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PubMed2005

Dosimetric impact of geometric errors due to respiratory motion prediction on dynamic multileaf collimator-based four-dimensional radiation delivery.

The synchronization of dynamic multileaf collimator (DMLC) response with respiratory motion is critical to ensure the accuracy of DMLC-based four dimensional (4D) radiation delivery. In practice, however, a finite time delay (response time) between the acquisition of tumor position and multileaf collimator response necessitates predictive models of respiratory tumor motion to synchronize radiation delivery. Predicting a complex process such as respiratory motion introduces geometric errors, which have been reported in several publications. However, the dosimetric effect of such errors on 4D radiation delivery has not yet been investigated. Thus, our aim in this work was to quantify the dosimetric effects of geometric error due to prediction under several different conditions. Conformal and intensity modulated radiation therapy (IMRT) plans for a lung patient were generated for anterior-posterior/posterior-anterior (AP/PA) beam arrangements at 6 and 18 MV energies to provide planned dose distributions. Respiratory motion data was obtained from 60 diaphragm-motion fluoroscopy recordings from five patients. A linear adaptive filter was employed to predict the tumor position. The geometric error of prediction was defined as the absolute difference between predicted and actual positions at each diaphragm position. Distributions of geometric error of prediction were obtained for all of the respiratory motion data. Planned dose distributions were then convolved with distributions for the geometric error of prediction to obtain convolved dose distributions. The dosimetric effect of such geometric errors was determined as a function of several variables: response time (0-0.6 s), beam energy (6/18 MV), treatment delivery (3D/4D), treatment type (conformal/IMRT), beam direction (AP/PA), and breathing training type (free breathing/audio instruction/visual feedback). Dose difference and distance-to-agreement analysis was employed to quantify results. Based on our data, the dosimetric impact of prediction (a) increased with response time, (b) was larger for 3D radiation therapy as compared with 4D radiation therapy, (c) was relatively insensitive to change in beam energy and beam direction, (d) was greater for IMRT distributions as compared with conformal distributions, (e) was smaller than the dosimetric impact of latency, and (f) was greatest for respiration motion with audio instructions, followed by visual feedback and free breathing. Geometric errors of prediction that occur during 4D radiation delivery introduce dosimetric errors that are dependent on several factors, such as response time, treatment-delivery type, and beam energy. Even for relatively small response times of 0.6 s into the future, dosimetric errors due to prediction could approach delivery errors when respiratory motion is not accounted for at all. To reduce the dosimetric impact, better predictive models and/or shorter response times are required.

Fluoroscopy↗

PubMed1992

Dissociation between dyspnea and respiratory effort.

Breathlessness induced by hypercapnia may be related to the sensation of respiratory effort or to the central or peripheral effects of CO2. To examine the relationship among breathlessness, respiratory effort, and hypercapnia, we studied eight normal naive subjects. By using a visual feedback system, subjects maintained a constant ventilation of 50-60 L/min. PETCO2 was held at 40 mm Hg during the first 2 min of each trial (control period), then for 4 min (test period) was either kept at 40 mm Hg or elevated to 50 mm Hg. At the end of each control and test period, subjects were asked to give separate ratings for dyspnea (an unpleasant urge to breathe) and for the sense of respiratory effort (analogous to lifting a weight) on a 50-cm visual analog scale. Hypercapnia was associated with a significant reduction in effort ratings (-7.3 +/- 6.4, mean +/- SD, p < 0.05) and a concomitant increase in dyspnea (+6.6 +/- 6.0, p < 0.05). We conclude that dyspnea associated with hypercapnia is dissociated from changes in respiratory effort, and that CO2 has a direct central effect that leads to breathlessness. Our data also suggest that the sense of effort at a given level of ventilation is less when the ventilation is the result of "reflex" stimuli to breathe rather than "voluntary" signals to the respiratory muscles.

Adult↗

PubMed1997

Positioning errors produced by persons with knee joint disease when vision is occluded.

