Automatic Imitation in Rhythmical Actions Figure 5. Intentional imitation experiment: Cycle times (ms). Mean cycle times (with standard error of the mean) for the factors habitual speed, distractor speed, and the availability of vision of the hand, displayed relative to the actual distractor speeds. doi:10.1371/journal.pone.0046728.g005 imitation bias was reduced, although still present, when the action type and/or plane differed across the observed and executed actions. Third, these effects were obtained at three separate time points: during execution immediately after offset of the distractor stimulus, and when there was a delay of 4 and 8 s between action observation and execution. Fourth, the effects were equally present when participants had no vision of their executing hand, as well as Figure 6. Automatic vs. intentional imitation: Cycle time ratios (%). Mean cycle time ratios (with standard error of the mean) for the factors intention, action type compatibility, and plane compatibility (SA = Same Action; DA = Different Action; SP = Same Plane; DP = Differ- ent Plane). The cycle time ratio in the distractor actions was 150%. doi:10.1371/journal.pone.0046728.g006 when online visual feedback was available. Before we discuss these main findings for automatic imitation in greater detail, we briefly turn to performance in intentional imitation, which served as a reference condition as it informed about possible deviations from the criterion movements, despite explicit instructions to copy them. Intentional Imitation Experiment Subsequent to the main experiment, participants were asked to intentionally imitate the cycle times of the distractor movies with the same factorial manipulations as in the main study, except that only the 0 s delay condition was used. Overall, imitation performance was acceptably high. A few points are worth noting, however: For the habitually fast actions, participants slightly undershot the ratio of 150% between slow and fast ‘distractor’ speeds (140%), and more so for the habitually slow actions (132%). Such variance from the criterion range is not atypical for intentional imitation (e.g., [20,23,40– 41]. Visual monitoring of the hand during execution tended to speed-up execution in intentional imitation, relative to perfor- mance with eyes closed, but this was at the cost of reduced cycle time ratios (Figure 5). Since these effects were relatively small and were not echoed in automatic imitation, we do not discuss them further. Importantly, with the exception of one just-significant effect that did not involve the distractor speed, cycle times were copied regardless of the compatibility between the imperative and distractor actions (Figure 6). This indicates that participants can easily extract the cycle time from an action in a different plane, or from an action that is different to the one they plan to execute, and can map it onto their own motor performance when instructed to do so. PLOS ONE | www.plosone.org 8 October 2012 | Volume 7 | Issue 10 | e46728