Sensorimotor memory for fingertip forces during object lifting: the role of the primary motor cortex.

When an object is repetitively lifted, the scaling of grip force is influenced by the mechanical properties of the preceding lift, suggesting the formation of a sensorimotor memory. Similar effects on force scaling are observed when the subsequent lift is performed with the hand opposite to the preceding lift. We used neuronavigated rTMS over the hand area of the dominant primary motor cortex to investigate its role in setting up sensorimotor memory. After ten lifts of a novel object with the dominant hand either rTMS or a period of motor rest commenced, until another set of lifts was performed with either the same or opposite hand. Compared to motor rest, rTMS caused underestimation of the object's weight when given 10 or 30s after the previous set of lifts, but overestimation of the object's weight when applied 60 or 120 s after the previous set of lifts, regardless of the hand performing the lift. Our interpretation of the data is that (a) the primary motor cortex is essential for setting up sensorimotor memory related to the mechanical object properties during manipulation and (b) rTMS can induce bidirectional changes of grip efficiency within the dynamics of sensorimotor integration.