm and hand position as well as on target shape and location at specific times during the movement. The present study manipulated object shape in order to produce differentiable patterns of finger placement along two orthogonal "dimensions" (flexion/extension and abduction/adduction), and manipulated the amount of available visual information during a grasp. Normal subjects were asked to reach to and grasp a set of objects presented in a randomized fashion at a fixed spatial location in three visual feedback conditions: Full Vision (both hand and target visible), Object Vision (only the object was visible but not the hand) and No Vision (vision of neither the hand nor the object during the movement). Flexion/extension angles of the metacarpophalangeal and proximal interphalangeal joints of the index, ring, middle and pinkie fingers as well as the abduction/adduction angles between the index-middle and middle-ring fingers were recorded. Kinematic analysis revealed that as visual feedback was reduced, movement duration increased and time to peak aperture of the hand decreased, in accord with previously reported studies. Analysis of the patterns of joint flexion/extension and abduction/adduction per object shape revealed that preshaping based on the abduction/adduction dimension occurred early during the reach for all visual feedback conditions (~45% of normalized movement time). This early preshaping across visual feedback conditions suggests the existence of mechanisms involved in the selection of basic hand configurations. Furthermore, while configuration changes in the flexion/extension dimension resulting in well-defined hand configurations occurred earlier during the movement in the Object Vision and No Vision conditions (45%), those in the Full Vision condition were observed only after 75% of the movement, as the moving hand entered the central region of the visual field. The data indicate that there are at least two control mechanisms at work during hand preshaping, an early predictive phase during which grip selection is attained regardless of availability of visual feedback and a late responsive phase during which subjects may use visual feedback to optimize their grasp.     

The interaction of visual and proprioceptive inputs in pointing to actual and remembered targets in Parkinson's disease.

We previously reported that Parkinson's disease patients could point with their eyes closed as accurately as normal subjects to targets in three-dimensional space that were initially presented with full vision. We have now further restricted visual information in order to more closely examine the individual and combined influences of visual information, proprioceptive feedback, and spatial working memory on the accuracy of Parkinson's disease patients. All trials were performed in the dark. A robot arm presented a target illuminated by a light-emitting diode at one of five randomly selected points composing a pyramidal array. Subjects attempted to "touch" the target location with their right finger in one smooth movement in three conditions: dark, no illumination of arm or target during movement; movement was to the remembered target location after the robot arm retracted; finger, a light-emitting diode on the pointing fingertip was visible during the movement but the target was extinguished; again, movement was to the remembered target location; and target, the target light-emitting diode remained in place and visible throughout the trial but there was no vision of the arm. In the finger condition, there is no need to use visual-proprioceptive integration, since the continuously visualized fingertip position can be compared to the remembered location of the visual target. In the target condition, the subject must integrate the current visible target with arm proprioception, while in the dark condition, the subject must integrate current proprioception from the arm with the remembered visual target. Parkinson's disease patients were significantly less accurate than controls in both the dark and target conditions, but as accurate as controls in the finger condition. Parkinson's disease patients, therefore, were selectively impaired in those conditions (target and dark) which required integration of visual and proprioceptive information in order to achieve accurate movements. In contrast, the patients' normal accuracy in the finger condition indicates that they had no substantial deficits in their relevant spatial working memory. Final arm configurations were significantly different in the two subject groups in all three conditions, even in the finger condition where mean movement endpoints were not significantly different. Variability of the movement endpoints was uniformly increased in Parkinson's disease patients across all three conditions.The current study supports an important role for the basal ganglia in the integration of proprioceptive signals with concurrent or remembered visual information that is needed to guide movements. This role can explain much of the patients' dependence on visual information for accuracy in targeted movements. It also underlines what may be an essential contribution of the basal ganglia to movement, the integration of afferent information that is initially processed through multiple, discrete modality-specific pathways, but which must be combined into a unified and continuously updated spatial model for effective, accurate movement.     

