RAPID ADAPTATION TO CORIOLIS-FORCE PERTURBATIONS OF ARM TRAJECTORY

1. Forward reaching movements made during body rotation generate tange ntial Coriolis forces that are proportional to the cross product of th e angular velocity of rotation and the linear velocity of the arm. Cor iolis forces are inertial forces that do not involve mechanical contac t. Virtually no constant centrifugal forces will be present in the bac kground when motion of the arm generates transient Coriolis forces if the radius of body rotation is small. 2. We measured the trajectories of arm movements made in darkness to a visual target that was extingui shed as movement began. The reaching movements were made prerotation, during rotation at 10 rpm in a fully enclosed rotating room, and postr otation. During testing the subject was seated at the center of the ro om and pointed radially. Neither visual nor tactile feedback about mov ement accuracy was present. 3. In experiment 1, subjects reached at a fast or slow rate and their hands made contact with a horizontal surfa ce at the end of the reach. Their initial perrotary movements were hig hly significantly deviated relative to prerotation in both trajectorie s and endpoints in the direction of the transient Coriolis forces that had been generated during the reaches. Despite the absence of visual and tactile feedback about reaching accuracy, all subjects rapidly reg ained straight movement trajectories and accurate endpoints. Postrotat ion, transient errors of opposite sign were present for both trajector ies and endpoints. 4. In a second experiment the conditions were ident ical except that subjects pointed just above the location of the extin guished target so that no surface contact was involved. All subjects s howed significant initial perrotation deviations of trajectories and e ndpoints in the direction of the transient Coriolis forces. With repea ted reaches the trajectories, as viewed from above, again became strai ght, but there was only partial restoration of endpoint accuracy, so t hat subjects reached in a straight line to the wrong place. Aftereffec ts of opposite sign were transiently present in the postrotary movemen ts. 5. These observations fail to support current equilibrium point mo dels, both alpha and lambda, of movement control. Such theories would not predict endpoint errors under our experimental conditions, in whic h the Coriolis force is absent at the beginning and end of a movement. Our results indicate that detailed aspects of movement trajectory are being continuously monitored on the basis of proprioceptive feedback in relation to motor commands. Adaptive compensations can be initiated after one perturbation despite the absence of either visual or tactil e feedback about movement trajectory and endpoint error. Moreover, mov ement trajectory and endpoint can be remapped independently. 6. We int erpret these results as emphasizing that movement trajectory and endpo int are continuously monitored. A model illustrating how this might be done is presented; it shows how proprioceptive, motor, and somatosens ory factors could be used in updating movement control and compensatin g for changes in effective limb inertia and dynamics.