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Visual feedback normally helps guide movements to their goal. When moving one’s hand, such guidance has to deal with a sensorimotor delay of about 100 ms. When moving a cursor, it also has to deal with a delay of tens of milliseconds that arises between the hand moving the mouse and the cursor moving on the screen. Moreover, the cursor is presented at a certain rate, so only positions corresponding with the position of the mouse at certain moments are presented. How does the additional delay and the rate at which cursor positions are updated influence how well the cursor can be guided to the goal? We asked participants to move a cursor to consecutive targets as quickly as they could. They did so for various additional delays and presentation rates. It took longer for the mouse to reach the target when the additional delay was longer. It also took longer when a lower presentation rate was achieved by not presenting the cursor all the time. The fraction of the time during which the cursor was present was more important than the rate at which the cursor’s position was updated. We conclude that the way human arm movements are guided benefits from continuous access to recent visual feedback.

Reaching movements are guided by estimates of the target object’s location. Since the precision of instantaneous estimates is limited, one might accumulate visual information over time. However, if the object is not stationary, accumulating information can bias the estimate. How do people deal with this trade-off between improving precision and reducing the bias? To find out, we asked participants to tap on targets. The targets were stationary or moving, with jitter added to their positions. By analysing the response to the jitter, we show that people continuously use the latest available information about the target’s position. When the target is moving, they combine this instantaneous target position with an extrapolation based on the target’s average velocity during the last several hundred milliseconds. This strategy leads to a bias if the target’s velocity changes systematically. Having people tap on accelerating targets showed that the bias that results from ignoring systematic changes in velocity is removed by compensating for endpoint errors if such errors are consistent across trials. We conclude that combining simple continuous updating of visual information with the low-pass filter characteristics of muscles, and adjusting movements to compensate for errors made in previous trials, leads to the precise and accurate human goal-directed movements.

Various studies have demonstrated a role for cognition on self-motion perception. Those studies all concerned modulations of the perception of a physical or visual motion stimulus. In our study, however, we investigated whether cognitive cues could elicit a percept of oscillatory self-motion in the absence of sensory motion. If so, we could use this percept to investigate if the resulting mismatch between estimated self-motion and a lack of corresponding sensory signals is motion sickening. To that end, we seated blindfolded participants on a swing that remained motionless during two conditions, apart from a deliberate perturbation at the start of each condition. The conditions only differed regarding instructions, a secondary task and a demonstration, which suggested either a quick halt (“Distraction”) or continuing oscillations of the swing (“Focus”). Participants reported that the swing oscillated with larger peak-to-peak displacements and for a longer period of time in the Focus condition. That increase was not reflected in the reported motion sickness scores, which did not differ between the two conditions. As the reported motion was rather small, the lack of an effect on the motion sickness response can be explained by assuming a subthreshold neural conflict. Our results support the existence of internal models relevant to sensorimotor processing and the potential of cognitive (behavioral) therapies to alleviate undesirable perceptual issues to some extent. We conclude that oscillatory self-motion can be perceived in the absence of related sensory stimulation, which advocates for the acknowledgement of cognitive cues in studies on self-motion perception.

When asked to move their unseen hand-to-visual targets, people exhibit idiosyncratic but reliable visuo-proprioceptive matching errors. Unsurprisingly, vision and proprioception quickly align when these errors are made apparent by providing visual feedback of the position of the hand. However, retention of this learning is limited, such that the original matching errors soon reappear when visual feedback is removed. Several recent motor learning studies have shown that reward feedback can improve retention relative to error feedback. Here, using a visuo-proprioceptive position-matching task, we examined whether binary reward feedback can be effectively exploited to reduce matching errors and, if so, whether this learning leads to improved retention relative to learning based on error feedback. The results show that participants were able to adjust the visuo-proprioceptive mapping with reward feedback, but that the level of retention was similar to that observed when the adjustment was accomplished with error feedback. Therefore, similar to error feedback, reward feedback allows for temporary recalibration, but does not support long-lasting retention of this recalibration.

