entrain, both for within-person [16] and for between-person coordination [17], even when unintended [18]. Oullier et al. [19] further showed that, following phase-entrainment, dyads preserved their new shared frequency (without instruction to do so) when vision was occluded over a 1 min observation period; an effect those authors termed ‘social memory’. While dynamical systems research has focused on online synchronisation, a separate branch of behavioural research has demonstrated an imitation bias in offline imitation paradigms (i.e., motor execution subsequent to action observation). To date, the following kinematic parameters have been studied: finger-tapping frequency [20], grip force [21–22], pointing velocity [23–24] and accuracy [25], object-transport velocity [26], reach-grasp trajec- tory [14,27], as well as its velocity and grip aperture [15,28–31]. These studies have shown that the imitation of kinematic details is sensitive to the observer’s a priori goals and to the action context [15], to the presence of spatial targets in the demonstrated action [24], to the proximity of the observed action to the observer’s peripersonal space [14], to changes in observed biological over non-biological velocity profiles [23], and to observed movement errors [25]. Furthermore, the imitation bias is not effector- dependent [28], and it can also influence the subsequent estimation of object properties [31]. However, in most of these studies the kinematic fidelity (i.e., the degree of similarity) between observed and executed actions was not quantified. In contrast, this was the principal aim of the present offline imitation study. Next we expand on the two main objectives for our study, and highlight a few key findings from the research listed above. Our first aim was to quantify automatic imitation for rhythmical actions as the kinematic fidelity (a) between observed and executed actions, and (b) relative to performance in intentional imitation of the observed rhythm. We used an extended SRC paradigm where participants performed one out of a set of eight everyday rhythmical actions (‘imperative action’) after observation of a short, action-irrelevant rhythmical distractor movie of the same or a different action. Across trials, the cycle times of the distractor actions were subtly manipulated (slow or fast). This allowed us to quantify kinematic fidelity as the ratio between executed and observed cycle times. Two recent studies provide data related to our first aim. In Bisio et al.’s [23] study, participants executed discrete pointing actions to one of two targets after observing a model perform the same action. Covert manipulation of the model’s reach velocity biased response velocities, relative to a condition without action observation. The magnitude of this imitation bias was roughly half of that observed for intentional imitation. Bove et al.’s [20] participants passively observed either a slow (1 Hz) or a fast (3 Hz) rhythmical finger-tapping action for 10 min. Immediately there- after, finger-tapping frequencies in both the slow and fast groups differed from a no-action-observation group. Across action observation groups, this bias was approximately 65% of that for intentional imitation. However, the data from these two studies might not provide adequate indices for the kinematic fidelity of automatic imitation, since their distractor actions were always the same as the to-be-performed action (albeit with different speed parameters). Consequently, participants could have reinterpreted these ‘task-irrelevant’ distractors as a task-relevant guide for action. This strategic coupling of motor preparation to the available visual input could then also have included the kinematic properties of the distractor. As a result, the effects reported in these two studies will most likely not reflect the kinematic fidelity of automatic imitation, given that the defining characteristic of the effect is that it occurs independently of intention [3]. We addressed this issue by assessing the imitation bias both for distractor actions that Automatic Imitation in Rhythmical Actions resembled the imperative action (as in the above studies), and for distractor actions that differed from the imperative actions along two dimensions, as discussed next. Our second aim was to study the imitation bias for two dimensions of compatibility, namely action type (or action goal), and plane of movement (horizontal or vertical). At present we are unaware of any studies that have directly manipulated action type compatibility in this context. It is thus unclear to what extent incompatible distractor actions might affect the imitation bias relative to compatible actions. To this end, we displayed Different Actions performed in the Same Plane (DA/SP) of movement as the imperative stimuli, in addition to a fully compatible Same Action/Same Plane (SA/SP) condition. A second factor that might modulate the imitation bias is spatial compatibility. For example, the kinematics of rhythmical arm movements have been shown to be biased by the simultaneous presentation of an orthogonal rhythmical distractor [32–35]. In addition, Hove et al. [36] showed a tighter coupling between finger-tapping and a visual rhythm when these were spatially compatible (e.g., simultaneously downwards), rather than in orthogonal planes. While these studies demonstrate spatial compatibility effects in online synchronisation, we studied these effects in our offline paradigm, using a Same Action/Different Plane (SA/DP) condition. According to the above considerations, one would expect that both compatibility dimensions, action type and plane, affect the strength of the imitation bias to some extent, where genuine automatic imitation is assessed only via the incompatible conditions. Furthermore, we were interested in whether the detrimental effects of action and plane compatibility, if found, would combine in an additive manner when action type and plane were both incompatible in a fourth, Different Action/Different Plane (DA/DP) condition. Note that we have just described the possible effects of incompatible action types and of incompatible planes separately from our earlier argument, namely that in trials where instructed and distractor actions are compatible, participants might simply copy the distractor action. However, these two explanations most likely coincide, at least in the present paradigm: the very reason why incompatible action types and planes give rise to smaller effects than compatible distractors might be that the former are not suitable for concurrent action planning. This point will be developed further in the Discussion. In addition to the two main research aims described above, we manipulated two further variables: the delay between distractor presentation and execution, and the opportunity for visual monitoring of one’s own hand during execution (the latter as a between-subjects factor). For brevity, the rationale for these additional manipulations is explained, together with the related results, in the Discussion. In summary then, we addressed the following four research questions in the present study: 1. What is the kinematic fidelity of the automatic imitation of rhythm, assessed (a) (b) as the gain between cycle time ratios in observed and executed actions, and relative to motor performance in intentional imitation of the observed rhythms? 2. Are automatic imitation effects reduced when imperative and distractor actions differ regarding (a) action type, (b) plane, and (c) both compatibility dimensions? 3. Do automatic imitation effects persist over a 4 s and 8 s delay between action observation and execution? PLOS ONE | www.plosone.org 2 October 2012 | Volume 7 | Issue 10 | e46728