Praise Enhances Offline Skill Improvements Table 1. Examples of the comments from evaluation clips. Valence of comment Direction of evaluation Content Positive Positive Positive Neutral Performance, Social Ranking Attitude, Social Ranking Performance, Social Ranking Performance, Social Ranking Hi, I observed your performance and attitude during the motor task. The tapping became more rhythmical over time. Your performance was great. The number of the pressed buttons in the last trial might be the highest of all the participants I have observed. Thank you. Hello. I would like to comment on your wonderful performance. First, I think your motor performance on the last trial was the highest of all the participants. In addition, you concentrated very well during the motor task. Thanks for your participation. I can imagine that other evaluators will also give you good feedback. Actually, your performance was amazing and deserves praise. Among the previous participants in the experiment, your performance was the best. Thank you. Thanks for your participation. The total number of tapped buttons and the speed of tapping increased as practice progressed. Your performance was average relative to the other participants. doi:10.1371/journal.pone.0048174.t001 between trials, whereas the retest on day 2 consisted of five trials with the same rest interval. Finger tapping performance was evaluated by the number of correctly tapped sequences per 30-s trial. The offline performance improvement following a night of sleep was defined as the percent increase in mean performance from the last three trials during training on day 1 compared with the first three retest trials on day 2 [18–20,39,40]. On day 2, participants also performed the sequence that they had not received training on during day 1 (that is, a participant who trained on sequence A on day 1 performed sequence B on day 2), and the randomly-ordered tapping task, in which stimuli were presented in a random order. Both tasks consisted of five 30-s trials with a 30-s rest period between trials. Performance for the non- trained sequence (NEW) and the randomly-ordered (RAN) tapping was calculated based on the mean number of correctly tapped sequences (NEW) or correctly pressed buttons (RAN) during the five trials. Manipulation of Praise After the training on day 1, participants in the Self and Other groups watched a movie in which evaluators praised the training performance. We adopted a movie instead of live praise because predetermined movie can totally control out the variability of evaluators’ comments and non-verbal information such as facial expression and intonation. Participants in the Self group were told that the movie represented the evaluation of their own perfor- mance during training. The movie consisted of three components: one introduction clip, 12 evaluation clips, and happiness ratings for each clip. In the introduction clip, a man greeted the participant by name to make the evaluation appear more believable and meaningful. Each movie clip was pre-recorded using six actors and six actresses. Ten movie clips contained positive feedback, and two neutral movie clips were included to maintain the attention of participants by making the evaluation less predictable. In the evaluation movies, praise was directed at the participant’s training performance, their attitude during training, or their social ranking relative to other participants (see Table 1 for examples of evaluators’ comments used in this experiment). To rule out the possibility that simply watching the movie might influence the offline improvement in motor skill, we included the Other group, in which participants watched the same movie clips but were told that they represented the evaluation of another participant’s training performance. In the introduction clip seen by the Other group, a man used another participant’s name. In both the Self and Other groups, regardless of the target of praise, the participants were asked to rate how happy they felt upon watching each movie clip using a seven-point scale (1 = very unhappy, 4 = neutral, and 7 = very happy; the responses for one participant were not collected due to technical difficulties). The order of the evaluation clips was fixed across participants. After the experiment on day 2, the participants were interviewed to determine whether they had any doubts about the evaluation movies they watched. After this, all participants were fully debriefed. Working Memory Task A subset of the participants (n = 35) performed an object working memory task on day 2. A previous study indicated that performance on working memory tasks is highly sensitive to a participant’s motivational state [41]. In the delayed-matching working memory task, participants were asked to remember three irregular polygons, and were then required to decide while whether a probe stimulus matched any of the three target stimuli (for details, see [41]). The task was presented in a total of 84 trials. Alertness, Concentration, and Fatigue During Training and Retest As it was possible that the subjective state of the participants during training and retest might influence their performance, they completed questionnaires to rate their level of alertness (Stanford Sleepiness Scale rating, [42], translated into Japanese), concen- tration (1 = not at all, 7 = very concentrated), and fatigue (1 = high level of fatigue, 7 = no fatigue, [43]) using a seven-point scale at the end of the training and retest periods. Sleep Duration and Quality the Nights Before and After Training Because sleep plays an important role in the offline improve- ment of motor skills [16,18–21], sleep duration the night after training was measured by subjective reports and actimetry. Participants were also asked to report the time that they went to bed both the night before and after training, and the time that they woke up on the training and retest mornings. In addition, to confirm the validity of the subjective sleep-duration reports, the PLOS ONE | www.plosone.org 5 November 2012 | Volume 7 | Issue 11 | e48174