Oculo-manual coordination control: Ocular and manual tracking of visual targets with delayed visual feedback of the hand motion

Summary The aim of this study was to examine coordination control in eye and hand tracking of visual targets. We studied eye tracking of a self-moved target, and simultaneous eye and hand tracking of an external visual target moving horizontally on a screen. Predictive features of eye-hand coordination control were studied by introducing a delay (0 to 450 ms) between the Subject's (S's) hand motion and the motion of the hand-driven target on the screen. In self-moved target tracking with artificial delay, the eyes started to move in response to arm movement while the visual target was still motionless, that is before any retinal slip had been produced. The signal likely to trigger smooth pursuit in that condition must be derived from non-visual information. Candidates are efference copy and afferent signals from arm motion. When tracking an external target with the eyes and the hand, in a condition where a delay was introduced in the visual feedback loop of the hand, the Ss anticipated with the arm the movement of the target in order to compensate the delay. After a short tracking period, Ss were able to track with a low lag, or eventually to create a lead between the hand and the target. This was observed if the delay was less than 250–300 ms. For larger delays, the hand lagged the target by 250–300 ms. Ss did not completely compensate the delay and did not, on the average, correct for sudden changes in movement of the target (at the direction reversal of the trajectory). Conversely, in the whole range of studied delays (0–450 ms), the eyes were always in phase with the visual target (except during the first part of the first cycle of the movement, as seen previously). These findings are discussed in relation to a scheme in which both predictive (dynamic nature of the motion) and coordination (eye and hand movement system interactive signals) controls are included.     

IMRT delivery to a moving target by dynamic MLC tracking: delivery for targets moving in two dimensions in the beam's eye view

A new modification of the dynamic multileaf collimator (dMLC) delivery technique for intensity-modulated therapy (IMRT) is outlined. This technique enables the tracking of a target moving through rigid-body translations in a 2D trajectory in the beam's eye view. The accuracy of the delivery versus that of deliveries with no tracking and of 1D tracking techniques is quantified with clinically derived intensity-modulated beams (IMBs). Leaf trajectories calculated in the target-reference frame were iteratively synchronized assuming regular target motion. This allowed the leaves defined in the lab-reference frame to simultaneously follow the target motion and to deliver the required IMB without violation of the leaf maximum-velocity constraint. The leaves are synchronized until the gradient of the leaf position at every instant is less than a calculated maximum. The delivered fluence in the target-reference frame was calculated with a simple primary-fluence model. The new 2D tracking technique was compared with the delivered fluence produced by no-tracking deliveries and by 1D tracking deliveries for 33 clinical IMBs. For the clinical IMBs normalized to a maximum fluence of 200 MUs, the rms difference between the desired and the delivered IMB was 15.6 +/- 3.3 MU for the case of a no-tracking delivery, 7.9 +/- 1.6 MU for the case where only the primary component of motion was corrected and 5.1 +/- 1.1 MU for the 2D tracking delivery. The residual error is due to interpolation and sampling effects. The 2D tracking delivery technique requires an increase in the delivery time evaluated as between 0 and 50% of the unsynchronized delivery time for each beam with a mean increase of 13% for the IMBs tested. The 2D tracking dMLC delivery technique allows an optimized IMB to be delivered to moving targets with increased accuracy and with acceptable increases in delivery time. When combined with real-time knowledge of the target motion at delivery time, this technique facilitates improved target conformality relative to no-tracking deliveries and allows PTV margin reduction.     

Hitting moving targets: effects of target speed and dimensions on movement time

To hit moving targets, one not only has to arrive at the right place but also at the right time. Moving quickly reduces spatial precision but increases temporal precision. This may explain why people usually move more quickly toward fast targets than toward slow ones, because arriving at the right time is more important when hitting fast targets. The temporal accuracy required depends not only on the target's speed but also on its length in the direction of motion; it decreases with increasing length. Here we investigate the effects of variations in the target's speed and dimensions on the subject's movement time. We asked subjects to hit targets that moved from left to right as quickly as possible with their index finger. The targets varied in length in the direction of motion (width: affecting both spatial and temporal demands), in length in the orthogonal direction (height: affecting spatial demand), and in speed (affecting temporal demand). Targets were presented in random order during one session and in blocks of trials with identical targets during another session. In the latter session subjects could optimize their strategy for each target separately. In the random condition subjects hit fast targets more quickly than slow ones. Their movement time was also affected by the target's size (the spatial demand), but not by the direction of the elongation. For the blocked condition, subjects did consider the direction of the elongation. We conclude that people do not consider an object's orientation to estimate the temporal demands of an interception task, but that they use the object's size and speed, and their experience from previous trials.     

