Seeing action simulation as it unfolds: The implicit effects of action scenes on muscle contraction evidenced through the use of a grip-force sensor.

Action simulation is a cognitive process that mentally simulates a motor act without performing it in the true external world. Simulation mechanisms play a key role in perceiving, feeling and understanding actions executed by others. However, very little is known about the process dynamics because of the absence of a behavioral tool to probe directly the action simulation process as it unfolds. Twenty-seven healthy adults were required to hold a force sensor in a relaxed pinch-grip while viewing action videos of different intensities: wait (null); touch (low); move (medium); crush (high). When contrasting the variations in grip force (GFv) across conditions, results indicated that GFv started to increase and peaked respectively 200 and 400 ms after the moment of effector-object contact. In the wait condition, GFv remained flat throughout the trial confirming an absence of simulation engagement. Peak GFv was greater for the high and medium than for the low intensity videos suggesting greater brain activity overflow to the peripheral motor system when simulating more effortful body movements. These effects were negatively correlated with the motor imagery abilities of the participants, with greater GFv in the poor imagers as determined by the Movement Imagery Questionnaire. Our results confirm the possibility of using a non-invasive grip force sensor to detect not only when individuals are cognitively engaged in action simulation but also to reveal the dynamics of the process. With various sets of videos, this paradigm offers new perspectives in the study of action simulation and its role in human cognition.

Efficiency of grip force adjustments for impulsive loading during imposed and actively produced collisions.

During object manipulation, both predictive feedforward and reactive feedback mechanisms are available to adjust grip force (GF) levels to compensate for the destabilizing effects of load force changes. During collisions, load force increases impulsively (< 20 ms). Thus, only predictive control of GF can be used to ensure grasp stabilization. A collision paradigm is here used to investigate the effects of practice and vision on the efficiency of the predictive control of GF. Subjects actively produced or received an imposed collision with a pendulum. Subjects were more efficient (used smaller GF for identical loads) when producing than when receiving the collisions. Effects of practice were evident in the active producing task only, with GF levels reducing over repetitions, suggesting that sensorimotor memory for the task was used to adjust GF more efficiently. With imposed collisions, GF levels did not reduce with repetition, which suggests that a direct relation between motor action and sensory feedback may be necessary to improve efficiency. Nevertheless, in this condition GF was lower with visual feedback, indicating potential for more efficient grip possibly associated with subjects degree of confidence. We discuss the implications of these results for accounts of the predictive and the reactive control of movement.