The inertial anisotropy of the arm is accurately predicted during movementplanning

An important theoretical concept in motor control is the idea that the CNS uses an internal model of the motor system and environment to predict the s ensory consequences of motor commands. In arm movement control, a critical factor affecting the transformation from motor commands to sensory conseque nces is limb dynamics, including the inertial anisotropy of the arm, which refers to the fact that the inertial resistance of the arm depends on hand movement direction. Here we show that the CNS maintains an accurate interna l model of the inertial anisotropy of the arm by demonstrating that the mot or system can precisely predict direction-dependent variations in hand acce leration. Subjects slid an object, held beneath the index finger, across a frictionless horizontal surface to radially located targets. We recorded th e normal (vertical) force exerted by the fingertip, as well as the tangenti al (horizontal) force proportional to hand acceleration. We found that norm al force was precisely scaled in anticipation of tangential force, which, a s expected, varied with direction. The peak rates of change of the normal a nd tangential forces, observed early in the movement, were highly correlate d. Similar results were obtained regardless of whether the start position o f the hand was located directly in front of the subject or rotated 45 degre es to the right. Finally, we observed reduced force correlations under reac tion time conditions. This suggests that the process of prediction, based o n an internal model of the limb, is not fully completed within the reaction time interval.