Limited persistence of the sensorimotor memory when transferred across prehension tasks.

A recent study has shown that the sensorimotor memory for the fingertip forces used to grasp and lift an object can be shared across two prehension tasks. However, the persistence (or decay) of these memory resources is not known. Reports of within-task sensorimotor memory indicate persistence of lifting forces, with evidence for reduced persistence of grip forces. Here we investigated the temporal dynamics of the transfer of memory related to vertical lifting forces across prehension tasks. Young adult participants in two separate experimental groups first held the object placed on their palm and 'hefted' it (moved it up and down) followed by lifting the object using a precision grip (thumb-finger) with the dominant hand after a delay of 10 s, or 20 min. The Control group lifted the object with the dominant hand using a precision grip and then did so again 20 min later. The Control group used higher load force rates (LFR) for their first lift compared to subsequent lifts, both before and after the 20-min delay. This suggests that the Control group initially overestimated the weight of the object, corrected their LFRs, and then was able to retain this corrected force scaling after the 20-min delay. The Experimental 10-second delay group accurately scaled their LFRs upon their first lift, indicating that they obtained an accurate memory for LFR scaling during hefting, and transferred it to the lift task. In contrast, the Experimental 20-minute delay group was unable to scale their LFRs upon their first lift, as indicated by high LFRs that were no different than those of the Control group. Thus, the memory related to the production of LFR remained stable over 20 min when obtained from the same task, while that obtained from a different task decayed completely within 20 min. This decay may indicate weakened sensorimotor memories related to prehension forces due to its dependence not only on the memory for object mechanical properties, but also on sensory signals generated during the prehension act, along with strong visual prior estimates of a size-weight relationship.