other three incompatible conditions (in all cases p,0.001). In contrast, responses did not differ across the three incompatible conditions (all ps $0.99). Importantly, running separate simple effect analyses on the cycle time (ms) data confirmed a significant main effect of distractor speed in each of the three incompatible conditions (all ps ,0.001, all gp2s $0.60). The second significant interaction was that between delay and habitual speed, F(2, 36) = 3.66, p,0.05, gp2 = 0.17. Simple effect analyses revealed that while the main effect of delay was not significant for the habitually fast actions, F(2, 36) = 1.02, p.0.05, this effect was marginally significant for the habitually slow actions, F(2, 29) = 3.39, p = 0.057, gp2 = 0.16. More detailed simple effect analyses showed that the main effect of delay was only significant for habitually slow actions when action type was compatible, F(2, 36) = 5.41, p,0.01, gp2 = 0.23. Pairwise compar- isons showed that the slow:fast ratios were more pronounced in the 0 s delay condition than in the 4 s and 8 s delay conditions (112%, 108%, and 109%, respectively, both ps ,0.05). These results indicate a pocket of stronger distractor effects for immediate vs. delayed execution in a subset of the data (compatible action type, and habitually slow actions). Response ratios were not significantly different across the two extended delay conditions. Experiment 2: Intentional Imitation The three-factorial ANOVA on the cycle time (ms) data yielded significant main effects of distractor speed, F(1, 18) = 408.33, p,0.001, gp2 = 0.96, and of habitual speed, F(1, 18) = 892.15, p,0.001, gp2 = 0.98 (see Figure 5). Note that in intentional imitation participants were instructed to imitate the ‘distractor’ cycle times. For reasons of consistency, we continue using the term ‘distractor’ for these analyses. As one might expect for intentional imitation, the interaction between dis- tractor speed and habitual speed was also significant, F(1, 18) = 49.74, p,0.001, gp2 = 0.73, which was in line with the stronger cycle time differences (in ms) for the habitually slow over fast actions in the distractor movies. Again, the main effect for the availability of vision was not significant, F(1, 18) = 2.07, p.0.05. However, the availability of vision interacted both with distractor speed, F(1, 18) = 7.13, p,0.05, gp2 = 0.31, and with habitual speed, F(1, 18) = 6.06, p,0.05, gp2 = 0.25, while the three-way interaction was not significant. Simple effect analyses regarding the first of these two interactions showed that the availability of vision did not modulate intentional imitation of the fast distractor speeds, F(1, 18) = 0.002, p.0.05. But for the slow distractor speeds, response cycle times tended to be shorter when vision was available compared to unavailable (780 ms vs. 835 ms, respectively; criterion = 750 ms), F(1, 18) = 3.79, p = 0.067, gp2 = 0.17; particularly for the habitually slow actions (945 vs. 1035 ms; criterion = 1000 ms), F(1, 18) = 4.86, p,0.05, gp2 = 0.34. Next, simple effect analyses for the interaction between vision and habitual speed revealed a similar pattern, namely that the availability of vision did not modulate the habitually fast actions, F (1, 18) = 0.004, p.0.05, but for habitually slow actions, full-vision tended to speed-up response cycle times over no-vision (845 ms vs. 899 ms; criter- ion = 833 ms), F(1, 18) = 3.77, p = 0.068, gp2 = 0.17. In summary, full-vision generally increased response speeds in intentional imitation, but also decreased the difference between response cycle times executed after both slow and fast distractors (see also below). The four-factorial ANOVA conducted on the ratio (%) data for intentional imitation used the same factors as the related ANOVA for automatic imitation, except that the delay factor was not included. This ANOVA yielded a significant main effect of Automatic Imitation in Rhythmical Actions habitual speed, F(1, 18) = 11.16, p,0.01, gp2 = 0.38. The slow:fast cycle time ratio was larger for the habitually fast (140%) over slow actions (132%), while both undershot the to-be-imitated ‘dis- tractor’ ratio of 150%. Thus, while the ms-data reflected the time differences in the displays in both automatic and intentional imitation, this difference did not affect cycle time ratios in automatic imitation, but it did increase the ratios in intentional imitation for the habitually fast over slow actions. Also, the availability of vision significantly modulated the cycle time ratios, F(1, 18) = 8.14, p,0.05, gp2 = 0.31, with a more pronounced mean ratio found in the no-vision (141%) compared to the vision condition (131%). The main effect of action type compatibility was not significant, F(1, 18) = 0.01, p.0.05, as was the main effect of plane compatibility, F(1, 18) = 0.93, p.0.05. The only significant interaction was between habitual speed and plane compatibility, F(1, 18) = 4.44, p,0.05, gp2 = 0.20. Pairwise comparisons showed that for habitually slow actions the slow:fast ratio was marginally more pronounced for trials with compatible compared to incompatible plane (131 vs. 133%; p = 0.055). Automatic vs. Intentional Imitation The five-factorial ANOVA performed on the ratio (%) data used the same factors as the four-factorial ANOVA for intentional imitation, except with the inclusion of a fifth factor ‘automatic vs. intentional imitation’. This analysis yielded a number of significant results that replicated our previous findings. As can expected from our separate analyses of the ratio scores for automatic and intentional imitation, significant main effects were found for action type, F(1, 18) = 8.12, p = 0.01, gp2 = 0.31, and plane compatibility, F(1, 18) = 12.11, p,0.01, gp2 = 0.40. Furthermore, two interaction results con- firmed the already reported differences between automatic and intentional imitation: The significant interaction between inten- tion and visual monitoring, F(1, 18) = 19.72, p,0.001, gp2 = 0.52, confirmed that visual monitoring only affected cycle time ratios in intentional imitation, and the significant interaction between intention and habitual speed, F(1, 18) = 27.47, p,0.001, gp2 = 0.60, reflected that cycle time ratios were only affected by habitual speed in intentional imitation. Importantly, the main effect of intention was significant, F(1, 18) = 517, p,0.001, gp2 = 0.97, where cycle time ratios were closer to the display ratio for intentional compared to automatic imitation (136 vs. 108%, respectively; see Figure 6). Intention interacted significantly with action compatibility, F(1, 18) = 6.87, p,0.05, gp2 = 0.28, as well as with plane compatibility, F(1, 18) = 5.87, p,0.05, gp2 = 0.25. These results confirm that there were no differences across the four compatibility conditions for intentional imitation, and that in automatic imitation, the distractor ratio was more pronounced in the fully-compatible SA/SP condition over each of the incompatible condition. The three-way interaction involving intention, action type, and plane compatibility was only marginally significant, F(1, 18) = 3.44, p = 0.08, gp2 = 0.16. Discussion The present study contributes to a growing body of research demonstrating that observed kinematics automatically bias move- ment kinematics. First, our data show that observing a task- irrelevant rhythmical action during a brief motor preparation phase significantly biased the cycle time of subsequently executed rhythmical actions. However, this imitation bias was only a small fraction of the differences in the observed cycle times, as well as of the differences produced in intentional imitation. Second, the PLOS ONE | www.plosone.org 7 October 2012 | Volume 7 | Issue 10 | e46728