performance from day 1 to day 2 performance. If this were the case, however, it would be expected that the uneven performance between the three groups would occur not only for the trained sequence but also on the other tasks. However, the present results showed no significant group differences in these tasks, suggesting that the effects of praise following training were specific to the trained sequence rather than a more general effect on experi- mental task performance. Praise is regarded as a reward [26], because praise has two essential components of reward, that is, hedonic and motivational [27]. Praise can induce a feeling of happiness (hedonic compo- nent), and also promotes motivation (motivational component) [8,10,11]. A recent human neuroimaging study demonstrated that praise activates reward-related areas of the brain, specifically the ventral striatum [26]. Rewards are associated with increased dopaminergic activity in the midbrain and striatum, in which dopamine-dependent long-term potentiation (LTP) [28–30] has an important role in memory consolidation. The cortico-striatal system plays a critical role in the automatization of the type of motor sequence learning used in the present study [18,31,32]. Synaptic plasticity represented by LTP at cortico-striatal synapses strongly depends on the activation of dopamine circuits [33]. As the ventral striatum is the part of the reward system driven by dopamine [34], rewards are expected to affect motor skill consolidation. Taken together, present findings suggest that praise functions as ‘‘social reward’’ that induces the dopamine transmis- sion in the striatum, resulting in an enhancement of the motor skill consolidation. Sleep is another possible contributing factor. There is mounting evidence that sleep is necessary for the offline improvement in the sequential finger-tapping task used in the present investigation [16–21]. Although this study was not designed to determine whether sleep is necessary for the praise-related enhancement of skill consolidation, it is reasonable to expect that this enhancement selectively occurs during sleep. Consolidation of a new motor sequence during sleep appears to rely on the covert re-activation of the brain regions that were initially involved in learning the motor skill [35]. Recent human neuroimaging studies have shown that several brain areas that were activated during the execution of a memory task are significantly re-activated during sleep [35–37], and that such re-activation facilitates memory consolidation [35,36]. Furthermore, a previous animal study revealed that sleep-dependent re-activation of firing patterns in the ventral striatum took place after reward-related learning [38]. In line with these findings, it is conceivable that the cortico-striatal loop that is modified by praise after the training is then re-activated during sleep, which in turn contributes to the praise-related enhancement of offline, overnight consolidation. This working hypothesis will be the focus of future experimental investigations. In summary, the present study demonstrated that social rewards directly enhance skill consolidation in humans, and suggests that they have a novel functional effect on the human motor memory system. Further understanding of the effects of social rewards on skill consolidation could help to develop protocols to improve motor skills in educational and rehabilitative contexts. Materials and Methods Participants Written informed consent was obtained from all participants before participation in the experiment and the study conducted according to the Declaration of Helsinki. If participant was a minor (i.e., 18 or 19 year-old), two different experimenters ensured their ability to make decision and obtained their written informed Praise Enhances Offline Skill Improvements consent to the participation of this experiment, which were approved by the internal review board of Research Center for Advanced Science and Technology, The University of Tokyo. Fifty-eight healthy volunteers (39 male and 19 females, mean [M] 6 standard deviation [SD] = 22.664.67 years) participated in this study. None of the participants had a history of neurological, psychiatric, or sleep disorders, and none had had previous training in playing the piano. Based on interviews after the experiments, five participants were excluded from the analyses because they physically or mentally practiced the trained motor sequence after the end of training on day 1. Another five participants were excluded because they noticed or suspected that the evaluation movies that they watched were predetermined. Thus, data from 48 participants (35 males and 13 females; M 6 SD = 22.865.17 years) were used for analysis (Self group, n = 17; Other group, n = 15; No-praise group, n = 16). Experimental Procedure Participants came to the laboratory on two subsequent days. All participants trained on a sequential finger-tapping task [12,18– 21,39,40] on day 1. The participants were told that evaluators in another room were monitoring their performance through a web camera above the computer monitor, and would comment on their performance after training. However, in reality, their performance was not monitored. After training, all participants received visual feedback about their performance (for example, their learning curve). The participants were then divided into three groups to systematically manipulate the praise that they experi- enced: 1) participants who watched a movie in which evaluators praised their training performance (Self group); 2) participants who watched the same movie as the Self group, but who were told that it reflected the evaluation of another participant’s perfor- mance (Other group); and 3) participants who did not watch the movie and who received no praise (No-praise group). Unbeknownst to the participants, the contents of the movie were predetermined and prerecorded, with actors and actresses portraying the evaluators. At the end of the experiment on day 1, participants were told that they would perform a different task on the next day. On the following day, however, all participants performed a ‘‘surprise’’ retest of the trained sequence; this was intended to minimize the possibility that the participants either physically or mentally practiced the trained sequence prior to the retest, or that those in the Self group, in particular, were more motivated to perform the tasks on day 2. We then examined the effect of the manipulation of praise on the retest performance of the trained sequence. After the retest, the participants also performed a non-trained sequence, a randomly-ordered tapping task and completed a working memory task. These additional tasks were included to investigate whether the effects of praise were specific to the offline improvement in the trained sequence or induced a more general feeling of happiness that increased motivation to perform well on day 2. If praise enhanced general motivation in the Self group, performance on all additional tasks on day 2 should be better in the Self group than in the Other and No-praise groups. Sequential Finger Tapping Task The sequential finger tapping task required participants to press four numeric keys on a standard computer keyboard repeatedly with the fingers of their non-dominant (left) hand as quickly and as accurately as possible for 30-s periods (for details, see [19,21]). On day 1, one-half of the participants trained on sequence A (‘‘4-1-3- 2-4’’), whereas the others trained on sequence B (‘‘2-3-1-4-2’’). Training on day 1 consisted of 12 30-s trials with 30-s rest periods PLOS ONE | www.plosone.org 4 November 2012 | Volume 7 | Issue 11 | e48174