Relations between the errors of targeted movements and inexactitude of visual perception of space

Errors of targeted movements of the arm to the places of presentation of light targets (in darkness) were studied in healthy subjects kept in a vertical position or laying on their backs. An error along theY axis (corresponding to the longitudinal body axis) changed its sign depending on the body orientation with respect to the gravitation vector. In the vertical position, the arm shifted to the feet at the movement’s termination, while in the laying position it shifted to the head. AnX error showed no dependence on the position of the body in space. The errors reached their maxima in the absence of visual control, but became two-three times smaller when the tested subject could observe the position of an indicator (light diodes) fixed on the end of the index finger (or of a pointer rod). When the spatial positions of targets were reconstructed according to verbal “indications”, the amplitudes ofX andY errors appeared similar to those at real movements (indication under visual control). In this case, the sign ofY errors also depended on the body orientation, but their direction was opposite. We suppose that systematicY errors at the targeted arm movements are determined not only by an antigravitation component of the motor program, but also by shifting of a sensory visual estimations of the spatial target position.     

Seeing your error alters my pointing: observing systematic pointing errors induces sensori-motor after-effects

During the procedure of prism adaptation, subjects execute pointing movements to visual targets under a lateral optical displacement: as consequence of the discrepancy between visual and proprioceptive inputs, their visuo-motor activity is characterized by pointing errors. The perception of such final errors triggers error-correction processes that eventually result into sensori-motor compensation, opposite to the prismatic displacement (i.e., after-effects). Here we tested whether the mere observation of erroneous pointing movements, similar to those executed during prism adaptation, is sufficient to produce adaptation-like after-effects. Neurotypical participants observed, from a first-person perspective, the examiner's arm making incorrect pointing movements that systematically overshot visual targets location to the right, thus simulating a rightward optical deviation. Three classical after-effect measures (proprioceptive, visual and visual-proprioceptive shift) were recorded before and after first-person's perspective observation of pointing errors. Results showed that mere visual exposure to an arm that systematically points on the right-side of a target (i.e., without error correction) produces a leftward after-effect, which mostly affects the observer's proprioceptive estimation of her body midline. In addition, being exposed to such a constant visual error induced in the observer the illusion "to feel" the seen movement. These findings indicate that it is possible to elicit sensori-motor after-effects by mere observation of movement errors.     

Effects of head position on errors in 3-D pointing to remembered locations

The accuracy of pointing movements performed under different head positions to remembered target locations in 3-D space was studied in healthy persons. The subjects fixated a visual target, then closed their eyes and after 1.0 sec performed the targeted movement with their right arm. The target (a point light source) was presented in random order by a programmable robot arm at one of five space locations. The accuracy of pointing movements was examined in a spherical coordinate system centered in respect with the shoulder of the responding arm. The pointing movements were most accurate under natural eye-head coordination. With the head fixed in the straight-ahead position, both the 3-D absolute error and its standard deviation increased significantly. At the same time, individual components of spatial error (directional and radial) did not change significantly. With the head turned to the rightmost or leftmost position, the pointing accuracy was disturbed within larger limits than under head-fixed condition. The main contributors to the 3-D absolute error were the changes in the azimuth error. The latter depended on the direction of the head-turn: the rightmost turn either increased leftward or decreased rightward shift, and conversely, the left turn increased rightward shift or decreased leftward shift of the target-directed movements.It is suggested that the increased inaccuracy of pointing under head-fixed condition reflected the impairment of the eye-head coordination underlying gaze orientation, and increased inaccuracy under the head-turned condition may be explained by changes in the internal representation of the head and target position in space.Neirofiziologiya/Neurophysiology, Vol. 26, No. 2, pp. 122–131, March–April, 1994.     

