Proprioception is sufficient to coordinate simple bimanual lifts of unanticipated asymmetrically loaded objects.

The relative importance of proprioception and vision in control of bimanual lifting of a box with varied weight and weight distribution was studied to test whether availability of vision improved the lifting motion in terms of box orientation and temporal synchrony of lifting the two sides of the box. Twelve right-handed young adults performed a random sequence of bimanual lifts of a box with varied weight and weight distribution. When lifting a symmetrically loaded box there was excellent temporal synchrony of the motions of the left and right side and minimal deviation from a level box orientation. When lifting an asymmetrically loaded box there was delayed onset of lifting the heavier side of the box but temporal synchrony of right and left side motions subsequently improved such that left and right sides ended motion nearly synchronously. There was also greater deviation from a level box orientation during the lift that was corrected to nearly level at lift termination. Adaptations to changes in load symmetry from heavier load on one side to the other side occurred over 2-3 lifts to produce nearly synchronous lifts of the left and right sides with little deviation from a level box orientation. Allowing vision did not improve maintenance of box orientation or temporal synchronization of left and right side lifting motions. These results show that non-visual information is sufficient to control lifting a box to a level orientation when there is no prior knowledge of box weight or its distribution within the box.