Hand Aperture Research Articles

The dynamics of coordinated reaching and grasping: Scanning prehension properly

Prehension is the act of coordinated reaching and grasping. S.A. Wallace, E. Stevenson, A. Spear, D.L. Weeks [Hum. Movement Sci. 13 (1994) 255–289] proposed to assess the stability of this coordination by scanning the dynamics of prehension. The scanning method proposed by these authors prompted participants to position their peak hand aperture at different locations along the trajectory. Comparing the actual performance with the required performance, Wallace and colleagues concluded that there was a single stable pattern of coordination, albeit different for each individual. Here, we show that the method developed by Wallace et al. (1994, loc. cit.), and later used by C. Button, S. Bennett, K. Davids [Hum. Movement Sci. 17 (1998) 801–820] was flawed. We propose two potential fixes to the method and report a replication of the original experiment. Interpretation of the data with a corrected scanning technique showed that the method does not seem to be able to result in an unambiguous and precise assessment of the locus of stability.

Discriminating the role of binocular information in the timing of a one-handed catch

Previous work using prediction-motion tasks has indicated that time-to-contact estimates are based exclusively on binocular information when the approaching object is small. The aim of this study was to examine the influence of object size on the use of binocular information sources in specifying time-to-contact in one-handed catching. Subjects (n = 10) were required to time their grasp to catch different-sized balls (60, 80, and 100 mm in diameter) approaching with a constant spatial trajectory and constant velocity of 2.0 m/s. Binocular information was manipulated with a telestereoscope that increased the individuals' effective interocular separation. Subjects performed six trials with each of the different-sized balls in normal and telestereoscopic viewing conditions. It was found that subjects closed the hand earlier when catching the small and medium balls under telestereoscopic viewing compared with normal viewing; furthermore, that subjects closed their hand earlier for the small and medium balls compared with the large ball under telestereoscopic viewing. With regard to hand aperture, there was an effect of ball size, with the hand being opened to a wider aperture for each increase in ball size. Subjects also opened the hand to a wider aperture when catching the small and medium balls under telestereoscopic viewing. These findings are consistent with the notion of an increased reliance on the use of binocular information when the approaching object is small. Furthermore, they indicate that binocular and monocular sources of information are not used in isolation, but rather in combination to support the timing of one-handed catching.

The effect of wrist angle on electrically evoked hand opening in patients with spastic hemiplegia.

This paper studied the effect of wrist angle on the amount of hand opening achieved by electrical stimulation in people with spastic hemiplegia. With their forearm in pronation, subjects were asked to relax while their affected wrist was passively moved in steps of about 15 degrees from full flexion into extension. Trains of stimuli were applied to the long finger extensor muscles through surface electrodes on the forearm. At each wrist position stimulation was turned on for a few seconds until hand opening equilibrated. Wrist angle and fingertip positions were recorded using a three-dimensional (3-D) motion analysis system. Maximal displacements between thumbtip and each fingertip occurred when the wrist was fully flexed. As the wrist was extended, hand aperture achieved by electrical stimulation progressively declined, reaching zero at 40 degrees of wrist extension. We conclude that electrical stimulation can significantly increase the grasp aperture of the hemiplegic hand, but this is strongly dependent on wrist posture and accompanying voluntary effort.

Prehension With the Ipsilesional Hand After Unilateral Brain Damage

Sensorimotor deficits in the hand ipsilateral to a brain lesion have been reported in different motor tasks. We evaluated performance of the ipsilesional hand in 12 patients with either left (LBD) or right brain damage (RBD) by kinematic analysis in order to precisely characterize possible deficits in the two components of prehension (transport and grasp). Both patient groups exhibited performance deficits in the main kinematic parameters, e.g., reduced velocity of the transport component and prolonged movement time. However, while LBD patients showed a more general slowing, RBD patients prolonged in particular the last phase of the movement toward the object. We suggest that relevant visuospatial representations and the adequate mapping of motor processes may be impaired after RBD. In contrast, LBD caused a more unspecific disturbance pattern, supporting the view that the precise parameterization of motor programs is impaired. Maximum grip aperture was normal in both patient groups. However, since aperture could be biased by slowed movement, the notion that the grasp component was preserved remains speculative. The patient's ability to scale the maximum velocity of the transport component to adapt to changes in movement amplitude and to scale the maximum hand aperture of the grasp component to adapt to object size was preserved in both groups. Thus both hemispheres can have competence for this scaling mechanism.