Clonidine as adjuvant for oxybuprocaine, bupivacaine or dextrorphan has a significant peripheral action in intensifying and prolonging analgesia in response to local dorsal cutaneous noxious pinprick in rats

The roles of visual and somatosensory information in arm movement planning remain enigmatic. Previous studies have examined these roles by dissociating visual and somatosensory cues about limb position prior to movement onset and examining the resulting effects on movements performed in the horizontal plane. Here we examined the effects of misaligned limb position cues prior to movement onset as reaches were planned and executed along different directions in the vertical plane. Movements were planned with somatosensory and visual feedback aligned at the starting position of the reach or with visual feedback displaced horizontally (Experiment 1) or vertically (Experiment 2). As in the horizontal plane, changes in movement directions induced by misaligned feedback indicated that vision and proprioception were both generally taken into account when planning vertical plane movements. However, we also found evidence that the contributions of vision and proprioception differed across target directions and between directions of displaced visual feedback. These findings suggest that the contributions of vision and proprioception to movement planning in the vertical plane reflect the unique multisensory and biomechanical demands associated with moving against gravity.     

Limb position drift: implications for control of posture and movement

In the absence of visual feedback, subject reports of hand location tend to drift over time. Such drift has been attributed to a gradual reduction in the usefulness of proprioception to signal limb position. If this account is correct, drift should degrade the accuracy of movement distance and direction over a series of movements made without visual feedback. To test this hypothesis, we asked participants to perform six series of 75 repetitive movements from a visible start location to a visible target, in time with a regular, audible tone. Fingertip position feedback was given by a cursor during the first five trials in the series. Feedback was then removed, and participants were to continue on pace for the next 70 trials. Movements were made in two directions (30 degrees and 120 degrees ) from each of three start locations (initial shoulder angles of 30 degrees, 40 degrees, 50 degrees, and initial elbow angles of 90 degrees ). Over the 70 trials, the start location of each movement drifted, on average, 8 cm away from the initial start location. This drift varied systematically with movement direction, indicating that drift is related to movement production. However, despite these dramatic changes in hand position and joint configuration, movement distance and direction remained relatively constant. Inverse dynamics analysis revealed that movement preservation was accompanied by substantial modification of joint muscle torque. These results suggest that proprioception continues to be a reliable source of limb position information after prolonged time without vision, but that this information is used differently for maintaining limb position and for specifying movement trajectory.     

Integration of proprioceptive and visual position-information: An experimentally supported model

To localize one's hand, i.e., to find out its position with respect to the body, humans may use proprioceptive information or visual information or both. It is still not known how the CNS combines simultaneous proprioceptive and visual information. In this study, we investigate in what position in a horizontal plane a hand is localized on the basis of simultaneous proprioceptive and visual information and compare this to the positions in which it is localized on the basis of proprioception only and vision only. Seated at a table, subjects matched target positions on the table top with their unseen left hand under the table. The experiment consisted of three series. In each of these series, the target positions were presented in three conditions: by vision only, by proprioception only, or by both vision and proprioception. In one of the three series, the visual information was veridical. In the other two, it was modified by prisms that displaced the visual field to the left and to the right, respectively. The results show that the mean of the positions indicated in the condition with both vision and proprioception generally lies off the straight line through the means of the other two conditions. In most cases the mean lies on the side predicted by a model describing the integration of multisensory information. According to this model, the visual information and the proprioceptive information are weighted with direction-dependent weights, the weights being related to the direction-dependent precision of the information in such a way that the available information is used very efficiently. Because the proposed model also can explain the unexpectedly small sizes of the variable errors in the localization of a seen hand that were reported earlier, there is strong evidence to support this model. The results imply that the CNS has knowledge about the direction-dependent precision of the proprioceptive and visual information.     

Differential influence of vision and proprioception on control of movement distance

The purpose of this study was to investigate the contribution of proprioceptive and visual information about initial limb position in controlling the distance of rapid, single-joint reaching movements. Using a virtual reality environment, we systematically changed the relationship between actual and visually displayed hand position as subjects’ positioned a cursor within a start circle. No visual feedback was given during the movement. Subjects reached two visual targets (115 and 125° elbow angle) from four start locations (90, 95, 100, and 105° elbow angle) under four mismatch conditions (0, 5, 10, or 15°). A 2×4×4 ANOVA enabled us to ask whether the subjects controlled the movement distance in accord with the virtual, or the actual hand location. Our results indicate that the movement distance was mainly controlled according to the virtual start location. Whereas distance modification was most extensive for the closer target, analysis of acceleration profiles revealed that, regardless of target position, visual information about start location determined the initial peak in tangential hand acceleration. Peak acceleration scaled with peak velocity and movement distance, a phenomenon termed “pulse-height” control. In contrast, proprioceptive information about actual hand location determined the duration of acceleration, which also scaled with peak velocity and movement distance, a phenomenon termed “pulse-width” control. Because pulse-height and pulse-width mechanisms reflect movement planning and sensory-based corrective processes, respectively, our current findings indicate that vision is used primarily for planning movement distance, while proprioception is used primarily for online corrections during rapid, unseen movements toward visual targets.     