Haptic Guidance Needs to Be Intuitive Not Just Informative to Improve Human Motor Accuracy

Humans make both random and systematic errors when reproducing learned movements. Intuitive haptic guidance that assists one to make the movements reduces such errors. Our study examined whether any additional haptic information about the location of the target reduces errors in a position reproduction task, or whether the haptic guidance needs to be assistive to do so. Holding a haptic device, subjects made reaches to visible targets without time constraints. They did so in a no-guidance condition, and in guidance conditions in which the direction of the force with respect to the target differed, but the force scaled with the distance to the target in the same way. We examined whether guidance forces directed towards the target would reduce subjects’ errors in reproducing a prior position to the same extent as do forces rotated by 90 degrees or 180 degrees, as it might because the forces provide the same information in all three cases. Without vision of the arm, both the accuracy and precision were significantly better with guidance directed towards the target than in all other conditions. The errors with rotated guidance did not differ from those without guidance. Not surprisingly, the movements tended to be faster when guidance forces directed the reaches to the target. This study shows that haptic guidance significantly improved motor performance when using it was intuitive, while non-intuitively presented information did not lead to any improvements and seemed to be ignored even in our simple paradigm with static targets and no time constraints.     

Prediction of a moving target's position in fast goal-directed action

 Subjects made fast goal-directed arm movements towards moving targets. In some cases, the perceived direction of target motion was manipulated by moving the background. By comparing the trajectories towards moving targets with those towards static targets, we determined the position towards which subjects were aiming at movement onset. We showed that this position was an extrapolation in the target’s perceived direction from its position at that moment using its perceived direction of motion. If subjects were to continue to extrapolate in the perceived direction of target motion from the position at which they perceive the target at each instant, the error would decrease during the movements. By analysing the differences between subjects’ arm movements towards targets moving in different (apparent) directions with a linear second-order model, we show that the reduction in the error that this predicts is not enough to explain how subjects compensate for their initial misjudgements. Received: 10 February 1995/Accepted in revised form: 30 May 1995     

Adjustment errors of feedback guided human forearm movements elicited by random position signal sequences

The effects of visual and auditory relevant feedback on human forearm movements elicited by random position signal sequences were examined. We observed the adjustment error sequences when the arm moved briskly and accurately to the target position. As the results, the mean adjustment errors for each speed (clock interval) of random position signal sequence are different in the left-right target positions, and also different in the flexion-extension movements. In faster speed (1s), the adjustment errors are approximately flat as compared with those in other speeds, on the other hand, in moderate speed (2s) or slower speed (3s), these indicate a upward tendency from left to right positions, and also indicate a upward tendency from flexion to extension movements. However, in the reaction times, the moving times, and the adjusting times, there are no significant differences for the left-right target positions and for the flexion-extension movements.     

Behavioral Investigation on the Frames of Reference Involved in Visuomotor Transformations during Peripheral Arm Reaching

Background

Several psychophysical experiments found evidence for the involvement of gaze-centered and/or body-centered coordinates in arm-movement planning and execution. Here we aimed at investigating the frames of reference involved in the visuomotor transformations for reaching towards visual targets in space by taking target eccentricity and performing hand into account.

Methodology/Principal Findings

We examined several performance measures while subjects reached, in complete darkness, memorized targets situated at different locations relative to the gaze and/or to the body, thus distinguishing between an eye-centered and a body-centered frame of reference involved in the computation of the movement vector. The errors seem to be mainly affected by the visual hemifield of the target, independently from its location relative to the body, with an overestimation error in the horizontal reaching dimension (retinal exaggeration effect). The use of several target locations within the perifoveal visual field allowed us to reveal a novel finding, that is, a positive linear correlation between horizontal overestimation errors and target retinal eccentricity. In addition, we found an independent influence of the performing hand on the visuomotor transformation process, with each hand misreaching towards the ipsilateral side.

Conclusions

While supporting the existence of an internal mechanism of target-effector integration in multiple frames of reference, the present data, especially the linear overshoot at small target eccentricities, clearly indicate the primary role of gaze-centered coding of target location in the visuomotor transformation for reaching.     

