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This paper examines how individuals track targets that move in relatively unpredictable trajectories. Gaze and behavioural data were captured as twenty two participants learned a simulated competitive marksmanship task known colloquially as the Death Star over six training days. Participants spontaneously selected one of two consistent target-tracking strategies with approximately equal probability. Participants employed either chasing behaviour, in which gaze follows a target's trajectory before a shot, or ambushing behaviour, wherein gaze anticipates the trajectory and the participant intercepts a moving target predictively. All participants improved in task performance measures (completion time and number of shots), but did so at the expense of accuracy in missed shot attempts. Surprisingly, neither behavioural strategy offered a significant advantage in task performance measures, indicating that either may be equally effective in tackling a hand-eye coordination task with complex target motion such as the Death Star.

Background: Virtual reality (VR) offers unprecedented opportunity as a scientific tool to study visuomotor interactions, training, and rehabilitation applications. However, it remains unclear if haptic-free hand-object interactions in a virtual environment (VE) may differ from those performed in the physical environment (PE). We therefore sought to establish if the coordination structure between the transport and grasp components remain similar whether a reach-to-grasp movement is performed in PE and VE., Method: Reach-to-grasp kinematics were examined in 13 healthy right-handed young adults. Subjects were instructed to reach-to-grasp-to-lift three differently sized rectangular objects located at three different distances from the starting position. Object size and location were matched between the two environments. Contact with the virtual objects was based on a custom collision detection algorithm. Differences between the environments were evaluated by comparing movement kinematics of the transport and grasp components., Results: Correlation coefficients, and the slope of the regression lines, between the reach and grasp components were similar for the two environments. Likewise, the kinematic profiles of the transport velocity and grasp aperture were strongly correlated across the two environments. A rmANOVA further identified some similarities and differences in the movement kinematics between the two environments - most prominently that the closure phase of reach-to-grasp movement was prolonged when movements were performed in VE., Conclusions: Reach-to-grasp movement patterns performed in a VE showed both similarities and specific differences compared to those performed in PE. Additionally, we demonstrate a novel approach for parsing the reach-to-grasp movement into three phases- initiation, shaping, closure- based on established kinematic variables, and demonstrate that the differences in performance between the environments are attributed to the closure phase. We discuss this in the context of how collision detection parameters may modify hand-object interactions in VE. Our study shows that haptic-free VE may be a useful platform to study reach-to-grasp movements, with potential implications for haptic-free VR in neurorehabilitation.

Reaching and grasping movements rely on visual information regarding the target characteristics (e.g. size) and the hand position during the action execution. Changes in the visual representation of the body (e.g. increase in the perceived size of the hand) can modify action performance, but it is still unclear how these modifications interact with changes in the external environment. We investigated this topic by manipulating the perceived size of both hand and target objects and the degree of visual feedback available during the movement execution. Ten young adults were asked to reach and grasp geometrical objects in four different conditions: (i) with normal vision with the light on, (ii) with normal vision in the dark, (iii) using magnifying lenses in the light and (iv) using magnifying lenses in the dark. In contrast with previous works, our results show that movement execution is longer in magnified vision compared to normal when the action is executed in the light, but the grasping component was not affected by changes in size in this condition. On the contrary, when the visual feedback of the hand was removed and participants performed the action in the dark, movements were faster and the distances across fingers larger in the magnified than normal vision. This pattern of data suggests that grasping movements adapt rapidly and compensate for changes in vision when this process depends on the degree of visual feedback and/or environmental cues available. In the debate regarding the dissociation between action and perception, our data suggest that action may overcome changes in perception when visual feedback is available, but perception may trick action in situations of reduced visual information., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Replanning ongoing movements following perturbations requires the accurate and immediate estimation of the motor response based on sensory input. Previous studies have used transcranial magnetic stimulation (TMS) in humans to demonstrate the participation of the anterior intraparietal sulcus (aIPS) and ventral premotor cortex (PMv) in visually mediated state estimation for grasping. Here, we test the role of parietofrontal circuits in processing the corrective responses to haptic perturbations of the finger during prehension. Subjects reached to grasp an object while having to compensate for a novel and unpredictable haptic perturbation of finger extension. TMS-based transient disruptions to the PMv and aIPS were delivered 0, 50, or 100 ms after the perturbation. TMS to the PMv delivered 50 ms after the perturbation (but not 0 or 100 ms, or in unperturbed trials) led to an overestimation of grasp aperture. No effects on grasp aperture were noted for the aIPS. Our results indicate that the PMv (but not aIPS) is involved in the deployment of the compensatory response in the presence of haptic perturbations during prehension. Our data also identify the time window of neural processing in the PMv when reprogramming occurs to be 50-100 ms following the perturbation onset., (Copyright © 2015 the authors 0270-6474/15/352112-06$15.00/0.)

