Coordination between the transport and the grasp components during prehension movements

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Abstract
In this study, the possible influence of the transport on the grasp component of prehension movements was investigated. The first phase of the transport (acceleration phase) and of the grasp (finger aperture phase) kinematics were studied under conditions of visual and non-visual object presentation (prehension experiment). In the non-visual condition, object size was estimated by haptics and object position was estimated by proprioception. Eight subjects were required to reach and grasp three objects of different size located at two distances. An additional experiment (matching experiment) was carried out to control the scaling of object size in the two conditions. The results showed that in the matching experiment size estimation for large objects was similar in the two conditions, whereas small stimuli were underestimated in the haptic condition. In the prehension experiment, maximal finger aperture and velocity of finger aperture were greater in the non-visual than in the visual condition, and the difference was greater for small than for large stimuli. Moreover, in both conditions, finger opening was larger for prehension movements directed to the far than to the near objects, but only for smaller stimuli. Hand trajectory variability increased in the non-visual condition and with the distance, whereas finger opening variability was only affected by the non-visual condition. For smaller stimuli, increased finger opening with distance was positively correlated with the increase in wrist variability in the visual condition, but not in the non-visual condition. Furthermore, increased finger opening between visual and non-visual conditions was correlated with the increase in wrist variability, for smaller objects at the near object location. No positive correlations were found between finger opening and grip variability. These results are interpreted in favour of the dependence of finger opening on transport, when control requirements during reaching increase.