Quantification of interplay effects of scanned particle beams and moving targets

Scanned particle beams and target motion interfere. This interplay leads to deterioration of the dose distribution. Experiments and a treatment planning study were performed to investigate interplay. Experiments were performed with moving radiographic films for different motion parameters. Resulting dose distributions were analyzed for homogeneity and dose coverage. The treatment planning study was based on the time-resolved computed tomography (4DCT) data of five lung tumor patients. Treatment plans with margins to account for respiratory motion were optimized, and resulting dose distributions for 108 different motion parameters for each patient were calculated. Data analysis for a single fraction was based on dose-volume histograms and the volume covered with 95% of the planned dose. Interplay deteriorated dose conformity and homogeneity (1-standard deviation/mean) in the experiments as well as in the treatment-planning study. The homogeneity on radiographic films was below approximately 80% for motion amplitudes of approximately 15 mm. For the treatment-planning study based on patient data, the target volume receiving at least 95% of the prescribed dose was on average (standard deviation) 71.0% (14.2%). Interplay of scanned particle beams and moving targets has severe impact on the resulting dose distributions. Fractionated treatment delivery potentially mitigates at least parts of these interplay effects. However, especially for small fraction numbers, e.g. hypo-fractionation, treatment of moving targets with scanned particle beams requires motion mitigation techniques such as rescanning, gating, or tracking.     

Coordination and control of bimanual prehension: effects of perturbing object location

The purpose of the present study was to investigate the coordination of the two effectors when one or both targets were displaced in a bimanual prehension task. Sixteen right-handed volunteers were asked to reach 20 cm to grasp and lift two cubic objects with the right and left hands. Upon initiation of the reach: (1) both objects could remain at the initial position (NN); (2) the right object could be displaced toward the subject (NJ); (3) the left object could be displaced (JN); or (4) both objects could be displaced (JJ). Generally, the results indicated that the hand moving to the perturbed object was reorganized to reach the target efficiently, but hovered to somewhat couple object lift for the two hands. In contrast, adjustments were seen in the velocity profiles of the hand moving to the non-perturbed target, including a premature deceleration phase and corrective movements to reach the target location. Together, these results indicate that when the perturbation of one object occurs during the performance of a bimanual prehension task, visual information is used to independently update the control process for the limb moving to the perturbed object. Additionally, interference causes the limb moving to the non-perturbed target to be inappropriately adjusted in response to the perturbation. Our results also indicated that perceptual and motor factors such as time allotted for the use of feedback and the direction of movement may play a role in the independence/dependence relationship between the hands during bimanual tasks. Furthermore, subjects’ expectations about the performance and goal of the task could have a further influence on the level of interference seen during bimanual movements. Finally, despite interference effects which caused multiple accelerations and decelerations, the hand moving to the non-perturbed target still achieved the target location in the same movement time as during control conditions. This final result indicates the efficiency with which subjects can reorganize both limbs in the face of altered task requirements.

Intercepting moving targets: a little foresight helps a lot

Behavioral studies suggest that humans intercept moving targets by maintaining a constant bearing angle (CBA). The purely feedback-driven CBA strategy has been contrasted with the strategy of predicting the eventual time and location of the future interception point. This study considers an intermediate anticipatory strategy of moving so as to produce a CBA a short duration into the future. Subjects controlled their speed of self-motion along a linear path through a simulated environment to intercept a moving target. When targets changed speed midway through the trial in Experiment 1, subjects abandoned an ineffective CBA strategy in favor of a strategy of anticipating the most likely change in target speed. In Experiment 2, targets followed paths of varying curvature. Behavior was inconsistent with both the CBA and the purely predictive strategy. To investigate the intermediate anticipatory strategy, human performance was compared with a model of interceptive behavior that, at each time-step t, produced the velocity adjustment that would minimize the change in bearing angle at time t + ∆t, taking into account the target’s behavior during that interval. Values of ∆t at which the model best fit the human data for practiced subjects varied between 0.5 and 3.5 s, suggesting that actors adopt an anticipatory strategy to keep the bearing angle constant a short time into the future.