Grasping in Children with and without Cerebral Palsy: A Kinematic Analysis

This study investigates how children with and without cerebral palsy (CP) adapt their hand grasp to changes in the size of the object involved in a prehension task. Specific questions include: does the maximum hand aperture (MHA) change synchronously with the size of the object to be grasped? Does the shape of the hand aperture form during the transportation phase to the object? In what way do children with CP differ from typically developing children? Thirteen children with spastic CP and 13 children with typical development participated in this study. Participants were engaged in a game that involved reaching to grasp wooden spheres of two sizes. Grasping movements were measured kinematically using a three-dimensional movement analysis system, Optotrak. Children in both groups exhibited larger hand apertures when grasping large objects than when grasping smaller objects. Furthermore, children in both groups reached their MHA earlier during the transport phase when grasping the smaller object than when grasping the larger one. Children with CP tended to have more exaggerated hand apertures and reached MHA earlier than children in the typically developing group. Finally, most of the children with CP in this sample possess the anticipatory control mechanism necessary for adapting grasping movements to objects of different size.

On the Hand Transport Component of Prehensile Movements

Jeannerod (1981) proposed that prehensile movements involve two independent visuomotor channels that are responsible for hand transport and hand aperture. In many studies, the movement of a marker placed on the wrist has been used as an index of hand transport because wrist movement is unaffected by the movements of the digits responsible for hand aperture. In the present study, the spatial paths of the wrist, index finger, and thumb of 5 adults, each performing 50 reaching movements, were measured with a WATSMART movement tracking system, and their variability was analyzed. The measures of movement variability suggest that the motor system is more concerned with thumb position than with wrist position during hand transport. Although the wrist is a technically convenient index of hand transport, the thumb may be a more appropriate index from the point of view of motor control.

What can and what cannot be adjusted in the movement patterns of cerebellar patients?

This commentary reviews the case of a patient who could alter the coordination of her prehensile movements when removal of visual feedback reduced her kinetic tremor, but could not coordinate her hand aperture with her hand transport within a single movement. This suggests a dissociation between different subtypes of cerebellar context-response linkage, rather than a single, general association function, [THACH]

Coordination of aimed movements in a case of unilateral cerebellar damage

We have studied multi-joint arm movements in a patient with right cerebellar damage. Our patient's kinetic tremor was reduced in the absence of visual feedback, but increased when performing a concurrent cognitive task. We suggest that her kinetic tremor reflects use of a slow cortical feedback circuit for error-corrections during movement. Analysing the spatial relation between hand aperture and hand transport in prehensile movements, we found preserved strategic, trial-to-trial coordination, but impaired reactive, within-trial coordination. We conclude that the proprioceptive representations provided by the normal cerebellum play an important role in coordinating multi-joint movement.

The speed-accuracy trade-off in manual prehension: effects of movement amplitude, object size and object width on kinematic characteristics.

Earlier studies have suggested that the size of an object to be grasped influences the time taken to complete a prehensile movement. However, the use of cylindrical objects in those studies confounded the effects of object size-extent orthogonal to the reach axis-and object width-extent along the reach axis. In separating these effects, the present study demonstrates that movement time is not affected by manipulation of object size, as long as the latter does not approach the maximal object size that can be grasped. Object width, on the other hand, is shown to exert a systematic influence on movement time: Smaller object widths give rise to longer movement times through a lengthening of the deceleration phase of the movement, thus reproducing the effect of target width on the kinematics of aiming movements. As in aiming, movement amplitude also affects the movement time in prehension, influencing primarily the acceleration phase (i.e. peak velocity attained). The effects of object width and movement amplitude were found to combine in a way predicted by Fitts' law, allowing a generalisation of the latter to the transport component in prehensile actions. With respect to the grasp component, both object size and object width are shown to affect peak hand aperture. Increasing object width thus lowers the spatial accuracy demands on the transport component, permitting a faster movement to emerge. At the same time, the hand opens to a larger grip in order to compensate for eventual directional errors that result.(ABSTRACT TRUNCATED AT 250 WORDS)

Remote responses to perturbation in human prehension

This experiment investigated how the control of hand transport and of hand aperture are coordinated in reaching and grasping movements. An electric actuator attached to the subject's right arm delivered mechanical perturbations randomly during a quarter of the experimental trials. A remote, compensatory adjustment of hand aperture followed the immediate, mechanical affects of the perturbation of hand transport. The invariant spatial and temporal characteristics of the response suggest how the two systems may be coordinated during prehension.

Influence of different types of grasping on the transport component of prehension movements

The main aim of the present study was to clarify whether different types of grasping may affect the transport component of prehension movements. To this purpose two experiments were carried out. In the first experiment the kinematics of arm movements (transport and manipulation components) were studied in eight normal subjects instructed to reach and grasp different objects located either 20 or 30 cm from their hand. The objects employed required two different types of grip: prehension with the whole hand and prehension with the index finger and the thumb (precision grip). In the second experiment subjects were instructed to point to the same objects employed in the first experiment. This experiment served as a control for the precision requirements related to the object size. The results showed that, once the precision requirements were taken into account, the transport component remained unmodified with the different types of grip. The time course of the manipulation component and its temporal relations with the transport component changed with the type of grasping. The maximal hand aperture was reached earlier in the precision grip than in the whole hand prehension and the temporal coupling with the transport component was weaker in the former condition than in the latter. The data are interpreted as further evidence in favour of independence between the transport and the manipulation “channels”.