P27 Does the way we grasp objects affect how we perceive our hands?
Butler AA, van Eijk T, Héroux ME, Gandevia SG
NeuRA, Randwick, NSW, Australia
Active muscle contraction and its intensity influence perceived joint position of single joints. Here we investigated how grasp intensity, and whether grasp is active or passive, affects perceived grasp aperture (spacing between the thumb and index finger). In experiment 1, 20 subjects grasped a 6.5cm unseen object at three different grasp intensities (1, 5 and 10% of maximal grasp force) either actively (i.e. voluntary) or passively (external force applied to grasping digits). Subjects were asked to report perceived grasp aperture using a visual chart. Experiment 2 addressed whether Experiment 1 results were influenced by the external pressure applied during the passive grasp. Subjects (n=20) grasped the object with either a very light active or passive force, eliminating the need for external pressure on the digits. Compared to published results in single joints, in Experiment 1 there was no effect of grasp intensity on perceived aperture (-0.02 [-0.05 to 0.01]; mean [95% CI]). However, perceived grasp aperture was narrower by 0.34cm [0.13 to 0.54]) when the object was grasped passively compared with actively. Experiment 2 revealed there was no difference in perceived grasp aperture between active and passive conditions (-0.18cm [-0.55 to 0.19]). The intensity of a grasp and whether or not it is performed actively does not impact how we perceive our hand. Although various factors can, in isolation, impact how we perceive our body, our results further support the view that the brain maintains a stable representation of the hand, highlighting its importance in primates.