Control strategies in directing the hand to moving targets

Summary We have evaluated the use of visual information about the movement of a target in two tasks tracking and interceptions — involving multi-joint reaching movements with the arm. Target velocity was either varied in a pseudorandom order (random condition) or was kept constant (predictable condition) across trials. Response latency decreased as target velocity increased in each condition. A simple model that assumes that latency is the sum of two components — the time taken for target motion to be detected, and a fixed processing time — provides a good fit to the data. Results from a step-ramp experiment, in which the target stepped a small distance immediately preceding the onset of the ramp motion, were consistent with this model. The characteristics of the first 100 ms of the response depended on the amount of information about target motion available to the subject. In the tracking task with randomly varied target velocities, the initial changes in hand velocity were largely independent of target velocity. In contrast, when the velocity was predictable the initial hand velocity depended on target velocity. Analogously, the initial changes in the direction of hand motion in the interception task were independent of target velocity in the random condition, but depended on target velocity in the predictable condition. The time course for development of response dependence was estimated by controlling the amount of visual information about target velocity available to the subject before the onset of limb movement. The results suggest that when target velocity was random, hand movement started before visual motion processing was complete. The response was subsequently adjusted after target velocity was computed. Subjects displayed idiosyncratic strategies during the catch-up phase in the tracking task. The peak hand velocity depended on target velocity and was similar for all subjects. The time at which the peak occurred, in contrast, varied substantially among subjects. In the interception task the hand paths were straighter in the predictable than in the random condition. This appeared to be the result of making adjustments in movement direction in the former condition to correct for initially inappropriate responses.     

Eye tracking of self-moved targets in the absence of vision

Summary Smooth pursuit eye movements have been described as resulting from the tracking of self-moved targets in total darkness. This study investigated the nature of the signal responsible for the release of smooth pursuit in this particular situation.Simultaneous monitoring of eye and hand positions shows that in total darkness smooth pursuit can only be released if the imagined target is either passively or actively moved by the subject's hand. An ischaemic block applied at the level of the biceps allowed us to selectively remove the afferent signal preferentially to the efferent copy in tasks involving eye tracking of an imaginary target actively or passively moved. The results show that an afferent signal was necessary and sufficient to release smooth pursuit, whereas the efferent copy alone could not trigger smooth pursuit. However, the efferent copy could play an important role in the phase relationship (prediction) between eye and finger events and in the activation of the concomitantly active saccadic system.Analysis of the eye movement characteristics, in various non-visually guided, load-affected situations, suggested that the main input to the smooth pursuit system was derived, in a non-graded way, from the position detector activation of the target-moving structure.     

Saccades to stationary and moving targets differ in the monkey

Saccade characteristics in response to moving and stationary targets were studied in three monkeys (Macaca mulatta) that had been trained to look at a target, which after an initial jump either remained in place or moved forward or backward with constant velocity (10°/s). Eye movements were recorded using a search coil. The contribution of smooth pursuit to the saccade amplitude was small (<0.25°). Saccades having the same amplitude (5.67–6.83° for different monkeys) to forward and backward moving targets were compared. Peak velocity was higher (37–42°/s on average for different monkeys) and saccade duration was shorter (8–10 ms on average) for backward saccades than for forward saccades These differences were highly significant (t-test: P<0.001). Thus, forward and backward saccades are not on the same main sequence. This suggests that saccade dynamics are affected not only by the retinal position error but also by target motion. Further analysis revealed that saccade peak velocity mainly depends on the retinal position error, but saccade amplitude also depends on a stimulus-related velocity factor, which affects the saccade mainly during deceleration. This velocity factor could be retinal slip or target velocity, which was the same under our conditions. Our results experimentally support recent models that propose that the saccade acceleration in response to moving targets might be controlled by the superior colliculus, whereas the deceleration changes are fine-tuned by the cerebellum. This prediction must still be tested on a neuronal level.Both Yanfang Guan and Thomas Eggert contributed equally     

The effect of expectations on hitting moving targets: influence of the preceding target's speed

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.     