Arm–trunk coordination in the absence of proprioception

During trunk-assisted reaching to targets placed within arms length, the influence of trunk motion on the hand trajectory is compensated for by changes in the arm configuration. The role of proprioception in this compensation was investigated by analyzing the movements of 2 deafferented and 12 healthy subjects. Subjects reached to remembered targets (placed ~80° ipsilateral or ~45° contralateral to the sagittal midline) with an active forward movement of the trunk produced by hip flexion. In 40% of randomly selected trials, trunk motion was mechanically blocked. No visual feedback was provided during the experiment. The hand trajectory and velocity profiles of healthy subjects remained invariant whether or not the trunk was blocked. The invariance was achieved by changes in arm interjoint coordination that, for reaches toward the ipsilateral target, started as early as 50 ms after the perturbation. Both deafferented subjects exhibited considerable, though incomplete, compensation for the effects of the perturbation. Compensation was more successful for reaches to the ipsilateral target. Both deafferented subjects showed invariance between conditions (unobstructed or blocked trunk motion) in their hand paths to the ipsilateral target, and one did to the contralateral target. For the other deafferented subject, hand paths in the two types of trials began to deviate after about 50% into the movement, because of excessive elbow extension. In movements to the ipsilateral target, when deafferented subjects compensated successfully, the changes in arm joint angles were initiated as early as 50 ms after the trunk perturbation, similar to healthy subjects. Although the deafferented subjects showed less than ideal compensatory control, they compensated to a remarkably large extent given their complete loss of proprioception. The presence of partial compensation in the absence of vision and proprioception points to the likelihood that not only proprioception but also vestibulospinal pathways help mediate this compensation.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Online control of the direction of rapid reaching movements

Online visual control of the direction of rapid reaching movements was assessed by evaluating how human subjects reacted to shifts in seen hand position near movement onsets. Participants (N=10) produced saccadic eye and rapid arm movements (mean duration = 328 ms) towards a peripheral visual target in complete darkness. During the saccade, visual feedback of hand position could be shifted by 1, 2, 3 or 4 cm perpendicularly to the main movement direction. The resulting discrepancies between visual and proprioceptive information about hand position were never consciously perceived by the subjects. Following the shifts, hand trajectories deviated from those produced in a control condition (without shift) in order to bring seen hand position closer to the target. Globally, the deviations corresponded to 45% of the shifts, regardless of their magnitude or movement duration. This finding highlights not only the efficiency of visual feedback processing in online motor control but also underlines the significant contribution of limb proprioception.     

Viewing the hand prior to movement improves accuracy of pointing performed toward the unseen contralateral hand

 It is now well established that the accuracy of pointing movements to visual targets is worse in the full open loop condition (FOL; the hand is never visible) than in the static closed loop condition (SCL; the hand is only visible in static position prior to movement onset). In order to account for this result, it is generally admitted that viewing the hand in static position (SCL) improves the movement planning process by allowing a better encoding of the initial state of the motor apparatus. Interestingly, this wide-spread interpretation has recently been challenged by several studies suggesting that the effect of viewing the upper limb at rest might be explained in terms of the simultaneous vision of the hand and target. This result is supported by recent studies showing that goal-directed movements involve different types of planning (egocentric versus allocentric) depending on whether the hand and target are seen simultaneously or not before movement onset. The main aim of the present study was to test whether or not the accuracy improvement observed when the hand is visible before movement onset is related, at least partially, to a better encoding of the initial state of the upper limb. To address this question, we studied experimental conditions in which subjects were instructed to point with their right index finger toward their unseen left index finger. In that situation (proprioceptive pointing), the hand and target are never visible simultaneously and an improvement of movement accuracy in SCL, with respect to FOL, may only be explained by a better encoding of the initial state of the moving limb when vision is present. The results of this experiment showed that both the systematic and the variable errors were significantly lower in the SCL than in the FOL condition. This suggests: (1) that the effect of viewing the static hand prior to motion does not only depend on the simultaneous vision of the goal and the effector during movement planning; (2) that knowledge of the initial upper limb configuration or position is necessary to accurately plan goal-directed movements; (3) that static proprioceptive receptors are partially ineffective in providing an accurate estimate of the limb posture, and/or hand location relative to the body; and (4) that static visual information significantly improves the representation provided by the static proprioceptive channel. Received: 23 July 1996 / Accepted: 13 December 1996     

