Trends in Neurosciences
The cutaneous contribution to adaptive precision grip
Section snippets
Precision grip as an experimental model for skilled movement control
The ability to grasp an object precisely between the thumb and index finger enables humans and some other primates to perform a wide range of complex and delicate hand movements. Flexibility is an essential aspect of the dexterous manipulation of small and delicate objects, and the skilful use of tools extends the functions of the hand still further. The biomechanical advantage of an opposable thumb, combined with direct cortical control and abundant sensory feedback, enable an amazing
Precision grip as model for predictive control
Prediction of the consequences of a motor command is a key feature of volitional motor control [10]. An important example of such prediction is the anticipatory increase in grip force that occurs with self-initiated lifting of an object [11]. This predictive grip force response can be contrasted with a reflexive grip force modulation when the loading increase is externally generated. However, reflexive grip force modulations might be influenced by cutaneous feedback in similar ways to
Movement control in altered dynamics
Prediction is central to acquiring new skills and adapting existing skills to novel environments. Predictions of the dynamics of manipulated objects are learned during development. When manipulating objects, children show an increasing ability with age to adjust grip force correctly to the load in lifting an object, with the coupling increasing towards adult values by 8 years of age [14]. The early variability in force production could reflect continual adaptation to a system that is rapidly
Neural basis for the predictive control of grip force
Neurological impairments, functional brain imaging and single-cell recordings from behaving monkeys have all contributed to our understanding of the neural organization of grip forces during object manipulation. Comparing hand function in healthy subjects with that in neurologically impaired individuals suggests that the predictive control of grip force is achieved by several brain regions, such as the cerebellum, basal ganglia and cerebral cortex 18, 19. However, the individual contributions
Cutaneous feedback is crucial in setting, maintaining and adapting the control of grip force
If a hand-held object is very slippery, cutaneous afferents will detect slip events related to low friction and instigate not only a signal to increase grip force [28] but also a signal to reduce the acceleration of the hand to muscles at the shoulder and elbow [29]. It appears that feedback from small regions of fingertip skin can influence not only the muscles of prehension but also the muscles of the whole arm. Moreover, when called upon, these same cutaneous afferents can provide
Neural mechanism for processing cutaneous feedback
Individuals with only moderately impaired tactile sensation caused by chronic median nerve compression at the wrist show no impairments in their ability to adapt the force ratio to various frictional conditions and no impairments in the skilful performance of precision tasks 36, 37. Even with severe abnormalities of median nerve conduction and discrete abnormalities of fingertip sensation, the ability to regulate grip force timing and magnitude might be conserved if some residual cutaneous
Concluding remarks
Feedback from cutaneous afferents is crucial for successful feedforward control of hand–arm coordination. Recent studies provide additional evidence from behavioural, neurological impairment and electrophysiological approaches for the important contribution of tonic, continuous cutaneous afferent feedback (Figure 1). These studies demonstrate the interdependence of prediction and feedback in anticipatory grip force control, and provide a useful means of examining the role of afferent
Acknowledgements
We acknowledge the helpful and constructive criticism of Steven P. Wise. We also thank Claude Gauthier for help with computer graphics. A.G.W. is funded by a Wellcome Trust Advanced Training Fellowship. A.M.W. and A.G.W. were supported by the UK Medical Research Council Grant G9901257 and European Union Information Society Technologies contract IST 2001–38040. J.L.T. gratefully acknowledges a grant from Prodex, OSTC (Belgian Federal Office for Scientific, Technical and Cultural Affairs) and
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