Thresholding in PET images of static and moving targets

Continued therapeutic gain in the treatment of non-small-cell lung cancer (NSCLC) will depend upon our ability to escalate the dose to the primary tumour while minimizing normal tissue toxicity. Both these objectives are facilitated by the accurate definition of a target volume that is as small as possible. To this end, both tumour immobilizations via deep inspiratory breath-hold, along with positron emission tomography (PET), have emerged as two promising approaches. Though PET is an excellent means of defining the general location of a tumour focus, its ability to define exactly the geometric extent of such a focus strongly depends upon selection of an appropriate image threshold. However, in clinical practice, the image threshold is typically not chosen according to consistent, well-established criteria. This study explores the relationship between image threshold and the resultant PET-defined volume using a series of F-18 radiotracer-filled hollow spheres of known internal volumes, both static and under oscillatory motion. The effects of both image threshold and tumour motion on the resultant PET image are examined. Imaging data are further collected from a series of simulated gated PET acquisitions in order to test the feasibility of a patient-controlled gating mechanism during deep inspiratory breath-hold. This study illustrates quantitatively considerable variability in resultant PET-defined tumour volumes depending upon numerous factors, including image threshold, size of the lesion, the presence of tumour motion and the scanning protocol. In this regard, when using PET in treatment planning for NSCLC, the radiation oncologist must select the image threshold very carefully to avoid either under-dosing the tumour or overdosing normal tissues.     

IgG4-related lymphadenopathy and IgG4-related lymphoma: moving targets

IgG4 is the least common of all the IgG subclasses. In recent years an uncommon systemic disease characterized by variable manifestations that may include the presence of tumour-like sclerosing lesions in a variety of extranodal sites, lymphadenopathy, presence of increased numbers of IgG4+ plasma cells in affected tissues, elevated serum IgG4 level, frequent presence of autoantibodies and often, a good response to steroid therapy or Rituximab has been described. The disorder has been called IgG4-related sclerosing disease, IgG4-related systemic disease and IgG4-related autoimmune disease; currently the preferred designation is IgG4-related disease (IgG4-RD). In addition to other changes that may be found in association with IgG4-RD, some investigators have suggested that IgG4-RD may serve as a substrate for the development of lymphoma.This review focuses on lymphadenopathy associated with IgG4-RD and also explores the significance of isolated lymphadenopathy with increased numbers of IgG4+ plasma cells. Emerging data on lymphomas arising in association with IgG4-RD and the possibility of an increased risk of lymphoma in patients with IgG4-RD are also discussed.     

Saccades to moving targets in schizophrenia: Evidence for normal posterior cortex functioning

People diagnosed with schizophrenia have abnormalities of smooth pursuit eye movement initiation that could be attributable to dysfunction of posterior cortical areas and/or the smooth pursuit regions of frontal cortex. To evaluate whether schizophrenia patients' pursuit initiation performance is most consistent with pre- or postrolandic neuropathology, 25 schizophrenia patients and 25 nonpsychiatric individuals were presented step-ramp stimuli moving either away from or toward the fovea. Schizophrenia and nonpsychiatric individuals did not differ on position error of saccades to moving targets, suggesting that the schizophrenia patients did not have general difficulty with motion perception. During the initial 100 ms of smooth pursuit, however, schizophrenia patients had significantly slower eye velocities than did nonpsychiatric individuals. These results suggest that schizophrenia patients'smooth pursuit abnormalities are not associated with neuropathology of posterior cortical areas.     

Oculo-manual tracking of visual targets: control learning,coordination control and coordination model

Summary The processes which develop to coordinate eye and hand movements in response to motion of a visual target were studied in young children and adults. We have shown that functional maturation of the coordination control between eye and hand takes place as a result of training. We observed, in the trained child and in the adult, that when the hand is used either as a target or to track a visual target, the dynamic characteristics of the smooth pursuit system are markedly improved: the eye to target delay is decreased from 150 ms in eye alone tracking to 30 ms, and smooth pursuit maximum velocity is increased by 100%. Coordination signals between arm and eye motor systems may be responsible for smooth pursuit eye movements which occur during self-tracking of hand or finger in darkness. These signals may also account for the higher velocity smooth pursuit eye movements and the shortened tracking delay when the hand is used as a target, as well as for the synkinetic eye-arm motions observed at the early stage of oculo-manual tracking training in children. We propose a model to describe the interaction which develops between two systems involved in the execution of a common sensorimotor task. The model applies to the visuo-oculo-manual tracking system, but it may be generalized to other coordinated systems. According to our definition, coordination control results from the reciprocal transfer of sensory and motor information between two or more systems involved in the execution of single, goal-directed or conjugate actions. This control, originating in one or more highly specialized structures of the central nervous system, combines with the control processes normally operating in each system. Our model relies on two essential notions which describe the dynamic and static aspects of coordination control: timing and mutual coupling.     

