Fig. 1.

Panel (a): Experimental set-up. Two participants were seated opposite each other. Their task was to trace the shapes drawn on the glass panel in between them using their right index finger. Panel (b): Two squares of different colours were positioned on top of each other with a 45° rotation on the centroid. On congruent trials, the two participants in a pair traced the same shape. On incongruent trials, they each traced a different shape. They were instructed to pass the crossing points as synchronously as possible. These crossing points were used to calculate participants' coordination performance. Panel (c): schematic representation of the expected velocity profiles for congruent trials (upper row: shape segments; lower row: velocity profile in red for one participant and in blue for the other). When participants traced a corner of the shape (c, left graph), we expected their movement velocity to drop as the curvature is high; when participants traced straight-line segments (c, right graph) we expected a bell-shaped velocity profile (Flash & Hogan, 1985). Panel (d) schematic representation of the expected velocity profiles for incongruent trials: the velocity profiles of two participants are incongruent, as they are tracing a straight-line and a corner segment at the same time.