Causal Inference for Spatial Constancy across Saccades

Our ability to interact with the environment hinges on creating a stable visual world despite the continuous changes in retinal input. To achieve visual stability, the brain must distinguish the retinal image shifts caused by eye movements and shifts due to movements of the visual scene. This process appears not to be flawless: during saccades, we often fail to detect whether visual objects remain stable or move, which is called saccadic suppression of displacement (SSD). How does the brain evaluate the memorized information of the presaccadic scene and the actual visual feedback of the postsaccadic visual scene in the computations for visual stability? Using a SSD task, we test how participants localize the presaccadic position of the fixation target, the saccade target or a peripheral non-foveated target that was displaced parallel or orthogonal during a horizontal saccade, and subsequently viewed for three different durations. Results showed different localization errors of the three targets, depending on the viewing time of the postsaccadic stimulus and its spatial separation from the presaccadic location. We modeled the data through a Bayesian causal inference mechanism, in which at the trial level an optimal mixing of two possible strategies, integration vs. separation of the presaccadic memory and the postsaccadic sensory signals, is applied. Fits of this model generally outperformed other plausible decision strategies for producing SSD. Our findings suggest that humans exploit a Bayesian inference process with two causal structures to mediate visual stability.     

Positioning Errors in Simple Targeted Movements of the Forearm Realized in the Absence of Visual Control: Effects of Vibrational Stimulation of Antagonist Muscles

In 17 healthy subjects, we examined the characteristics of targeted movements of the forearm, flexion from the initial position of full extension taken as 0 deg to a 50 deg target angle in the elbow joint (flexor tests, FTs) and extension from the initial angle of 100 deg to the same target angle (extensor tests, ETs) with return to the initial positions. A standard movement (its trajectory corresponded to a simple trapezium) was performed under conditions of visual feedback (the value of the target angle and trajectory of the movement were visualized on the screen of a monitor); then, this movement should be reproduced by the subject (according to an acoustic signal) in the absence of visual control. Target-reaching test movements in the absence of visual feedback differed from the standard ones in a higher velocity. Blindfold reproduction of standard movements realized under kinesthetic control was accompanied in all subjects by noticeable positive systematic errors of targeted positioning (in the group, on average, 5.16 ± 0.55 and 4.83 ± 0.58 deg under FT and ET conditions, respectively). Vibrational stimulation of the muscles whose activity mainly provided the movement and positioning (m. biceps brachii in the FT cases and m. triceps brachii in the case of ETs) resulted in decreases of the errors of kinesthetic positioning; intragroup means of these errors were 2.55 ± 0.36 deg (FTs) and 2.26 ± 0.40 deg (ETs). The positioning errors demonstrated even greater decreases upon vibrational stimulation of the muscles, which were relatively inactive under conditions of the tests and underwent passive stretching in the course of the movements (m. triceps in FTs and m. biceps in ETs). Mean intragroup values of the errors in these cases were 0.46 ± 0.25 and 0.52 ± 0.31 deg, respectively. The nature of systematic positioning errors in the reproduction of targeted movements in the absence of visual control and the mechanisms underlying the influence of vibrational stimulation of the muscles involved in realization of these movements on the positioning errors under kinesthetic control are discussed.     

A Comparison of Sensorimotor Adaptation in the Visual and in the Auditory Modality

We compared sensorimotor adaptation in the visual and the auditory modality. Subjects pointed to visual targets while receiving direct spatial information about fingertip position in the visual modality, or they pointed to visual targets while receiving indirect information about fingertip position in the visual modality, or they pointed to auditory targets while receiving indirect information about fingertip position in the auditory modality. Feedback was laterally shifted to induce adaptation, and aftereffects were tested with both target modalities and both hands. We found that aftereffects of adaptation were smaller when tested with the non-adapted hand, i.e., intermanual transfer was incomplete. Furthermore, aftereffects were smaller when tested in the non-adapted target modality, i.e., intermodal transfer was incomplete. Aftereffects were smaller following adaptation with indirect rather than direct feedback, but they were not smaller following adaptation with auditory rather than visual targets. From this we conclude that the magnitude of adaptive recalibration rather depends on the method of feedback delivery (indirect versus direct) than on the modality of feedback (visual versus auditory).     

Effect of eccentric exercise on position sense at the human forearm in different postures.