There is an extensive literature debating whether perceived size is used to guide grasping. A possible reason for not using judged size is that using judged positions might lead to more precise movements. As this argument does not hold for small objects, and all studies showing an effect of the Ebbinghaus illusion on grasping used small objects, we hypothesized that size information is used for small objects but not for large ones. Using a modified diagonal illusion, we obtained an effect of about 10% on perceptual judgements, without an effect on grasping, irrespective of object size. We therefore reject our precision hypothesis. We discuss the results in the framework of grasping as moving digits to positions on an object. We conclude that the reported disagreement on the effect of illusions is because the Ebbinghaus illusion not only affects size, but –unlike most size illusions– also affects perceived positions.

The increased reliance on electronic devices such as smartphones in our everyday life exposes us to various delays between our actions and their consequences. Whereas it is known that people can adapt to such delays, the mechanisms underlying such adaptation remain unclear. To better understand these mechanisms, the current study explored the role of eye movements in interception with delayed visual feedback. In two experiments, eye movements were recorded as participants tried to intercept a moving target with their unseen finger while receiving delayed visual feedback about their own movement. In Experiment 1, the target randomly moved in one of two different directions at one of two different velocities. The delay between the participant’s finger movement and movement of the cursor that provided feedback about the finger movements was gradually increased. Despite the delay, participants followed the target with their gaze. They were quite successful at hitting the target with the cursor. Thus, they moved their finger to a position that was ahead of where they were looking. Removing the feedback showed that participants had adapted to the delay. In Experiment 2, the target always moved in the same direction and at the same velocity, while the cursor’s delay varied across trials. Participants still always directed their gaze at the target. They adjusted their movement to the delay on each trial, often succeeding to intercept the target with the cursor. Since their gaze was always directed at the target, and they could not know the delay until the cursor started moving, participants must have been using peripheral vision of the delayed cursor to guide it to the target. Thus, people deal with delays by directing their gaze at the target and using both experience from previous trials (Experiment 1) and peripheral visual information (Experiment 2) to guide their finger in a way that will make the cursor hit the target.

When reaching towards an object while standing, one's hand responds very quickly to visual perturbations such as the target being displaced or the background moving. Such responses require postural adjustments. When the background moves, its motion might be attributed to self-motion in a stable world, and thereby induce compensatory postural adjustments that affect the hand. The changes in posture associated with a given hand movement response may, therefore, be different for the two types of perturbations. To see whether they are, we asked standing participants to move their hand in the sagittal direction away from their body to targets displayed on a horizontal screen in front of them. The target displacements and background motion were in the lateral direction. We found hand movement responses that were in line with earlier reports, with a latency that was slightly shorter for target displacements than for background motion, and that was independent of target displacement size or background motion speed. The trunk responded to both perturbations with a modest lateral sway. The two main findings were that the upper trunk responded even before the hand did so and that the head responded to background motion but hardly responded to target displacements. These findings suggest that postural adjustments associated with adjusting the hand movement precede the actual adjustments to the movement of the hand, while at the same time, participants try to keep their head stable on the basis of visual information. ispartof: EXPERIMENTAL BRAIN RESEARCH vol:236 issue:6 pages:1573-1581 ispartof: location:Germany status: published

Our interaction with objects is facilitated by the availability of visual feedback. Here, we investigate how and when visual feedback affects the way we grasp an object. Based on the main views on grasping (reach-and-grasp and double-pointing views), we designed four experiments to test: (1) whether the availability of visual feedback influences the digits independently, and (2) whether the absence of visual feedback affects the initial part of the movement. Our results show that occluding (part of) the hand’s movement path influences the movement trajectory from the beginning. Thus, people consider the available feedback when planning their movements. The influence of the visual feedback depends on which digit is occluded, but its effect is not restricted to the occluded digit. Our findings indicate that the control mechanisms are more complex than those suggested by current views on grasping.

