Experimental apparatus and protocol.
(a) Players in a dyad were connected through a virtual spring. Both players were instructed to perform reaching movements in the vertical plane, between the same start point and the same target point but through different via-points (VP). Each player could only see his/her own VP, but not their partner’s. Both were instructed to keep the interaction force as low as possible during movement. The experimental protocol consisted of three phases: baseline, training and after-effect. During the baseline phase the interaction forces were turned off, and each player performed on their own (‘solo’ performance). The players were mechanically connected during the training phase, and the connection was permanently removed during the after-effect phase. (b) We manipulated the information available on partner’s actions by providing it either haptically, through the interaction force (Haptic group, H) or by additionally displaying the interaction force vector on the screen (Visuo-Haptic group, VH) or displaying partner’s cursor itself (Partner Visible group, PV). The yellow and white circles denote, respectively, the start and target position. The green circle is the cursor location. In the VH group, direction and magnitude of the interaction force is depicted by a line originating from the cursor. In the PV group, partner’s cursor is shown by black circle. (c) Movement paths in baseline (unconnected), early-training, late-training and washout phases of the experiment from three typical H, VH and PV group dyads.