This study examined the effect of occluding static vision on the ability of persons with knee joint disease and impaired position sense to actively reproduce knee angles in a one-legged standing position. The subjects, six women with knee joint disease, ages 54-75 years each performed a series of two knee repositioning tasks in the ranges of 20-40 degrees flexion on two different occasions, both with, and without visual feedback. On each occasion, the leg was marked with reflective markers and the resultant sagittal plane knee angles were photographed. Whilst the absolute differences between the criterion and the matching knee angles calculated from these records showed vision to have an effect on positioning accuracy (p = 0.04), there were no differences in mean absolute constant errors between the two experimental paradigms, indicating similar perceptual judgment in memory. However, variability in performance tended to be greater when vision was occluded (p = 0.08). These findings suggest that while visual sources of information may play a minor role in the knee positioning bias towards target overestimation apparent in this group, supplementary visual cues may help to promote more consistent positioning performance.

Aged↗

PubMed1997

Real-time feedback of body center of gravity for postural training of elderly patients with peripheral neuropathy.

A system that estimates and displays, in real-time, the location of the center of gravity of the human body relative to the feet was developed and then used in a biofeedback training program for improving the postural instability caused by deterioration of the proprioceptive system in elderly patients with significant diabetic sensory neuropathy during perturbations of a support platform. Training sessions were scheduled three times a week for four weeks and consisted of continuous perturbations of the support platform for the standing subject. Postural stability of each subject was evaluated before, during and after training on a weekly basis. After twelve sessions of training, subjects who received visual feedback showed greater reduction in total number of falls and in fall magnitude defined by force in the body suspension cord, and longer delay in time between perturbation and fall than NFB group.

Biofeedback, Psychology↗

PubMed1995

Are non-relevant objects represented in working memory? The effect of non-target objects on reach and grasp kinematics.

The role of visual information and the precise nature of the representations used in the control of prehension movements has frequently been studied by having subjects reach for target objects in the absence of visual information. Such manipulations have often been described as preventing visual feedback; however, they also impose a working memory load not found in prehension movements with normal vision. In this study we examined the relationship between working memory and visuospatial attention using a prehension task. In this study six healthy, right-handed adult subjects reached for a wooden block under conditions of normal vision, or else with their eyes closed having first observed the placement of the target. Furthermore, the role of visuospatial attention was examined by studying the effect, on transport and grasp kinematics, of placing task-irrelevant "flanker" objects (a wooden cylinder) within the visual field on a proportion of trials. Our results clearly demonstrated that the position of flankers produced clear interference effects on both transport and grasp kinematics. Furthermore, interference effects were significantly greater when subjects reached to the remembered location of the target (i.e., with eyes closed). The finding that the position of flanker objects influences both transport and grasp components of the prehension movement is taken as support for the view that these components may not be independently computed and that subjects may prepare a coordinated movement in which both transport and grasp are specifically adapted to the task in hand. The finding that flanker effects occur primarily when reaching to the remembered location of the target object is interpreted as supporting the view that attentional processes do not work efficiently on working memory representations.

Adult↗

PubMed2002

A ménage à trois: the eye, the hand and on-line processing.

This review summarizes research conducted in our laboratories over the past 5 years aimed at determining the temporal and spatial relationships between eye and hand movements and the amount of central processing that must occur before performing a manual aiming movement, relative to the amount of processing that is done online. All of our research to date points to a two-component model of speed-accuracy control in manual aiming. Several studies have shown that eye and hand movements in manual aiming are inextricably linked both temporally and spatially. Typically, the eye arrives in the vicinity of the target first; this coincides with peak acceleration of the finger during the initial impulse phase of a movement. There is also significant temporal and spatial coupling of the finger, elbow and shoulder in aiming, and movements appear to evolve in a proximal-to-distal fashion. Movements are endpoint driven and variability is reduced with distal approximation to the target. This movement control strategy means that visual information is not only available for use in modifying responses, but there is sufficient time available for its use. In sequential complex aiming movements, the use of visual feedback and on-line processing become even more important. Practice does not diminish the need for on-line processing; rather, its use appears to ensure greater movement efficiency.

Biomechanical Phenomena↗

PubMed2001

Movement prediction and movement production.

The prediction of future positions of moving objects occurs in cases of actively produced and passively observed movement. Additionally, the moving object may or may not be tracked with the eyes. The authors studied the difference between active and passive movement prediction by asking observers to estimate displacements of an occluded moving target, where the movement was driven by the observer's manual action or was passively observed. In the absence of eye tracking, they found that in the active condition, estimates are more anticipatory than in the passive conditions. Decreasing the congruence between motor action and visual feedback diminished but did not eliminate the anticipatory effect of action. When the target was tracked with the eyes, the effect of manual action disappeared. Results indicate distinct contributions of hand and eye movement signals to the prediction of trajectories of moving objects.

