Learning motor skills commonly requires repeated execution to achieve gains in performance. Motivated by memory reactivation frameworks predominantly originating from fear-conditioning studies in rodents, which have extended to humans, we asked the following: Could motor skill learning be achieved by brief memory reactivations? To address this question, we had participants encode a motor sequence task in an initial test session, followed by brief task reactivations of only 30 s each, conducted on separate days. Learning was evaluated in a final retest session. The results showed that these brief reactivations induced significant motor skill learning gains. Nevertheless, the efficacy of reactivations was not consistent but determined by the number of consecutive correct sequences tapped during memory reactivations. Highly continuous reactivations resulted in higher learning gains, similar to those induced by full extensive practice, while lower continuity reactivations resulted in minimal learning gains. These results were replicated in a new independent sample of subjects, suggesting that the quality of memory reactivation, reflected by its continuity, regulates the magnitude of learning gains. In addition, the change in noninvasive brain stimulation measurements of corticospinal excitability evoked by transcranial magnetic stimulation over primary motor cortex between pre- and postlearning correlated with retest and transfer performance. These results demonstrate a unique form of rapid motor skill learning and may have far-reaching implications, for example, in accelerating motor rehabilitation following neurological injuries.

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The ability of the human brain to successively learn or perform two competing tasks constitutes a major challenge in daily function. Indeed, exposing the brain to two different competing memories within a short temporal offset can induce interference, resulting in deteriorated performance in at least one of the learned memories [1-4]. Although previous studies have investigated online interference and its effects on performance [5-13], whether the human brain can enable long-term prevention of future interference is unknown. To address this question, we utilized the memory reactivation-reconsolidation framework [2, 12] stemming from studies at the synaptic level [14-17], according to which reactivation of a memory enables its update. In a set of experiments, using the motor sequence learning task [18] we report that a unique pairing of reactivating the original memory (right hand) in synchrony with novel memory trials (left hand) prevented future interference between the two memories. Strikingly, these effects were long-term and observed a month following reactivation. Further experiments showed that preventing future interference was not due to practice per se, but rather specifically depended on a limited time window induced by reactivation of the original memory. These results suggest a mechanism according to which memory reactivation enables long-term prevention of interference, possibly by creating an updated memory trace integrating original and novel memories during the reconsolidation time window. The opportunity to induce a long-term preventive effect on memories may enable the utilization of strategies optimizing normal human learning, as well as recovery following neurological insults.

Following initial acquisition, studies across domains have shown that memories stabilize through consolidation processes, requiring a post-acquisition temporal interval to allow their occurrence. In procedural skill memories, consolidation not only stabilizes the memory, but also simultaneously enhances it by accumulating additional gains in performance. In addition, explicit skill tasks were previously shown to consolidate through sleep, whereas implicit tasks were consolidated following a time interval which did not include a period of sleep. Although previous research has been instrumental in utilizing simple motor tasks designed to model skill learning, whether and how skill consolidation processes operate in complex real-life environments remains to be determined. Here, we tested consolidation in a complex motor skill, used to train execution of fine-motor movements. Since the complex task was explicit, we hypothesized that consolidation will be evident immediately following sleep, as in simple explicit motor skills. However, results show that even though participants were aware of the goal of the complex skill task, consolidation was evident only 24 hr following skill acquisition, and not following a shorter 12 hr interval, even when the latter included sleep. An additional experiment verified that without a temporal interval longer than 12hr, the same skill training does not undergo complete consolidation. These results suggest that task complexity is a crucial characteristic determining the proper terms allowing full consolidation. Due to the enhanced ecological validity of this study, revealing the differences between complex and simple motor skills could enable the facilitation of advanced rehabilitation methods following neurological injuries.

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