Visually induced motion sickness (VIMS) is a well-known sensation in virtual environments and simulators, typically characterized by a variety of symptoms such as pallor, sweating, dizziness, fatigue, and/or nausea. Numerous methods to reduce VIMS have been previously introduced; however, a reliable countermeasure is still missing. In the present study, the effect of airflow and seat vibration to alleviate VIMS was investigated. Eighty-two participants were randomly assigned to one of four groups (airflow, vibration, combined airflow and vibration, and control) and then exposed to a 15 min long video of a bicycle ride shot from first-person view. VIMS was measured using the Fast Motion Sickness Scale (FMS) and the Simulator Sickness Questionnaire (SSQ). Results showed that the exposure of airflow significantly reduced VIMS, whereas the presence of seat vibration, in contrast, did not have an impact on VIMS. Additionally, we found that females reported higher FMS scores than males, however, this sex difference was not found in the SSQ scores. Our findings demonstrate that airflow can be an effective and easy-to-apply technique to reduce VIMS in virtual environments and simulators, while vibration applied to the seat is not a successful method.

Two different ways to code a goal-directed movement have been proposed in the literature: vector coding and position coding. Assuming that the code is fine-tuned if a movement is immediately repeated, one can predict that repeating movements to the same endpoint will increase precision if movements are coded in terms of the position of the endpoint. Repeating the same movement vector at slightly different positions will increase precision if movements are coded in terms of vectors. Following this reasoning, Hudson and Landy (J Neurophys 108(10):2708–2716, 2012) found evidence for both types of coding when participants moved their hand over a table while the target and feedback were provided on a vertical screen. Do we also see evidence for both types of coding if participants repeat movements within a more natural visuo-motor mapping? To find out, we repeated the study of Hudson and Landy (J Neurophys 108(10):2708–2716, 2012), but our participants made movements directly to the targets. We compared the same movements embedded in blocks of repetitions of endpoints and blocks of repetitions of movement vectors. Within blocks, the movements were presented in a random order. We found no benefit of repeating either a position or a vector. We subsequently repeated the experiment with a similar mapping between movements and images to those used by Hudson and Landy and found that participants only clearly benefit from repeating a position. We conclude that repeating a position is particularly useful when dealing with unusual visuo-motor mappings.

When picking up objects using a pinch grip, there are usually numerous places at which one could place the thumb and index finger. Yet, people seem to consistently place them at or close to the centre of mass (COM), presumably to minimize torque and therefore the required grip force. People also prefer to grasp objects by parallel surfaces and ones with higher friction coefficients (rough surfaces), to prevent the object from slipping when they lift it. Here, we examine the trade-off between friction and COM. Participants were asked to grasp and lift aluminium bars of which one end was polished and therefore smooth and the other was rough. Their finger positions were recorded to determine how they grasped the objects. The bars were oriented horizontally in the frontal plane, with the centre aligned with the participants’ body midline. The bars varied in the horizontal offset between the COM and the edge of the rough region. The offset could be 0, 1 or 2 cm. We expected participants to grasp closer to the rough area than the centre of the bar. Completely rough bars and completely smooth bars served as control conditions. The slipperiness of the surface that was grasped affected the height of the grasping points, indicating that participants adjusted their grasping behaviour to the slipperiness of the surface. However, the tendency to grasp closer to the rough area was minimal. This shows that the judged COM largely determines how an object is grasped. Friction has very limited influence.

