Abstract
Motor planning and execution require a representational map of our body. Since the body can assume different postures, it is not known how it is represented in this map. Moreover, is the generation of the motor command favored by some body configurations? We investigated the existence of a centrally favored posture of the hand for action, in search of physiological and behavioral advantages due to central motor processing. We tested two opposite hand pinch grips, equally difficult and commonly used: forearm pronated, thumb-down, index-up pinch against the same grip performed with thumb-up. The former revealed faster movement onset, sign of faster neural computation, and faster target reaching. It induced increased corticospinal excitability, independently on pre-stimulus tonic muscle contraction. Remarkably, motor excitability also increased when thumb-down pinch was only observed, imagined, or prepared, actually keeping the hand at rest. Motor advantages were independent of any concurrent modulation due to somatosensory input, as shown by testing afferent inhibition. Results provide strong behavioral and physiological evidence for a preferred hand posture favoring brain motor control, independently by somatosensory processing. This suggests the existence of a baseline postural representation that may serve as an a priori spatial reference for body–space interaction.
Highlights
We all use our hands daily for a multitude of scopes, from object manipulation to interpersonal contacts, experiencing every possible posture
We found that the pinch started earlier when it was done with the thumb-down configuration showing a behavioral advantage in the computation of the action plan (main effect of factor hand posture (F(1,5498.1) = 127.460, P < 0.001; interaction hand posture ∗ forearm orientation (F(1,5498.1) = 15.694, P < 0.001))
The temporal advantage was detectable when starting the action with a reversed orientation of the arm, suggesting that the computational advantage for the hand posture is not determined by the synergic advantage of the position of the whole arm (Fig. 1)
Summary
We all use our hands daily for a multitude of scopes, from object manipulation to interpersonal contacts, experiencing every possible posture. We are extremely quick and precise in choosing and shaping grips and assuming postures (think, for example, pantomiming), even without full attentional or sensory (e.g., visual) control. We are able to localize precisely sensory stimuli delivered to the hand in the space and couple them with spatial visual information. Such abilities suggest that our brain owns a precise, dynamic spatial representation of our body, the socalled body representation. Motor control is modeled through algorithms implementing a forward model that makes a prediction of action outcome, an inverse model which estimates the motor command required to achieve the desired outcome and an internal model, where kinematic and dynamic features of the body and its interactions with the environment are stored (Kawato 1999; Wolpert and Ghahramani 2000)
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