Target capture strategy selection in a simulated marksmanship task.

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.

Coordination of reach-to-grasp in physical and haptic-free virtual environments.

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.

Magnifying vision improves motor performance in individuals with stroke.

Increasing perceived hand size using magnifying lenses improves tactile discrimination and motor performance in neurologically-intact individuals. We tested whether magnification of the hand can improve motor function in individuals with chronic stroke. Twenty-five individuals with a history of stroke more than 6 months prior to testing underwent a series of tasks exploring different aspects of motor performance (grip force, finger tapping, reaching and grasping, and finger matching) under two visual conditions: magnified or normal vision. Performance was also assessed shortly after visual manipulation to test if these effects persisted. Twenty-eight percent of individuals showed an immediate significant improvement averaged across all tasks with magnification; similar beneficial responses were also observed in 32% of individuals after a short delay. These results suggest that magnification of the image of the hand may be of utility in rehabilitation of individuals with stroke.

When perception trips action! The increase in the perceived size of both hand and target matters in reaching and grasping movements.

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.

Coordination of pincer grasp and transport after mechanical perturbation of the index finger.

Our understanding of reach-to-grasp movements has evolved from the original formulation of the movement as two semi-independent visuomotor channels to one of interdependence. Despite a number of important contributions involving perturbations of the reach or the grasp, some of the features of the movement, such as the presence or absence of coordination between the digits during the pincer grasp and the extent of spatio-temporal interdependence between the transport and the grasp, are still unclear. In this study, we physically perturbed the index finger into extension during grasping closure on a minority of trials to test whether modifying the movement of one digit would affect the movement of the opposite digit, suggestive of an overarching coordinative process. Furthermore, we tested whether disruption of the grasp results in the modification of kinematic parameters of the transport. Our results showed that a continuous perturbation to the index finger affected wrist velocity but not lateral displacement. Moreover, we found that the typical flexion of the thumb observed in nonperturbed trials was delayed until the index finger counteracted the extension force. These results suggest that physically perturbing the grasp modifies the kinematics of the transport component, indicating a two-way interdependence of the reach and the grasp. Furthermore, a perturbation to one digit affects the kinematics of the other, supporting a model of grasping in which the digits are coordinated by a higher-level process rather than being independently controlled.NEW & NOTEWORTHY A current debate concerning the neural control of prehension centers on the question of whether the digits in a pincer grasp are controlled individually or together. Employing a novel approach that perturbs mechanically the grasp component during a natural reach-to-grasp movement, this work is the first to test a key hypothesis: whether perturbing one of the digits during the movement affects the other. Our results support the idea that the digits are not independently controlled.

Disruption of activity in the ventral premotor but not the anterior intraparietal area interferes with on-line correction to a haptic perturbation during grasping.

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.

NeuroLab: A Set of Graphical Computer Simulations to Support Neuroscience Instruction at the High School and Undergraduate Level.

Young students struggle with concepts that involve the parallel activity of large numbers of similar entities, precisely the kind of concepts that abound in neuroscience. While a direct experience to laboratory work cannot be replaced, such activities include a steep learning curve and may be impractical in certain course settings. This article describes a set of computer simulations of a number of neural processes using NetLogo (Wilensky, 1999), a software environment for the design and implementation of multi-agent simulations that has an intuitive graphical interface and minimal learning curve. NeuroLab is a group of graphical simulations that portray ions, molecules, synapses or cells as individual recognizable agents with particular behaviors, depending on the level at which the particular process is simulated. On a typical assignment, students run the simulation a few times manipulating specific variables by means of buttons, switches and sliders and observe the results of their manipulations on the main window. Many simulations include one or more plots that help visualize statistical data in real time and allow for the testing of experimental hypotheses. Students may repeat the simulation as many times as they wish and collect data or answer questions based on their observations. Assignments may take just a few minutes to perform, but could conceivably be part of more involved activities as designed by the instructor.

Hand preshaping in Parkinson's disease: effects of visual feedback and medication state.

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.

Deficits in the evolution of hand preshaping in Parkinson's disease.

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.

Effects of object shape and visual feedback on hand configuration during grasping.

Normal subjects gradually preshape their hands during a grasping movement in order to conform the hand to the shape of a target object. The evolution of hand preshaping may depend on visual feedback about arm and hand position as well as on target shape and location at specific times during the movement. The present study manipulated object shape in order to produce differentiable patterns of finger placement along two orthogonal "dimensions" (flexion/extension and abduction/adduction), and manipulated the amount of available visual information during a grasp. Normal subjects were asked to reach to and grasp a set of objects presented in a randomized fashion at a fixed spatial location in three visual feedback conditions: Full Vision (both hand and target visible), Object Vision (only the object was visible but not the hand) and No Vision (vision of neither the hand nor the object during the movement). Flexion/extension angles of the metacarpophalangeal and proximal interphalangeal joints of the index, ring, middle and pinkie fingers as well as the abduction/adduction angles between the index-middle and middle-ring fingers were recorded. Kinematic analysis revealed that as visual feedback was reduced, movement duration increased and time to peak aperture of the hand decreased, in accord with previously reported studies. Analysis of the patterns of joint flexion/extension and abduction/adduction per object shape revealed that preshaping based on the abduction/adduction dimension occurred early during the reach for all visual feedback conditions (approximately 45% of normalized movement time). This early preshaping across visual feedback conditions suggests the existence of mechanisms involved in the selection of basic hand configurations. Furthermore, while configuration changes in the flexion/extension dimension resulting in well-defined hand configurations occurred earlier during the movement in the Object Vision and No Vision conditions (45%), those in the Full Vision condition were observed only after 75% of the movement, as the moving hand entered the central region of the visual field. The data indicate that there are at least two control mechanisms at work during hand preshaping, an early predictive phase during which grip selection is attained regardless of availability of visual feedback and a late responsive phase during which subjects may use visual feedback to optimize their grasp.