FUNCTIONAL STAGES IN THE FORMATION OF HUMAN LONG-TERM MOTOR MEMORY

Previous research has demonstrated that the primate CNS has the abilit y to learn and store multiple and conflicting visuomotor maps. Here we studied the ability of human subjects to learn to make reaching movem ents while interacting with one of two conflicting mechanical environm ents as produced by a robotic manipulandum. We demonstrate that two mo tor maps may be learned and retained, but only if the training session s in the tasks are separated by an interval of similar to 5 hr. If the interval is shorter, learning of the second map begins with an intern al model appropriate for the first task and performance in the second task is significantly impaired. Analysis of the after-effects suggests that with a short temporal distance, learning of the second task lead s to an unlearning of the internal model for the first. With the longe r temporal distance, learning of the second task starts with an unbias ed internal model, and performance approaches that of naives. Furtherm ore, the memory of the consolidated skill lasts for at least 5 months after training. These results argue for a distinct change in the state of resistance of motor memory (to disruption) within a few hours afte r acquisition. We suggest that motor practice results in memories that have at least two functional components: soon after completion of pra ctice, one component fades while another is strengthened. A further ex periment suggests that the hypothetical first stage is not merely a ga teway to long-term memory, but also temporary storage for items of inf ormation, whether new or old, for use in the near-term. Our results ra ise the possibility that there are distinct neuronal mechanisms for re presentation of the two functional stages of motor memory.