Adult↗

PubMed1996

Visual illusion and action.

The role of allocentric cues on movement control was investigated in the present study. Pointing movements directed to the more distant vertex of closed and open configurations of the Muller-Lyer illusion, as well as to the vertex of control lines, were studied in four experimental conditions. In the first (full-vision condition) subjects saw both stimulus and their hand before and during movement, in the second (non-visual feedback condition) they saw the stimulus, but not their hand during movement. In the two remaining conditions (no-vision conditions) vision of the scene and the hand was precluded. Pointing was executed 0 sec (no vision 0 sec delay condition) or 5 sec (no-vision 5 sec delay condition) after the light was switched off. The Muller-Lyer illusion affected pointing kinematics with respect to the control lines. Subjects undershot and overshot the vertex location, respectively, of the closed and open configuration. Correspondingly, the entire kinematics were changed. The main result was, however, a gradually increasing effect of the perceptual illusion when pointing was executed from memory compared to the full-vision condition. These data are discussed according to the hypothesis that the system underlying visual perception in the allocentric frame of reference and that involved in motor action can functionally interact. The strength of this interaction depends upon the efficiency of the egocentric frame of reference by which motor actions are constructed.

Adolescent↗

PubMed2001

Cephalopod chromatophores: neurobiology and natural history.

The chromatophores of cephalopods differ fundamentally from those of other animals: they are neuromuscular organs rather than cells and are not controlled hormonally. They constitute a unique motor system that operates upon the environment without applying any force to it. Each chromatophore organ comprises an elastic sacculus containing pigment, to which is attached a set of obliquely striated radial muscles, each with its nerves and glia. When excited the muscles contract, expanding the chromatophore; when they relax, energy stored in the elastic sacculus retracts it. The physiology and pharmacology of the chromatophore nerves and muscles of loliginid squids are discussed in detail. Attention is drawn to the multiple innervation of dorsal mantle chromatophores, of crucial importance in pattern generation. The size and density of the chromatophores varies according to habit and lifestyle. Differently coloured chromatophores are distributed precisely with respect to each other, and to reflecting structures beneath them. Some of the rules for establishing this exact arrangement have been elucidated by ontogenetic studies. The chromatophores are not innervated uniformly: specific nerve fibres innervate groups of chromatophores within the fixed, morphological array, producing 'physiological units' expressed as visible 'chromatomotor fields'. The chromatophores are controlled by a set of lobes in the brain organized hierarchically. At the highest level, the optic lobes, acting largely on visual information, select specific motor programmes (i.e. body patterns); at the lowest level, motoneurons in the chromatophore lobes execute the programmes, their activity or inactivity producing the patterning seen in the skin. In Octopus vulgaris there are over half a million neurons in the chromatophore lobes, and receptors for all the classical neurotransmitters are present, different transmitters being used to activate (or inhibit) the different colour classes of chromatophore motoneurons. A detailed understanding of the way in which the brain controls body patterning still eludes us: the entire system apparently operates without feedback, visual or proprioceptive. The gross appearance of a cephalopod is termed its body pattern. This comprises a number of components, made up of several units, which in turn contains many elements: the chromatophores themselves and also reflecting cells and skin muscles. Neural control of the chromatophores enables a cephalopod to change its appearance almost instantaneously, a key feature in some escape behaviours and during agonistic signalling. Equally important, it also enables them to generate the discrete patterns so essential for camouflage or for signalling. The primary function of the chromatophores is camouflage. They are used to match the brightness of the background and to produce components that help the animal achieve general resemblance to the substrate or break up the body's outline. Because the chromatophores are neurally controlled an individual can, at any moment, select and exhibit one particular body pattern out of many. Such rapid neural polymorphism ('polyphenism') may hinder search-image formation by predators. Another function of the chromatophores is communication. Intraspecific signalling is well documented in several inshore species, and interspecific signalling, using ancient, highly conserved patterns, is also widespread. Neurally controlled chromatophores lend themselves supremely well to communication, allowing rapid, finely graded and bilateral signalling.

Animals↗

PubMed1998

Influence of vision on upper limb reaching movements in patients with cerebellar ataxia.