This is a study of the ability of blindfolded human subjects to match the position of their forearms before and after eccentric exercise. The hypothesis tested was that the sense of effort contributed to forearm position sense. The fall in force after the exercise was predicted to alter the relationship between effort and force and thereby induce position errors. In the arms-in-front posture, subjects had their unsupported reference arm set to one of two angles from the horizontal, 30 or 60 degrees , and they matched its position by voluntary placement of their other arm. Matching errors were compared with a task where the arms were counterweighted, so could be moved in the vertical plane with minimal effort, and where the arms were moved in the horizontal plane. In these latter two tasks, the intention was to test whether removal of an effort sensation from holding the arm against gravity influenced matching performance. It was found that, although absolute errors for counterweighted and horizontal matching were no larger than for unsupported matching, their standard deviations, 6.1 and 6.8 degrees , respectively, were significantly greater than for unsupported matching (4.6 degrees ), indicating more erratic matching. The eccentric exercise led, the next day, to a fall in maximum voluntary muscle torque of >or=15%. This was accompanied by a significant increase in matching errors for the unsupported matching task from 2.7 +/- 0.5 to 0.8 +/- 0.7 degrees but not for counterweighted (1.4 +/- 0.2 to -0.2 degrees +/- 1.1 degrees ) or horizontal matching (-1.3 +/- 0.7 degrees to -1.8 +/- 0.7 degrees ). This, it is postulated, is because the reduced voluntary torque after exercise was accompanied by a greater effort required to support the arms, leading to larger matching errors. However, effort is only able to provide positional information for unsupported matching where gravity plays a role. In gravity-neutral tasks like counterweighted or horizontal matching, a change in the effort-force relationship after exercise leaves matching accuracy unaffected.     

Different programming modes of human saccadic eye movements as a function of stimulus eccentricity: Indications of a functional subdivision of the visual field

1. Voluntary saccadic eye movements were made toward flashes of light on the horizontal meridian, whose duration and distance from the point of fixation were varied; eye movements were measured using d.c.-electrooculography.—2. Targets within 10°–15° eccentricity are usually reached by one saccadic eye movement. When the eyes turn toward targets of more than 10°–15° eccentricity, the first saccadic eye movement falls short of the target by an angle usually not exceeding 10°. The presence of the image of the target off the fovea (visual error signal) subsequent to such an undershoot elicits, after a short interval, corrective saccades (usually one) which place the image of the target on the fovea. In the absence of a visual error signal, the probability of occurrence of corrective saccades is low, but it increases with greater target eccentricities. These observations suggest that there are different, eccentricity-dependent modes of programming saccadic eye movements.—3. Saccadic eye movements appear to be programmed in retinal coordinates. This conclusion is based on the observations that, irrespective of the initial position of the eyes in the orbit, a) there are different programming modes for eye movements to targets within and beyond 10°–15° from the fixation point, and b_ the maximum velocity of saccadic eye movements is always reached at 25° to 30° target eccentricity. —4. Distributions of latency and intersaccadic interval (ISI) are frequently multimodal, with a separation between modes of 30 to 40 msec. These observations suggest that saccadic eye movements are produced by mechanisms which, at a frequency of 30 Hz, process visual information. —5. Corrective saccades may occur after extremely short intervals (30 to 60 msec) regardless of whether or not a visual error signal is present; the eyes may not even come to a complete stop during these very short intersaccadic intervals. It is suggested that these corrective saccades are triggered by errors in the programming of the initial saccadic eye movements, and not by a visual error signal. —6. The exitence of different, eccentricity-dependent programming modes of saccadic eye movements, is further supported by anatomical, physiological, psychophysical, and neuropathological observations that suggest a dissociation of visual functions dependent on retinal eccentricity. Saccadic eye movements to targets more eccentric than 10°–15° appear to be executed by a mechanism involving the superior colliculus (perhaps independent of the visual cortex), whereas saccadic eye movements to less eccentric targets appear to depend on a mechanism involving the geniculo-cortical pathway (perhaps in collaboration with the superior colliculus).     

