Automatic grasp imitation following action observation affects estimation of intrinsic object properties

The present study aimed to determine whether the observation of different grasps of the same object elicits automatic imitation of the kinematics of those grasps and this process influences the estimation of intrinsic target properties. In experiments 1 and 2, participants reached and grasped differently sized spheres after observation of the same objects grasped using two different types of grasp (power and precision grasp) and hand kinematics. The observed grasp kinematics were imitated especially when the vision of the target and the acting hand were precluded. In experiments 3, 4 and 5 participants matched the diameter of the spheres, either perceived or imagined, by opening their thumb and index finger (i.e. the fingers used to grasp the objects) after observation of the two types of grasp. Finger opening was larger after observation of power grasp than precision grasp, consistently with the notion that power grasp and precision grasp are preferentially used to grasp large and small objects, respectively. However, the effect was poorly observed for the small object, this depending on the fact that the participants imitated also the final position of the thumb and index finger, which were closer to each other in the power grasp. Finally, those participants, for whom the effect was stronger, reported to have perceived more differently sized objects than those really presented. The results suggest that imitation evoked by a mirror system is involved in planning how to interact with an object and in the estimation of the properties extracted for sensory-motor integration.