The effects of vision on spatial and temporal characteristics of free unrestrained reaching movements of the arm were examined in 17 patients with ataxic syndromes due to degenerative disease of the cerebellum and its connections. Subjects were required to reach out and touch a visually presented target either in the dark or with the target and their finger visible. Overall, patients had prolonged reaction times and their movements were performed slower than normal. The spatial paths described by their fingertips were more circuitous, being of greater length than normal, a characteristic that was uninfluenced by visual conditions. Ataxic movements were less accurate than normal in two ways. First, there was greater spatial variability between repeat paths to the same target. The increased variability was present very early in the movement trajectory and at that stage was not influenced by visual feedback. Secondly, there were large constant errors at the end of movement, but only when moving in darkness. Patients with Friedreich's ataxia as well as those with intrinsic cerebellar degeneration showed the above abnormalities, although there were some quantitative differences between the two groups. We suggest these spatial errors arise because the cerebellum contributes either directly or indirectly to preparatory motor processes which, based on limb proprioceptive and retinal information, compute the pattern of muscle activity required to launch the limb accurately towards a target. Patients were largely successful at using visual guidance to make midflight adjustments to their movements in order to improve accuracy. This manifested as a reduction in spatial variability between repeat paths as the target was approached and a reduction in constant error. However, the visual correction mechanism did not appear normal. Under visual guidance, the end-phase of movement was often prolonged and characterized by excessive deviations or direction changes in the path. These deviations may be the expression of a visual guidance system producing corrections which themselves contain error requiring further correction. Thus, this process may be abnormal for the same reason that the initial pattern of muscle activity is misjudged.

Adult↗

PubMed2005

Saccade-vergence interactions in macaques. II. Vergence enhancement as the product of a local feedback vergence motor error and a weighted saccadic burst.

In the accompanying paper we reported that intrasaccadic vergence enhancement during combined saccade-vergence eye movements reflects saccadic dynamics, which implies the involvement of saccadic burst signals. This involvement was not predicted by the Multiply Model of Zee et al. We propose a model wherein vergence enhancement is the result of a multiplicative interaction between a weighted saccadic burst signal and a nonvisual short-latency estimate of the vergence motor error at the time of the saccade. The enhancement of vergence velocity by saccades causes the vergence goal to be approached more rapidly than if no saccade had occurred. The adjustment of the postsaccadic vergence velocity to this faster reduction in vergence motor error occurred with a time course too fast for visual feedback. This implies the presence of an internal estimate of the progress of the movement and indicates that vergence responses are under the control of a local feedback mechanism. It also implies that the vergence enhancement signal is included in the vergence feedback loop and is an integral part of the vergence velocity command. Our multiplicative model is able to predict the peak velocity of the vergence enhancement as a function of cyclopean saccadic dynamics, smooth vergence dynamics, and saccade-vergence timing with remarkable precision. It performed equally well for both horizontal and vertical saccades with very similar parameters, suggesting a common mechanism for all saccadic directions. A saccade-vergence additive model is also presented, although it would require external switching elements. Possible neural implementations are discussed.

Action Potentials↗

PubMed2002

Postural balance in children with cerebral palsy.

Postural control deficits have been suggested to be a major component of gait disorders in cerebral palsy (CP). Standing balance was investigated in 23 ambulatory children and adolescents with spastic diplegic CP, ages 5 to 18 years, and compared with values of 92 children without disability, ages 5 to 18 years, while they stood on a force plate with eyes open or eyes closed. The measurements included center of pressure calculations of path length per second, average radial displacement, mean frequency of sway, and Brownian random motion measures of the short-term diffusion coefficient, and the long-term scaling exponent. In the majority of children with CP (14 of 23) all standing balance values were normal. However, approximately one-third of the children with CP (eight of 23) had abnormal values in at least two of the six center of pressure measures. Thus, mean values for path length, average radial displacement, and diffusion coefficient were higher for participants with CP compared with control individuals with eyes open and closed (p<0.05). Mean values for frequency of sway and the long-term scaling exponent were lower for participants with CP compared with control participants (p<0.05). Increased average radial displacement was the most common (nine of 23) postural control deficit. There was no increase in abnormal values with eyes closed compared with eyes open for participants with CP, indicating that most participants with CP had normal dependence on visual feedback to maintain balance. Identification of those children with impaired standing balance can delineate factors that contribute to the patient's gait disorder and help to guide treatment.