Fix your eyes in the space you could reach: neurons in the macaque medial parietal cortex prefer gaze positions in peripersonal space

Interacting in the peripersonal space requires coordinated arm and eye movements to visual targets in depth. In primates, the medial posterior parietal cortex (PPC) represents a crucial node in the process of visual-to-motor signal transformations. The medial PPC area V6A is a key region engaged in the control of these processes because it jointly processes visual information, eye position and arm movement related signals. However, to date, there is no evidence in the medial PPC of spatial encoding in three dimensions. Here, using single neuron recordings in behaving macaques, we studied the neural signals related to binocular eye position in a task that required the monkeys to perform saccades and fixate targets at different locations in peripersonal and extrapersonal space. A significant proportion of neurons were modulated by both gaze direction and depth, i.e., by the location of the foveated target in 3D space. The population activity of these neurons displayed a strong preference for peripersonal space in a time interval around the saccade that preceded fixation and during fixation as well. This preference for targets within reaching distance during both target capturing and fixation suggests that binocular eye position signals are implemented functionally in V6A to support its role in reaching and grasping.     

Neuronal processing delays are compensated in the sensorimotor branch of the visual system

Moving objects change their position until signals from the photoreceptors arrive in the visual cortex. Nonetheless, motor responses to moving objects are accurate and do not lag behind the real-world position. The questions are how and where neural delays are compensated for. It was suggested that compensation is achieved within the visual system by extrapolating the position of moving objects. A visual illusion supports this idea: when a briefly flashed object is presented in the same position as a moving object, it appears to lag behind. However, moving objects do not appear ahead of their final or reversal points. We investigated a situation where participants localized the final position of a moving stimulus. Visual perception and short-term memory of the final target position were accurate, but reaching movements were directed toward future positions of the target beyond the vanishing point. Our results show that neuronal latencies are not compensated for at early stages of visual processing, but at a late stage when retinotopic information is transformed into egocentric space used for motor responses. The sensorimotor system extrapolates the position of moving targets to allow for precise localization of moving targets despite neuronal latencies.     

Effect of sitting postures and shoulder position on the cervicocephalic kinesthesia in healthy young males

Information about head orientation, position, and movement with respect to the trunk relies on the visual, vestibular, extensive muscular, and articular proprioceptive system of the neck. Various factors can affect proprioception since it is the function of afferent integration, and tuning of muscular and articular receptors. Pain, muscle fatigue, and joint position have been shown to affect proprioceptive capacity. Thus, it can be speculated that changes in body posture can alter the neck proprioception. This study was undertaken to investigate the effect of body posture on cervicocephalic kinesthetic sense in healthy subjects. Cervicocephalic kinesthetic sensibility was measured by the kinesthetic sensibility test in healthy young adults while in (a) habitual slouched sitting position with arms hanging by the side (SS), (b) habitual slouched sitting position with arms unloaded (supported) (SS-AS), and (c) upright sitting position with arms hanging by the side (US) during maximum and 30 degree right, left rotations, flexion, and extension. Thirty healthy male adults (mean age 27.83; SD 3.41) volunteered for this study. The least mean error was found for the SS-AS position (0.48; SD 0.24), followed by SS (0.60; SD 0.43) and US (0.96; SD 0.71), respectively. For all test conditions, there was significant difference in mean absolute error while head repositioning from maximum and 30 degree rotation during SS and SS-AS positions (p?相似文献   

The role of perceived relative position in pointing to objects apparently shifted by depth-contrast

This paper is concerned with the information used in open-loop pointing to visually perceived targets. Stereoscopic stimuli were used to produce illusory relative egocentric distances, which were inconsistent with the angles of vergence required to fuse the targets. One of the stimuli was a rectangle slanted around a vertical axis. Four participants in Experiment 1 reported its slant and pointed to its edges. The slant was hugely underestimated (condition A) unless the rectangle was flanked by other surfaces (condition B). The relative depth of a pair of dots placed in front of the rectangle was also misperceived due to depth-contrast effect. The critical finding is that pointing responses were not based on vergence but were consistent with depth estimates, both for the rectangle and for the dots. Experiment 2 revealed the conditions necessary for pointing to be consistent with perceived relative position. The different target distances were either randomised allowing inter-trial comparisons, or presented only one per session to prevent them. Pointing was similar to estimates only in the randomised condition showing the significance of inter-trial comparisons. It is proposed that participants used the remembered motor command and kinesthetic sensations of a previous movement as a reference, attempting to make the difference between successive movements the same as a visually perceived depth difference between successive targets.     