Proprioceptive guidance of saccades in eye-hand coordination

The saccade generator updates memorized target representations for saccades during eye and head movements. Here, we tested if proprioceptive feedback from the arm can also update handheld object locations for saccades, and what intrinsic coordinate system(s) is used in this transformation. We measured radial saccades beginning from a central light-emitting diode to 16 target locations arranged peripherally in eight directions and two eccentricities on a horizontal plane in front of subjects. Target locations were either indicated 1) by a visual flash, 2) by the subject actively moving the handheld central target to a peripheral location, 3) by the experimenter passively moving the subject's hand, or 4) through a combination of the above proprioceptive and visual stimuli. Saccade direction was relatively accurate, but subjects showed task-dependent systematic overshoots and variable errors in radial amplitude. Visually guided saccades showed the smallest overshoot, followed by saccades guided by both vision and proprioception, whereas proprioceptively guided saccades showed the largest overshoot. In most tasks, the overall distribution of saccade endpoints was shifted and expanded in a gaze- or head-centered cardinal coordinate system. However, the active proprioception task produced a tilted pattern of errors, apparently weighted toward a limb-centered coordinate system. This suggests the saccade generator receives an efference copy of the arm movement command but fails to compensate for the arm's inertia-related directional anisotropy. Thus the saccade system is able to transform hand-centered somatosensory signals into oculomotor coordinates and combine somatosensory signals with visual inputs, but it seems to have a poorly calibrated internal model of limb properties.     

Action and attentional load can influence aperture effects on motion perception

Goal-directed movements performed in a virtual environment pose serious challenges to the central nervous system because the visual and proprioceptive representations of one’s hand position are not perfectly congruent. The aim of the present study was to determine whether the vision of one’s hand or upper arm, compared with that of a cursor representing the tips of one’s index finger and thumb, optimizes the planning and modulation of one’s movement as the cursor nears the target. The participants performed manual aiming movements that differed by the source of static visual information available during movement planning and the source of dynamic information available during movement execution. The results revealed that the vision of one’s hand during the movement planning phase results in more efficient online control processes than when the movement planning was based on a virtual representation of one’s initial hand location. This observation was seen regardless of the availability of online visual feedback during movement execution. These results suggest that a more reliable estimation of the initial hand position results in more accurate estimation of the position of the cursor/hand at any one time resulting in more accurate online control.     

Pointing to oneself: active versus passive proprioception revisited and implications for internal models of motor system function

We re-examined the issue of active versus passive proprioception to more fully characterize the accuracy afforded by proprioceptive information in natural, unconstrained, movements in 3-dimensions. Subjects made pointing movements with their non-dominant arm to various locations with eyes closed. They then proprioceptively localized the tip of its index finger with a prompt pointing movement of their dominant arm, thereby bringing the two indices in apposition. Subjects performed this task with remarkable accuracy. More remarkably, the same subjects were equally accurate at localizing the index finger when the arm was passively moved and maintained in its final position by an experimenter. Two subjects were also tested with eyes open, and they were no more accurate than with eyes closed. We also found that the magnitude of the error did not depend on movement duration, which is contrary to a key observation in support of the existence of an internal forward model-based state-reconstruction scheme. Three principal conclusions derive from this study. First, in unconstrained movements, proprioceptive information provides highly accurate estimates of limb position. Second, so-called active proprioception does not provide better estimates of limb position than passive proprioception. Lastly, in the active movement condition, an internal model-based estimation of limb position should, according to that hypothesis, have occurred throughout the movement. If so, it did not lead to a better estimate of final limb position, or lower variance of the estimate, casting doubt on the necessity to invoke this hypothetical construct.     