Adolescent↗

PubMed1999

The remembered pursuit task: evidence for segregation of timing and velocity storage in predictive oculomotor control.

Regular, repeated presentation of identical constant-velocity target motion stimuli (ramps) appears to allow build up of an internal store, release of which can be used to generate anticipatory smooth pursuit prior to subsequent target onset. Here, we examine whether release of the anticipatory response can be controlled by timing cues unrelated to the motion stimulus itself. In experiment 1, the target moved in alternate directions and was exposed for 480 ms as it passed through centre; otherwise subjects were in darkness. Inter-stimulus interval (ISI) was either regular (3.6 s) or randomized (2.7-4.3 s). Presentations were given with or without audio cues that occurred at a constant cue time (CT) prior to target appearance. Even when ISI was randomized, cues could be used to generate anticipatory smooth pursuit. Eye velocity (V100) measured 100 ms after target onset (just prior to visual feedback influence) was greater with cues than without and decreased significantly as CT increased from 240-960 ms. In experiment 2, we assessed the effects of fixation between presentations and eccentricity of target starting position, using unidirectional ramps. The target was visible for 400 ms and started on, ended on or straddled the midline. Subjects held fixation on the midline until an audio cue signalled that preparation for ensuing target appearance could begin. There was no difference in V100 between starting positions or between presence/absence of fixation. In experiment 3, we compared the effects of using audio, visual or tactile cues. All types of cue evoked anticipatory smooth pursuit, but the response to the visual cue was significantly delayed compared with the others. However, V100 was not significantly different between cues. In all experiments, V100 was scaled in proportion to target velocity over the range 12.5-50 degrees /s, showing that this was a truly predictive response. The results provide evidence that timing and velocity storage can be independently controlled through different sensory channels and suggest that the two functions are probably carried out by separate neural mechanisms.

Acceleration↗

PubMed1994

An initial experience with lighted ureteral catheters during laparoscopic colectomy.

Ureteral catheters are recommended when a difficult pelvic dissection is anticipated to minimize the risk of ureteral injuries. With the introduction of laparoscopic-assisted colectomy, it has become necessary to replace tactile with visual feedback. Lighted ureteral catheters (LUC) have been advocated for major laparoscopic pelvic surgery as a means of enhancing ureteral identification. However, the use of LUC has been anecdotal to this point. We present the first series of selectively used LUC during laparoscopic-assisted colectomy. Forty-nine consecutive laparoscopic-assisted colectomies were reviewed in which patients had LUC placed or not (NC) [LUC,24(49%); NC, 25(51%)]. Indications for catheter insertion included complicated diverticular disease, previous pelvic surgery, and obesity. The need for LUC was at the discretion of the surgeon. Data collected included catheter visualization, type of procedure, operative time, operating room cost, and catheter or ureteral complications. Catheters were visualized in 5 of 6 (83%) right colectomies and 15 of 18 (83%) left colectomies. Nonvisualization was because of migration to the bladder (2 cases) and dense inflammatory reaction (2 cases). No catheter complications or ureteral injuries occurred in either group. Operative time (LUC, 192 +/- 11.3 min, NC, 161.4 +/- 9.5 min) was significantly longer in the LUC group as a result of the time for catheter insertion and the greater complexity of the case. The operating room cost was similar in the two groups (LUC, $3488.63 +/- 259.01; NC, $3537.56 +/- 313.43). The results indicate that selected use of LUC does significantly increase operating time without significantly increasing operating room cost or operative morbidity.(ABSTRACT TRUNCATED AT 250 WORDS)

Colectomy↗

PubMed1988

The preparation and production of isometric force in Parkinson's disease.

Subjects with Parkinson's disease (PD) and age-matched controls performed an isometric force production task, aiming at different target force levels without concurrent force feedback. Overall, PD subjects were as accurate as controls in attaining the target force levels, but executed the task differently. They had longer times to peak force and contraction durations, larger impulses and lower rates of force development, and force-time profiles with many more irregularities. They also initiated lower force contractions with longer latencies, unlike controls. The data suggest that PD subjects are deficient in the regulation of force and time parameters, rather than simply in force production. The ability to produce peak forces accurately limits the generality of previous assertions that PD subjects are heavily dependent on concurrent visual feedback.

Aged↗

PubMed2005

Concurrent adaptations of left and right arms to opposite visual distortions.

