Functional relationships between grasp and transport components in a prehension task

doi:10.1016/0167-9457(90)90025-9

Human Movement Science

Volume 9, Issue 2, April 1990, Pages 149-176

https://doi.org/10.1016/0167-9457(90)90025-9 Get rights and content

Abstract

Prehension in adults is a highly developed motor skill that affords the study of how components of a movement are coordinated to produce the near endless variety of acts that serve to acquire objects in near body space. This study used three-dimensional movement analysis to describe the kinematic characteristics and coordination of the transport (the reach) and the manipulation (the grasp) components of prehension. Six subjects reached for and grasped 10 different sized wooden disks. Results indicated that the initial phase of the prehension movement could be considered as structured in advance of the movement in that, over the ten disk sizes, time of limb transport was a constant up to peak deceleration. The time after peak deceleration to object contact, however, increased as disk size decreased. This led to the movement trajectories being uniquely shaped for each disk size and as such did not support a proportional duration based model of motor programming. Other findings indicated that maximum grip aperture was reached progressively sooner as disk size was decreased and that maximum grip aperture was highly related to the size of the to-be-grasped disk. In terms of the coordination between the transport and grasp components, support was not found for a temporal linkage between them nor did their coordination appear to be dependent on a motor program. Rather, from an analysis of the spatial variability of the transport and grasp components, evidence was found for supporting the idea that the coordination between these two components was achieved by a sensorimotor process. Through this process, and given the goal of a prehension movement, it was argued that the two components are linked functionally rather than temporally or spatially. These results are discussed in terms of current sensorimotor models of motor control and prehension.