While the process of hand preshaping during grasping has been studied for over a decade, there is relatively little information regarding the organization of digit contact timing (DCT). This dearth of information may be due to the assumption that DCT while grasping exhibits few regularities or to the difficulty in obtaining information through traditional movement recording techniques. In this study, we employed a novel technique to determine the time of digit contacts with the target object at a high precision rate in normal healthy participants. Our results indicate that, under our task conditions, subjects tend to employ a radial to ulnar pattern of DCT which may be modulated by the shape of the target object. Moreover, a number of parameters, such as the total contact time, the frequency of first contacts by the thumb and index fingers and the number of simultaneous contacts, are affected by the relative complexity of the target object. Our data support the notion that a great deal of information about the object's physical features is obtained during the early moments of the grasp.

Several studies have found that Parkinson's disease (PD) disrupts the organization of complex motor sequences regardless of the influence of parkinsonian medications. A clear candidate for the neural bases of such deficits, which we term "coordinative," is the failure to integrate propioceptive and visual information by cortico-striatal circuits in a timed fashion. Recent reports, however, have indicated that deep-brain stimulation of the subthalamic nucleus (STN DBS) may result in an improvement in coordinative deficits beyond the amelioration of "intensive deficits" such as bradykinesia and scaling errors. The present study examined the spatio-temporal organization underlying the shaping of the hand during reaching to grasp objects differing in shape. Six PD patients ON and OFF their STN DBS when OFF their concomitant medications and six age-matched controls participated in this study. STN DBS improved the coordination involved in preshaping the hand while grasping. We discuss these results in light of our earlier work with PD patients on and off dopamine replacement therapy.

Previous studies in our laboratory examining pointing and reach-to-grasp movements of Parkinson's disease patients (PDPs) have found that PDPs exhibit specific deficits in movement coordination and in the sensorimotor transformations required to accurately guide movements. We have identified a particular difficulty in matching unseen limb position, sensed by proprioception, with a visible target. In the present work, we further explored aspects of complex sensorimotor transformation and motor coordination using a reach-to-grasp task in which object shape, visual feedback, and dopaminergic medication were varied. Normal performance in this task requires coordinated generation of appropriate reach, to bring the hand to the target, and differentiated grasp, to preshape the hand congruent with object form. In Experiment 1, we tested PDPs in the off-medication state. To examine the dependence of subjects on visual feedback and their ability to implement intermodal sensory integration, we required them to reach and grasp the target objects in three conditions: (1) Full Vision, (2) Object Vision with only the target object visible and, (3) No Vision with neither the moving arm nor the target object visible. PDPs exhibited two types of deficits. First, in all conditions, they demonstrated a generalized slowing of movement or bradykinesia. We consider this an intensive deficit, since it involves largely a modulation of the gain of specific task parameters: in this case, velocity of movement. Second, they were less able than controls to extract critical proprioceptive information and integrate it with vision in order to coordinate the reach and grasp components of movement. These deficits which involve the coordination of different inputs and motor components, we classify as coordinative deficits. As in our previous work, the PDPs' deficits were most marked when they were required to use proprioception to guide their hand to a visible target (Object Vision condition). But even in the full-vision condition, their performance only became fully accurate when both the target and effector (hand) were simultaneously visible. In Experiment 2, PDPs were tested on their dopaminergic replacement therapy. Dopaminergic treatment significantly ameliorated the bradykinesia of the PDPs, but produced no changes in the hand preshaping deficiencies of PDPs. These results suggest that adequate treatment of the PDPs may more readily compensate for intensive, than coordinative deficits, since the latter are likely to depend on specific and time-dependent neural interdependencies that are unlikely to be remediated simply by increasing the gain of a pathway.

Parkinson's disease (PD) results in various types of motor impairments including bradykinesia, tremor and rigidity. Recent research has implicated more fundamental processes at the source of the observed motor deficits. Among these, problems in the sequencing and/or timing of complex movements and in the execution of internally-guided tasks. Furthermore, PD patients exhibit procedural learning deficits which may complicate the interpretation of experimental results of studies involving novel sensorimotor tasks. The reach-to-grasp movement is a complex, overlearned sensorimotor task consisting of two semi-independent components, a relatively simple reach or transport phase and a more complex manipulation or prehension phase. In the present study, we used a novel technique in order to study the evolution of hand preshaping during the reach-to-grasp movement of PD patients and age-matched controls to objects of different shapes in three different spatial locations. Our results indicate that while PD patients are able to specify movement direction as well as controls, their hand preshaping exhibits substantial impairments. Other prehension measures, such as the time to peak aperture (TPA), indicate that PD patients delayed execution of the grasp until visual feedback of their hand was available. Overall, our results suggest that PD patients' internal guidance processes are severely disrupted, having to rely on visual feedback in order to modulate their hand shape to fit the contours of the target objects during a reach-to-grasp movement.