If the surrounding of a visual target unexpectedly starts to move during a fast goal-directed hand movement, the hand reflexively moves along with it. This is known as the 'manual following response'. One explanation for this response is that it is a compensation for inferred self-motion in space. Previous studies have shown that background motion gives rise to both postural responses and deviations in goal-directed hand movements. To evaluate whether compensation for inferred self-motion is responsible for the manual responses we examined whether galvanic stimulation of the vestibular system would give rise to similar deviations in hand movements. Standing participants tried to quickly tap on targets that were presented on a horizontal screen. Participants could infer self-motion on some of the trials, either from galvanic vestibular stimulation or from background motion. Both perturbations took place during the hand movement. It took both the head and hand about 45 ms longer to respond to background motion than to respond to galvanic stimulation. The head responded in a similar manner to both types of perturbations. The hand responded about as expected to galvanic stimulation, but much more vigorously to background motion. Thus, the manual response to background motion is probably not a direct consequence of trying to compensate for inferred self-motion. Perhaps the manual following response is a consequence of an error in binding motion information to objects. ispartof: EXPERIMENTAL BRAIN RESEARCH vol:237 issue:10 pages:2549-2558 ispartof: location:Germany status: published

Visual judgment of small numerosities (

It is known that our senses are influenced by contrast effects and aftereffects. For haptic perception, the curvature aftereffect has been studied in depth but little is known about curvature contrast. In this study we let observers explore two shapes simultaneously. The shape felt by the index finger could either be flat or convexly curved. The curvature at the thumb was varied to quantify the curvature of a subjectively flat shape. We found that when the index finger was presented with a convex shape, a flat shape at the thumb was also perceived to be convex. The effect is rather strong, on average 20% of the contrasting curvature. The contrast effect was present for both raised line stimuli and solid shapes. Movement measurements revealed that the curvature of the path taken by the metacarpus (part of the hand that connects the fingers) was approximately the average of the path curvatures taken by the thumb and index finger. A failure to correct for the movement of the hand could explain the contrast effect.

We studied the influence of haptics on visual perception of three-dimensional shape. Observers were shown pictures of an oblate spheroid in two different orientations. A gauge-figure task was used to measure their perception of the global shape. In the first two sessions only vision was used. The results showed that observers made large errors and interpreted the oblate spheroid as a sphere. They also misinterpreted the rotated oblate spheroid for a prolate spheroid. In two subsequent sessions observers were allowed to touch the stimulus while performing the task. The visual input remained unchanged: the observers were looking at the picture and could not see their hands. The results revealed that observers perceived a shape that was different from the vision-only sessions and closer to the veridical shape. Whereas, in general, vision is subject to ambiguities that arise from interpreting the retinal projection, our study shows that haptic input helps to disambiguate and reinterpret the visual input more veridically.

Are humans able to perceive the circularity of a cylinder that is grasped by the hand? This study presents the findings of an experiment in which cylinders with a circular cross-section had to be distinguished from cylinders with an elliptical cross-section. For comparison, the ability to distinguish a square cuboid from a rectangular cuboid was also investigated. Both elliptical and rectangular shapes can be characterized by the aspect ratio, but elliptical shapes also contain curvature information. We found that an elliptical shape with an aspect ratio of only 1.03 could be distinguished from a circular shape both in static and dynamic touch. However, for a rectangular shape, the aspect ratio needed to be about 1.11 for dynamic touch and 1.15 for static touch in order to be discernible from a square shape. We conclude that curvature information can be employed in a reliable and efficient manner in the perception of 3D shapes by touch.

The effects of visual contextual illusions on motor behaviour vary largely between experimental conditions. Whereas it has often been reported that the effects of illusions on pointing and grasping are largest when the movement is performed some time after the stimulus has disappeared, the effect of a delay has hardly been studied for saccadic eye movements. In this experiment, participants viewed a briefly presented Müller-Lyer illusion with a target at its endpoint and made a saccade to the remembered position of this target after a delay of 0, 0.6, 1.2 or 1.8 s. We found that horizontal saccade amplitudes were shorter for the perceptually shorter than for the perceptually longer configuration of the illusion. Most importantly, although the delay clearly affected saccade amplitude, resulting in shorter saccades for longer delays, the illusion effect did not depend on the duration of the delay. We argue that visually guided and memory-guided saccades are likely based on a common visual representation.