Previous research has shown that subjects can adapt with either arm to an opposite visual distortion, and the two adaptive states can then be used in sequence to control the respective arm. To extend this finding, we exposed the left and right arms of our subjects to opposite-directed rotations of the visual field alternately for 20 s each, and determined the time-course of adaptation, as well as aftereffects without visual feedback under uni- and bimanual conditions. Our data confirm that two adaptive states can co-exist in the sensorimotor system, one for each arm. We further found that the time-course of adaptive improvement was similar for both arms, that the improvement was present as early as the first movement after a change of arm and discordance, and that the magnitude of adaptation was similar to control data yielded by a single arm and discordance. Taken together, these findings suggest that the two adaptive states were formed concurrently, and without mutual interference. We also observed significant aftereffects. They were smaller but still appreciable under bimanual conditions; the two arms moved at the same time in different directions even though they were aimed at a common visual target. This outcome indicates that the two adaptive states were not merely of a strategic nature, but rather changed the rules by which sensory information was transformed into motor outputs; it also suggests that the two states not only co-exist, but can also be engaged concurrently in movement control. The reduced aftereffects observed under bimanual conditions can be attributed to the well-known phenomenon of bimanual coupling, which is unrelated to adaptation.

Adaptation, Physiological↗

PubMed1996

Use of an instrumented treadmill for real-time gait symmetry evaluation and feedback in normal and trans-tibial amputee subjects.

The purpose of this research was to evaluate a newly developed system for assessing and providing feedback of gait symmetry information in real time to subjects walking on a motorised treadmill (the CCF Treadmill). The advantages of the system are that it allows the rapid collection and comparison of temporal and kinetic parameters of gait for multiple successive strides, at a constant known speed, without forcing subjects to target their footsteps. Gait asymmetries of six normal (mean age 42.7 years) and six unilateral transtibial amputee subjects (mean age 41.7, and average 6.0 years using a prosthesis) were quantified. The amputee group was the reevaluated after receiving five minutes of training with each of three different types of real-time visual feedback (RTVF). Asymmetries in the measured parameters before feedback were 4.6 times greater in the amputee population than in the normal group, and were consistent with the finding of previous authors. Significant decreases in gait asymmetry were demonstrated for all forms of feedback after amputees received feedback training. Results, however, indicated that gait asymmetries for different variables are not necessarily related, and that more work needs to be done to identify those variables for which attaining a more symmetrical gait pattern is most beneficial. Further work also needs to be done to determine the long term effects of such RTVF training. The CCF Treadmill and RTVF were shown to be potentially useful tools both for defining rehabilitation targets and for quantifying patients' progress towards those goals.

Adult↗

PubMed1995

Proprioceptive control of interjoint coordination.

This paper reviews a series of experiments comparing intact controls with functionally deafferented patients to determine the role of proprioception in controlling dynamic interactions between limb segments during movement. We examine the control of hand path in a planar movement-reversal task and in a familiar three-dimensional gesture with similar biomechanical characteristics. In the planar task subjects had to move their hand out and back along a series of straight-line segments in the horizontal plane without visual feedback. The lengths and directions of the target line segments were chosen to require different amounts of shoulder motion while requiring the same elbow excursion. In controls, hand paths were, as required, straight with sharp bends at the outermost point. In patients, however, distinctive errors appeared at movement reversals, consisting of widened hand paths resulting from desynchronization in the reversals of elbow and shoulder motions. These errors reflected an inability to program elbow muscle contractions in accord with interaction torques produced at the elbow by variations in acceleration of the shoulder. The reversal errors were substantially reduced after patients had practiced for a few trials while visually monitoring movements of their arm. The improvement was not limited to the direction where they had practiced with vision, but also extended to other directions in which the elbow torques were different. This suggests that practice with vision of the arm served to improve the general rules that subjects used to plan movement, rather than simply improving the performance of a specific response. Similar to their performance on the planar task, the patients made errors in interjoint coordination during unconstrained three-dimensional gestures with movement reversals. We conclude (i) that both the planning and the learning of movement required an internal model of the dynamic properties of the limb that takes account of interaction torques acting at different joints; (ii) that this internal model is normally established and updated using proprioceptive information; but (iii) that when proprioception is lacking, vision of the limb in motion partially substitutes for proprioception.

Adult↗