Effects of long-term space flights on the organization of the horizontal gaze fixation reaction

The results of the Russian-Austrian space experiment Monimir, which was a part of the international space program Austromir, are presented. The characteristics of the horizontal gaze fixation reaction (hGFR) to the visual targets were studied during long-term space flights. Seven crewmembers of the space station Mir participated in our experiment. The subjects were tested four times before the flight, five times during the flight, and three to four times after landing. During the flight and after accomplishing, the characteristics of gaze fixation reaction changed regularly: the reaction time and coefficient of the gain of vestibular-ocular reflex increased; the velocities of eye-head movements increased and decreased. These changes were indicative of a disturbed control of the vestibular-ocular reflex under microgravity conditions because of variability of the vestibular input activity. The cosmonauts that had flight and non-flight professional specializations differed in strategies of their adaptation to the microgravity conditions. In the former, exposure to microgravity was accompanied by gaze hypermetry and inhibition of head movements; conversely, in the latter, the velocity of head movements increased, whereas that of saccades decreased.     

The information transmitted at final position in visually triggered forearm movements

Visually triggered forearm movements were analyzed by an Information Theory approach. Human subjects made smooth movements which were characterized by moderate speeds, ranging about 100 degrees per second, by continuity in the position and velocity traces, and attainment of final average EMG levels before completion of the movement. We calculated the information transmitted by final position, biceps EMG, triceps EMG, and the ratio of the EMGs. The results were: (1) The information transmitted by final joint angle increased with number of targets but gradually levelled off. The maximum value was slightly over 3 bits, corresponding to an equivalent number of less than nine independent arm positions for a single movement. (2) The information transmitted by the ratio of the EMGs exceeds that transmitted by the biceps or triceps alone. (3) A previous theoretical prediction based on a spring model (Sakitt, 1980a) gives a moderately good fit to the experimental EMG ratio as a function of final position over a large range of angles. Our results lend consistency to two ideas about the nature of visually triggered forearm movements. First, our finding about the EMG ratio suggests that the basic motor program for final position is probably in terms of relative allocation of innervations, rather than looking up individual values. Second, single movements of this kind transmit surprisingly little information. If this is the case, it suggests that very fine accuracy is not achieved by a single program but requires feedback in order to program and execute additional movement.Laboratoire de Physiologie Neurosensorielle, CNRS, Paris, France     

The proximity of visual landmarks impacts reaching performance

The control of goal-directed reaching movements is thought to rely upon egocentric visual information derived from the visuomotor networks of the dorsal visual pathway. However, recent research (Krigolson and Heath, 2004) suggests it is also possible to make allocentric comparisons between a visual background and a target object to facilitate reaching accuracy. Here we sought to determine if the effectiveness of these allocentric comparisons is reduced as distance between a visual background and a target object increases. To accomplish this, participants completed memory-guided reaching movements to targets presented in an otherwise empty visual background or positioned within a proximal, medial, or distal visual background. Our results indicated that the availability of a proximal or medial visual background reduced endpoint variability relative to reaches made without a visual background. Interestingly, we found that endpoint variability was not reduced when participants reached to targets framed within a distal visual background. Such findings suggest that allocentric visual information is used to facilitate reaching performance; however, the fidelity by which such cues are used appears linked to the proximity of veridical target location. Importantly, these data also suggest that information from both the dorsal and ventral visual streams can be integrated to facilitate the online control of reaching movements.     

Changes in the amplitude and direction of goal-directed hand movements in the lack of visual information

The goal of the present investigation was to determine the precision of goal-directed hand movements in the lack of visual information. The movement amplitude and direction was examined under different experimental conditions. Subjects were ten female and ten male university students. The motor test was drawing 10 cm long straight line and 24 cm long zigzag line in four different experimental conditions. 1) The drawing with open eyes was followed immediately with drawing with closed eyes. 2) The drawing was executed from memory in the lack of visual information. 3) Drawing with restricted amplitude or direction. 4) Drawing with verbal feedback. The errors of the target distance and the lateral deviations from the target were different under the different experimental conditions. The largest errors and underestimation of the target distance occurred in drawing horizontal straight line with closed eyes. No statistically significant gender differences were found. It is concluded that the practice, adjustment of single movement parameter to the target, and the verbal feedback assist better the accuracy of unseen goal-directed hand movement than the recent visual memory.