For intermanual tactual discrimination to occur, it is thought that neural signals have to cross the corpus callosum in some way at least once. In this note we address the question of whether this interhemispheric transfer affects grating discrimination in active touch. Eight right-handed participants made intermanual and intramanual tactual discriminations of sinusoidal gratings that had slightly different spatial periods. Intramanual discrimination included comparisons in which the gratings were felt with the same finger, as well as comparisons made with two different fingers of the same hand. During intermanual discrimination the gratings were felt with corresponding fingers or with non-corresponding fingers of different hands. We found that thresholds for intramanual conditions were lower than for intermanual conditions in active dynamic touch. This suggests that there is a component of the task that is unilateral, as might be mediated by a somatosensory region that has predominantly or exclusively contralateral receptive fields.

It is not evident how the small effects of the flankers of the Ebbinghaus figure on peak grip aperture (PGA) should be interpreted. One interpretation is that the flankers influence the estimated size, which in turn influences the grasp. If this interpretation is correct, then only the size-dependent aspects of the grasping movement should depend on the spatial positions of the flankers. An alternative interpretation is that the effect on grip aperture is caused by a change in judgement of the required precision, in which case various aspects of the grasping movement could be influenced by the size and position of the flankers. We presented subjects with a display consisting of a central disk surrounded by four large or small flankers. The array of circular flankers could be rotated by 45 degrees . There were two tasks: to reproduce the perceived size of the central disk, and to grasp the central disk. As in other studies, the reproduced size and the PGA were both influenced by the size of the flankers. The effect on reproduced size settings was independent of the flankers' spatial position. Nevertheless, the flankers' position did influence the final grip aperture and the grip orientation at PGA and at movement offset. Because the flankers changed more than only the PGA, we conclude that the effect of the flankers on prehension cannot only be because of misjudgement of the size of the central disk.

Pictorial illusions bias our judgments about certain visual attributes. Such illusions are therefore only expected to influence a task if these attributes are used to perform the task. When pointing to a position, different visual attributes could be used to guide the hand: direction and distance (or length) of the required displacement (vector coding) or the final position (position coding). In this study we used the Brentano illusion (an illusion of length) to determine which attributes are used in pointing. Several conditions were tested in which the visibility of the hand and the stimulus were varied. The illusion influenced movements between two points along the shaft of the figure, but not movements perpendicular to the shaft. When the hand and/or target were invisible during the movement, the influence of the illusion increased. Pointing movements under different visual conditions were based on different relative contributions of position and vector coding. The contribution of vector coding was always rather modest.

The on-line visual control of human movements can be exceptionally fast. Whether it is fast depends on the kind of visual information that is involved. In the present study we examine whether fast on-line control is specific to the magnocellular visual pathway. Fast manual responses become evident when an ongoing movement has to be adjusted, for instance because the target is displaced. We examined whether the response to such perturbations is faster for stimuli that only activate the magnocellular pathway than for equally conspicuous stimuli that only activate the parvocellular pathway. The response was indeed about 35 ms faster for stimuli that activate the magnocellular pathway. However, we argue that the slower response to stimuli that only stimulate the parvocellular pathway is due to the properties of the neurones involved and the less direct connection to the motor areas, rather than to fast reactions being driven exclusively by magnocellular input.

Previous work has indicated that people do not use their judgment of a target's speed to determine where to hit it. Instead, they use their judgment of the target's changing position and an expected speed (based on the speed of previous targets). In the present study we investigate whether people also ignore the target's apparent direction of motion, and use the target's changing position and an expected direction of motion instead. Subjects hit targets that moved in slightly different directions across a screen. Sometimes the targets disappeared after 150 ms, long before the subjects could reach the screen. This prevented subjects from using the target's changing position to adjust their movements, making it possible to evaluate whether subjects were relying on the perceived or an expected (average) direction to guide their movements. The background moved perpendicular to the average direction of motion in some trials. This influences the target's perceived direction of motion while leaving its perceived position unaffected. When the background was stationary, subjects hit disappearing targets along their trajectory, just as they hit ones that remained visible. Moving the background affected the direction in which subjects started to move their hand, in accordance with the illusory change in direction of target motion. If the target disappeared, this resulted in a hit that was systematically off the target's trajectory. If the target remained visible, subjects corrected their initial error. Presumably they did so on the basis of information about the target's changing position, because if the target disappeared they did not correct the error. We conclude that people do use the target's perceived direction of motion to determine where to hit it. Thus the perceived direction of motion is treated differently than the perceived speed. This suggests that the motion of an object is not broken down into speed components in different directions, but that speed and direction are perceived and used separately.