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Citation Excerpt :
Importantly, as already mentioned in section 1.1, the size of the object significantly modulates the grip aperture from the very beginning (∼10%) of the transportation phase [22]. The amplitude of the maximum grip aperture covaries linearly with the object size, i.e. the larger the object the wider the aperture [211,308,165,281,282,88,22], although an interesting modulation related to the ultimate goal of the actions sequence – i.e. grasp-to-eat vs. grasp-to-place identical targets (e.g., see Flindall & Gonzalez [102]) – has been reported as well. Grip aperture is gradually modulated also by the object shape [286], especially whenever undesirable collisions with some of its parts have to be avoided: The higher this risk is, the greater will be the maximum grip aperture (Cuijpers et al. [72]; Verheij et al. [328,327]).
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People with a visual impairment often find it difficult to detect information about distant objects they may want to grasp. To overcome this difficulty, we developed a sensory substitution glove that facilitates non-visual grasping. The glove includes two vibration motors, one on the index finger and one on the thumb. The motors vibrate whenever the corresponding finger points toward the object. The vibration intensity increases when the hand approaches the target. Three experiments were performed with the glove, with blindfolded participants. Experiment 1 tested the ability of participants to point to cylindrical targets. The absolute angular error (1.38°) was lower than the angular size of the object (3.82°). In Experiment 2, participants aimed to grasp differently sized cylinders at different distances. They were successful in 83% of the trials. The observed movements showed distinguishable reaching and grasping phases. Experiment 3 manipulated the direction, size, and distance of the targets, hence representing a more real-world situation. In this case, the action was completed successfully in 93% of the trials. An orientation phase preceded the reaching and grasping components. Oscillatory explorations were observed in all experiments, permitting the detection of the information that is needed to successfully complete the action.
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Whether the visuomotor coding of size in grasping obeys Weber's law is currently debated. Following up on previous work from our laboratory, here we investigated the precision associated with the maximum in-flight index-thumb aperture (MGA) in grasping small-to-medium sized objects. We report three main findings. First, grasp preparation was longer with 5 mm objects and became increasingly faster as object size increased from 10 to 20–40 mm. Second, MGA variable errors increased as sizes increased from 5 to 10–20 mm, whereas they decreased as size reached 40 mm. Third, MGA distributions were symmetrical with 5 mm objects, but became increasingly right-skewed as size increased. These results, as well as a re-analysis of previous findings, suggest that the precision of visuomotor representations varies as a function of size, consistent with the key principle underlying Weber's law. However, a fundamental constraint on precision grips (the MGA must always exceed physical size) changes the skew of the distribution and reduces the variability of MGAs as size increases from very small to medium.
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There is evidence that seeing a graspable object automatically elicits a preparatory motor process. However, it is unclear whether this implicit visuomotor process might influence the preparation of a successive grasp for a different object. We addressed the issue by implementing a combined behavioural and electrophysiological paradigm. Participants performed pantomimed grasps directed to small or large disks with either a two (pincer) or a five-finger (pentapod) grip, after the presentation of congruent (same size) or incongruent (different size) distractor disks. Preview reaction times (PRTs) and response-locked lateralized readiness potentials (R-LRPs) were recorded as online indices of motor preparation. Results revealed asymmetric effects of the distractors on PRTs and R-LRPs. For pincer grip disks, incongruent distractors were associated with longer PRTs and a delayed R-LRP peak. For pentapod grip disks, conversely, incongruent distractors were associated with shorter PRTs and a delayed R-LRP onset. Supporting an interpretation of these effects as tapping into motor preparation, we did not observe modulations of stimulus-locked LRP's (sensitive to sensory processing), or of the P300 component (related to reallocating attentional resources). These results challenge models (i.e., the “dorsal amnesia” hypothesis) which assume that visuomotor information presented before a grasp will not affect how we later perform that grasp.
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Higher-order somatosensory areas of the cerebral cortex have been divided anatomically and functionally into two distinct pathways: a ventral stream for object recognition by touch and a dorsal stream for monitoring actions of the hand and arm. This chapter reviews the anatomy and physiology of the dorsal stream for touch mediated through the posterior parietal cortex (PPC). Neurons in PPC integrate information from the somatic senses of touch and proprioception as well as from the visual system to initiate goal-directed actions of the hand and arm. They use the multisensory information to plan and monitor reach and grasp behaviors as humans and nonhuman primates interact with objects in the workspace. Reaching and grasping behaviors are mediated by distinct neural networks in medial and lateral zones of PPC that project to dorsal and ventral premotor cortices, but these behaviors are temporally coordinated by anatomical connections between the two zones. The timing of neural responses in PPC suggests that it receives convergent central and peripheral signals, allowing it to integrate and plan intended movements in a variety of coordinate systems and monitor their success or failure. Anatomical connections between the hand representations in PPC of the left and right hemispheres enable bilateral coordination of the two hands. Finally, active touch paradigms designed to acquire information needed for tactile discrimination use PPC circuits to direct the hand toward salient features of objects. The findings obtained from neurophysiological single unit studies, and functional imaging of the human brain, are consistent with the sensory and motor deficits seen in patients with damage to PPC.
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Functional relationships between grasp and transport components in a prehension task

Abstract

Human Movement Science

Control of complex motor gestures: Orofacial muscle responses to load perturbations of lip during speed

Journal of Neurophysiology

Control of multimovement coordination: Sensorimotor mechanisms in speech motor programming

Journal of Motor Behavior

Perceptual structures and distributed motor control

Schemas for the temporal organization of behavior

Human Neurobiology

Coordinated control programs for movements of the hand

Experimental Brain Research Supplement

Movement control characteristics of aiming responses

Ergonomics

Coordination of three-joint digit movements for rapid finger-thumb grasp

Journal of Neurophysiology

The information capacity of the human motor system in controlling the amplitude of movement

Journal of Experimental Psychology

Neural control of muscle length and tension

Opposition space as a structuring concept for the analysis of skilled hand movements

Intersegmental coordination during reaching at natural visual objects

The timing of natural prehension movements

Journal of Motor Behavior

Visuomotor mechanisms in reaching within extrapersonal space