The effect of removing visual information on reach control in young children

Visual information about the hand, the reach space, and a target can all contribute to the control of a reaching movement. When visual information is removed, both feedforward mechanisms (involved in planning the movement) and feedback mechanisms (involved in correcting errors) may be affected. This study looks at how 4- to 5-year-old children use visual information to guide reaching movements. Children reached for a toy object in four conditions—in the light, in the dark while the toy was glowing, and in complete darkness after a 0-s delay and a 4-s delay. When a reach in the glowing condition was compared with a reach in the light, reaches were more curved, had a longer duration, and earlier time-to-peak-velocity than a reach in the light but the number of grasping responses were comparable to in the light condition. When a reach in the two dark conditions (0- and 4-s) was compared with a reach in the light, the number of grasping responses decreased and 14 and 31 % of reaches resulted in a miss, that is, no contact was made with the object. While we did not find any significant kinematic differences between the 0- and 4-s dark conditions, there was a significantly larger number of misses in the 4-s dark condition, suggesting that memory of target position may decay over time. Overall, removing vision of the hand and reach space in the glowing condition appears to affect the planning of a reach (as vision of the hand was not available at reach initiation) and feedback control, while removing vision of the object in the dark conditions has an effect on endpoint response as we found that children experience difficulty retrieving the object in the dark. While young children demonstrate more adult-like reach control (i.e., relatively longer deceleration time, increased reach duration) under reduced feedback conditions, they have difficulty retrieving the object in the dark, particularly after a 4-s delay, and it is possible that mechanisms guiding predictive control and visual memory are still developing.     

Spatially valid proprioceptive cues improve the detection of a visual stimulus

Vision and proprioception are the main sensory modalities that convey hand location and direction of movement. Fusion of these sensory signals into a single robust percept is now well documented. However, it is not known whether these modalities also interact in the spatial allocation of attention, which has been demonstrated for other modality pairings. The aim of this study was to test whether proprioceptive signals can spatially cue a visual target to improve its detection. Participants were instructed to use a planar manipulandum in a forward reaching action and determine during this movement whether a near-threshold visual target appeared at either of two lateral positions. The target presentation was followed by a masking stimulus, which made its possible location unambiguous, but not its presence. Proprioceptive cues were given by applying a brief lateral force to the participant’s arm, either in the same direction (validly cued) or in the opposite direction (invalidly cued) to the on-screen location of the mask. The d′ detection rate of the target increased when the direction of proprioceptive stimulus was compatible with the location of the visual target compared to when it was incompatible. These results suggest that proprioception influences the allocation of attention in visual space.     

Basal ganglia network mediates the control of movement amplitude

In the present study we address the hypothesis that the basal ganglia are specifically involved in the planning of movement amplitude (or related covariates). This prediction has often been put forward based on the observation that Parkinson's disease (PD) patients exhibit hypokinesia. A close examination of the literature shows, however, that this commonly reported clinical symptom is not consistently echoed by experimental observations. When required to point to visual targets in the absence of vision of the moving limb, PD subjects exhibit various patterns of inaccuracy, including hypometria, hypermetria, systematic direction bias, or direction-dependent errors. They have even been shown to be as accurate as healthy, age-matched subjects. The main aim of the current study is to address the origin of these inconsistencies. To this end, we required nine patients presenting with advanced PD and 15 age-matched control subjects to perform planar reaching movements to visual targets. Eight targets were presented in equally spaced directions around a circle centered on the hand's starting location. Based on a previously validated parsing procedure, end-point errors were segmented into localization and planning errors. Localization errors refer to the existence of systematic biases in the estimation of the initial hand location. These biases can potentially transform a simple pattern of pure amplitude errors into a complex pattern involving both amplitude and direction errors. Results indicated that localization errors were different in the PD patients and the control subjects. This is not surprising knowing both that proprioception is altered in PD patients and that the ability to locate the hand at rest relies mainly on the proprioceptive sense, even when vision is available. Unlike normal subjects, localization errors in PD were idiosyncratic, lacking a consistent pattern across subjects. When the confounding effect of initial hand localization errors was canceled, we found that end-point errors were only due to the implementation of an underscaled movement gain (15%), without direction bias. Interestingly, the level of undershoot was found to increase with the severity of the disease (inferred from the Unified Parkinson's Disease Rating Scale, UPDRS, motor score). We also observed that movement variability was amplified (32%), but only along the main movement axis (extent variability). Direction variability was not significantly different in the patient population and the control group. When considered together, these results support the idea that the basal ganglia are specifically involved in the control of movement amplitude (or of some covariates). We propose that this structure participates in extent planning by modulating cortical activity and/or the tuning of the spinal interneuronal circuits.