Sensorimotor memory of weight asymmetry in object manipulation.

Using a precision grip-lifting task, we examined how sensorimotor memory for weight asymmetry transfers across changes in hand and object configuration. We measured object tilt when participants lifted a visually symmetric box with an offset centre of mass. Transfer was assessed after participants lifted the box 10 times, during which the large tilt observed in the first lift was reduced. Consistent with previous work of Salimi et al. (J Neurophysiol 84:2390-2397, 2000), we found that when the object was rotated 180 degrees , participants failed to update their sensorimotor memory appropriately. Instead, participants acted as if the object did not rotate and negative transfer was observed. However, when the hand was rotated 180 degrees around the object, participants were able to correctly update sensorimotor memory and positive transfer was observed. This finding argues against the hypothesis that sensorimotor memory is digit-specific because the rotation of the hand (like rotation of the object) changes the forces that each digit must generate to prevent tilt. Positive transfer was also observed when both the hand and object were rotated. This suggests that the rotation of the hand may facilitate rotation of an internal representation of the object. Finally, we found positive transfer of weight asymmetry across the two hands but only when the second hand was rotated such that homologous digits of each hand gripped the same contact surfaces. We suggest that good transfer is observed under these conditions because, when we pass objects from hand to hand, we typically place homologous digits of the two hands in similar locations on the object.