Sensorimotor memory for fingertip forces: evidence for a task-independent motor memory

Abstract

When repetitively lifting an object with randomly varying mechanical properties, the fingertip forces reflect the previous lift. We examined the specificity of this "sensorimotor memory" by observing the effects of an isolated pinch on the subsequent lift of a known object. In this case, the pinch force was unrelated to the fingertip forces necessary to grip the object efficiently. The peak grip force used to lift the test object (4 N weight) depended on the preceding task. Compared with repetitively lifting the 4 N test object, the peak grip force was 2 N greater when a lift of the same object was preceded by a lift in which a hidden mass was attached to the object to increase the weight to 8 N. This 2 N increase in grip force also occurred when subjects lifted the 4 N test object after pinching a force transducer with a force of 8 N. Thus, similar grip forces were stored in sensorimotor memory for both tasks, and reflected subjects' use of 7.9 +/- 1.1 N to lift the 8 N object. Similar effects occurred when the preceding pinch or lift was performed with the opposite hand. The peak lift force was unaffected by the isolated pinch, suggesting that a generalized increase in fingertip and limb forces did not occur. We conclude that the sensorimotor memory is not specific for lifting an object. It is doubtful that this particular memory stores the physical properties of objects or reflects a forward internal model for predictively controlling fingertip forces.

Fig. 1.

Novel test object. Subjects grasped and lifted the test object at the black, sandpaper-covered vertical surfaces using the thumb and index finger.

Fig. 2.

Single representative trials of a typical subject lifting the 4 N test object. Peak grip and lift forces (vertical lines) occur after the object has been lifted from the support surface.

Fig. 3.

The peak grip force when lifting the 4 N test object depends on the preceding action. A, Fingertip force records of single trials from a typical subject for lifts of the 4 N test object when preceded by an 8 N pinch (thin line), a lift of the 8 N test object (dashed line), or another 4 N lift of the test object (thick line). B, Average peak grip forces for the group across conditions (lines indicate SEM), when a lift of the 4 N object was preceded by an 8 N lift (white), an 8 N pinch (gray), or a 4 N lift (black). Average peak grip forces were significantly greater for lifts of the 4 N test object when preceded by 8 N pinches and 8 N lifts compared with repeated 4 N lifts. *p< 0.001.

Fig. 4.

Preceding actions performed with the right hand affected fingertip force programming for a subsequent lift using the left hand. A, Fingertip force records of single trials for a typical subject for lifts of the 2 N object when preceded by either 8 N pinches or 8 N object lifts (with the same or opposite hand), or repeated left-handed lifts of the 2 N object.B, Bar graphs of the peak grip force across subjects for lifts of the 2 N test object when preceded by 8 N pinches or 8 N lifts using either the right or the left hand [R8L (R), 8 N right-handed lift precedes a 2 N lift with the right hand; R8P (L), right-handed 8 N pinch precedes a 2 N lift with the left hand, etc.]. Regardless of the hand performing the preceding action, average peak grip forces were significantly greater (*p < 0.001) when the left-handed 2 N lift was preceded by either an 8 N pinch or 8 N lift, compared with repeated lifts of the 2 N object with the left hand.

Fig. 5.

Peak lift force when a lift of the 4 N test object was preceded by an 8 N lift (white), 8 N pinch (gray), or another 4 N lift (black). An 8 N pinch preceding a 4 N lift had no effect on the peak lift force. However, an 8 N lift preceding a 4 N lift produced significantly greater peak lift forces (*p < 0.001) compared with repeated 4 N lifts.