A Double Dissociation between Savings and Long-Term Memory in Motor Learning

Both declarative and procedural memories are easier to reacquire than learn from scratch. This advantage, known as savings, has been widely assumed to result from the reemergence of stable long-term memories. In fact, the presence of savings has often been used as a marker for whether a memory had been consolidated. However, recent findings have demonstrated that motor learning rates can be systematically controlled, providing a mechanistic alternative to the reemergence of a stable long-term memory, and recent work has reported conflicting results about whether implicit contributions to savings in motor learning are present, absent, or inverted, suggesting a limited understanding of the underlying mechanisms. In order to elucidate the mechanism responsible for savings in motor learning, we investigate the relationship between savings and long-term memory by determining how they depend on different components of motor learning. To accomplish this, we experimentally dissect motor adaptation based on short-term (1-minute) temporal persistence. Surprisingly, we find that a temporally-volatile component of implicit learning leads to savings whereas temporally-persistent learning does not, but that temporally-persistent learning leads to long-term memory at 24 hours whereas temporally-volatile learning does not. Moreover, we find that temporally-persistent implicit learning not only fails to contribute to savings, but that it produces an anti-savings which acts to reduce the net savings, and we show that the balance between temporally-volatile and temporally-persistent components can explain seemingly inconsistent reports about implicit savings. The clear double dissociation between the mechanisms for savings and long-term memory formation challenges widespread assumptions about the connection between savings and memory consolidation, and provides new insight into the mechanisms for motor learning.