Kinematics of fast hemiparetic aiming movements toward stationary and moving targets

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

Manual interception of moving targets I. Performance and movement initiation

 We investigated the capacities of human subjects to intercept moving targets in a two-dimensional (2D) space. Subjects were instructed to intercept moving targets on a computer screen using a cursor controlled by an articulated 2D manipulandum. A target was presented in 1 of 18 combinations of three acceleration types (constant acceleration, constant deceleration, and constant velocity) and six target motion times, from 0.5 to 2.0 s. First, subjects held the cursor in a start zone located at the bottom of the screen along the vertical meridian. After a pseudorandom hold period, the target appeared in the lower left or right corner of the screen and traveled at 45o toward an interception zone located on the vertical meridian 12.5 cm above the start zone. For a trial to be considered successful, the subject’s cursor had to enter the interception zone within 100 ms of the target’s arrival at the center of the interception zone and stay inside a slightly larger hold zone. Trials in which the cursor arrived more than 100 ms before the target were classified as ”early errors,” whereas trials in which the cursor arrived more than 100 ms after the target were classified as ”late errors.” Given the criteria above, the task proved to be difficult for the subjects. Only 41.3% (1080 out of 2614) of the movements were successful, whereas the remaining 58.7% were temporal (i.e., early or late) errors. A large majority of the early errors occurred in trials with decelerating targets, and their percentage tended to increase with longer target motion times. In contrast, late errors occurred in relation to all three target acceleration types, and their percentage tended to decrease with longer target motion times. Three models of movement initiation were investigated. First, the threshold-distance model, originally proposed for optokinetic eye movements to constant-velocity visual stimuli, maintains that response time is composed of two parts, a constant processing time and the time required for the stimulus to travel a threshold distance. This model only partially fit our data. Second, the threshold-τ model, originally proposed as a strategy for movement initiation, assumes that the subject uses the first-order estimate of time-to-contact (τ) to determine when to initiate the interception movement. Similar to the threshold distance model, the threshold-τ model only partially fit the data. Finally, a dual-strategy model was developed which allowed for the adoption of either of the two strategies for movement initiation; namely, a strategy based on the threshold-distance model (”reactive” strategy) and another based on the threshold-τ model (”predictive” strategy). This model provided a good fit to the data. In fact, individual subjects preferred to use one or the other strategy. This preference was allowed to be manifested at long target motion times, whereas shorter target motion times (i.e., 0.5 s and 0.8 s) forced the subjects to use only the reactive strategy. Received: 5 July 1996 / Accepted: 4 March 1997     

Accuracies of saccades to moving targets during pursuit initiation and maintenance

The overall goals of the studies presented here were to compare (1) the accuracies of saccades to moving targets with either a novel or a known target motion, and (2) the relationships between the measures of target motion and saccadic amplitude during pursuit initiation and maintenance. Since resampling of position error just prior to saccade initiation can confound the interpretation of results, the target ramp was masked during the planning and execution of the saccade. The results suggest that saccades to moving targets were significantly more accurate if the target motion was known from the early part of the trial (e.g., during pursuit maintenance) than in the case of novel target motion (e.g., during pursuit initiation); both these types of saccades were more accuate than those when target motion information was not available. Using target velocity in space as a rough estimate of the magnitude of the extra-retinal signal during pursuit maintenance, the saccadic amplitude was significantly associated with the extra-retinal target motion information after accounting for the position error. In most subjects, this association was stronger than the one between retinal slip velocity and saccadic amplitude during pursuit initiation. The results were similar even when the smooth eye motion prior to the saccade was controlled. These results suggest that different sources of target motion information (retinal image velocity vs internal representation of previous target motion in space) are used in planning saccades during different stages of pursuit. The association between retinal slip velocity and saccadic amplitude is weak during initiation, thus explaining poor saccadic accuracy during this stage of pursuit.