We studied active haptic discrimination of the geometrical features of an object. The geometrical parameters under investigation were the amplitude and width of a gaussian-shaped surface. Haptic discrimination thresholds were measured with regard to three values of these geometrical parameters. We found that humans discriminate up to about 300 shapes when both amplitude and width are extrapolated to the range between 1 μm and 1 m. Over this range, which covers the span of the arms, the number of discriminations is small compared to the number across the full range of chromaticities in vision. Roughly speaking, humans are far better at discriminating sharp (extensive amplitude and little width) gaussian surfaces from smooth (small amplitude and extensive width) ones than they are at discriminating small (small amplitude and width) surfaces from large (extensive amplitude and width) ones. Our main conclusion is that discrimination in the geometrical domain is poorest when the proportion between amplitude and width is roughly the same for both shapes. Our results are in close agreement with results of earlier experiments on detection thresholds. This indicates that similar, or even the same, neural mechanisms were used for detection and discrimination of the geometrical parameters under investigation.

It has been argued that precision in throwing is limited by the precision in the timing of the release. When precision is the only goal, as in throwing darts, one could therefore expect people to throw in a way that reduces sensitivity for imprecision in timing. We show that subjects do not do so, but throw in a way that reduces the sensitivity for speed errors instead. They even appear to vary the timing of release to compensate for the errors in the hand's movement. Thus timing does not appear to be the limiting factor.

We investigated what information subjects use when trying to hit moving targets. In particular, whether only visual information about the target's position is used to guide the hand to the place of interception or also information about its speed. Subjects hit targets that moved at different constant speeds and disappeared from view after varying amounts of time. This prevented the subjects from updating position information during the time that the target was invisible. Subjects hit further ahead of the disappearing point when the target moved faster, but not as much as they should have on the basis of the target's speed. This could be because more time is needed to perceive and use the correct speed than was available before the target disappeared. It could also be due to a speed-related misperception of the target's final position. The results of a second experiment were more consistent with the latter hypothesis. In a third experiment we moved the background to manipulate the perceived speed. This did not affect the hitting positions. We conclude that subjects respond only to the changing target position. Target speed influences the direction in which the hand moves indirectly, possibly via a speed-related misperception of position.

Previous studies on how we hit moving targets have revealed that the direction in which we move our hand is continuously adjusted on the basis of the target's perceived position, with a delay of about 110 ms. In the present study we show that the acceleration of the hand is also under such continuous control. Subjects were instructed to hit moving targets (running spiders) as quickly as possible with a rod. We found that changing the velocity of the target influenced the speed with which the rod was moved. The influence was noticeable about 200 ms after the target's velocity changed. The extent of the influence was consistent with a direct dependence of the acceleration of the hand on the target's velocity. We conclude that the acceleration of the hand is continuously adjusted on the basis of the speed of the target, with a delay of about 200 ms.

