Abstract
We investigated bimanual coordination in a precision task that required reach-to-grasps to two different objects and locations at the same time. Participants exhibited a single simultaneous coordination during most of the movement, but then abandoned this for a sequential mode at the end. In our paradigm, one hand (Fixed) always moved to the same object at a fixed distance while the other hand (Variable) moved to objects of different width (3, 5, 7cm) and grip surface size (1, 2, 3cm) placed at different distances (20, 30, 40cm). The two hands always started moving synchronously, but the movement was found to consist of two phases: (i) an initial phase (IP) during which the hand moved forwards and the fingers pre-formed an appropriate aperture before stopping at the object; (ii) a completion phase (CP) during which the finger and thumb closed to contact the object. The IP duration of the Fixed hand was affected reliably by the distance moved by the Variable hand. The IP duration of the Variable hand was affected reliably by distance and surface size. The CP duration in the Fixed hand was unaffected by the Variable hand's task but the Variable hand's CP was affected by grip surface sizes, so there was no relationship between the CPs of the two hands. The total movement time was lawfully related to the average distance and the accuracy demands. The maximum grasp aperture (MGA) of the Fixed hand was affected reliably by the object width of the Variable hand. The MGA of the Variable hand was affected by both object width and surface size. These findings indicate a simultaneous coordination during the IP followed by a sequential coordination at the end of the movement. This mode change is an efficient organization necessarily using visual guidance that is limited by the spatial span of attention.