Reaching out for an object is often described as consisting of two components that are based on different visual information. Information on the object's position and orientation guides the hand to the object, while information on the object's shape and size determines how the fingers move relative to the thumb to grasp it. We propose an alternative description, which consists of determining suitable positions on the object - on the basis of its shape, surface texture, and so on - and then moving one's thumb and fingers to these positions. This could lead to the same performance without requiring distinct visual information on the object's orientation or size. If so, an illusory change in size need not influence the distance between thumb and fingers when reaching out for an object. However, as the object's size is used to estimate its weight, the illusory change in size should influence the force that is exerted to lift the object. To find out whether this is so, eight subjects were asked to pick up brass disks from a fixed position straight in front of them. The illusory change in size was brought about by presenting five converging lines in two different configurations under the disks. As predicted, the illusion influenced the force used to lift the disks, but not the distance between the subjects' thumbs and fingers when reaching for the disks.

'Subitizing' refers to fast and error-free numerosity judgment for small (

When hitting a target that is moving, the time for planning the interception is limited. Instead of waiting for all the necessary information about the target's position and speed before starting to move, subjects could use their previous experience with similar targets to make initial guesses and adjust as new information becomes available. In the present study we examined whether the speed of the preceding target influences a hitting movement. Subjects hit moving targets that appeared on a screen about 40 cm in front of them. The targets moved at 6, 12 or 18 cm/s. Both the hand's initial movement direction and the final hitting error depended on the speed of the preceding target. We conclude that people control the way they hit moving targets on the basis of the speed of the preceding target.

In the haptic filled-space illusion for active dynamic touch, observers move their fingertip across an unfilled extent or an extent filled with intermediate stimulations. Previous researchers have reported lengths of filled extents to be overestimated, but the parameters affecting the strength of the illusion are still largely unknown. In the current research, we show that the illusion persists when intermediate stimulations do not provide information about the extent's length. In addition, the results show that the strength of the illusion increases with the number of filler elements. In contrast with earlier research, we control for movement speed differences between filled and unfilled extents. The results suggest that the strength of the illusion is independent of the overall average movement speed. Insight into factors affecting the strength of the illusion may provide a better understanding of the kinematic mechanisms underlying haptic length perception.

What humans haptically perceive as parallel is often far from physically parallel. These deviations from parallelity are highly significant and very systematic. There exists accumulating evidence, both psychophysical and neurophysiological, that what is haptically parallel is decided in a frame of reference intermediate to an allocentric and an egocentric one. The central question here concerns the nature of the egocentric frame of reference. In the literature, various kinds of egocentric reference frames are mentioned for haptic spatial tasks, such as hand-centered, arm-centered, and body-centered frames of reference. Thus far, it has not been possible to distinguish between body-centered, arm-centered, and hand-centered reference frames in our experiments, as hand and arm orientation always covaried with distance from the body-midline. In the current set of experiments the influence of body-centered and hand-centered reference frames could be dissociated. Subjects were asked to make a test bar haptically parallel to a reference bar in five different conditions, in which their hands were oriented straight ahead, rotated to the left, rotated to the right, rotated outward or rotated inward. If the reference frame is body-centered, the deviations should be independent of condition. If, on the other hand, the reference frame is hand-centered, the deviations should vary with condition. The results show that deviation size varies strongly with condition, exactly in the way predicted by the influence of a hand-centered egocentric frame of reference. Interestingly, this implies that subjects do not sufficiently take into account the orientation of their hands.

Reaching out for objects with an unseen arm involves using both visual and kinesthetic information. Neither visual nor kinesthetic information is perfect. Each is subject to both constant and variable errors. To evaluate how such errors influence performance in natural goal-directed movements, we asked subjects to align a real 5-cm cube, which they held in their hand but could not see, with a three-dimensional visual simulation of such a cube. The simulated cube was presented at one of four target locations at the corners of an imaginary tetraeder. Subjects made successive, self-paced movements between these target locations. They could not see anything except the simulated cube throughout the experiment. Initial analysis of the spatial dispersion of movement endpoints demonstrated that the major source of errors under these conditions was visual. Further analysis of the relationship between variability of the starting positions and endpoints showed that the errors were primarily in judging the endpoint, rather than the direction or amplitude of the required movement vector. The findings support endpoint control of human goal-directed movements.