Automatic Imitation Experiment: Kinematic Fidelity and the Effects of Incompatible Distractor Stimuli In the automatic imitation experiment we found robust effects of distractor speed for both habitually slow and habitually fast actions (Figure 3). These effects were markedly smaller than those in intentional imitation, despite identical stimulus conditions (modulation of 150%). In the compatible condition (SA/SP), the ratio of participants’ slow:fast cycle times in response to the different distractor speeds was 112%, and only 106% across the incompatible conditions (Figure 6). Expressed relative to inten- tional imitation, the modulation by the distractor speeds equated to 37% for the compatible condition, and to 16% for the incompatible conditions. Similarly, for discrete pointing move- ments, Bisio et al. [23] showed that reaching velocities in their implicit (compatible) imitation condition considerably undershot the modulations in the stimuli. A first general conclusion from this and our work is that modulations in distractor displays are typically not copied with high kinematic fidelity, in the sense of a 1:1 match. Rather, participants’ kinematics normally only exhibit small, but reliable biases towards the distractor. Crucially, our study further demonstrates automatic imitation effects when the distractor action was different from the instructed action, as discussed next. We correctly anticipated that the imitation bias would be attenuated in the three incompatible conditions, relative to the fully-compatible SA/SP condition. This result contrasts with intentional imitation, where participants copied the rhythms in compatible and incompatible conditions alike. The significant (though reduced) distractor effects in the incompatible conditions are particularly important in light of our analysis of the putative ‘distractor effects’ in earlier studies (e.g., [20,23], see Introduction). In those studies, as well as in our compatible SA/SP condition, participants could have utilised the ‘task-irrelevant’ distractor as a valid guide for their own actions. Accordingly, we would submit that only the present incompatible conditions can be taken as evidence for genuine automatic imitation. Why were distractor effects reduced (while still statistically significant) in the present incompatible conditions, relative to the compatible SA/SP condition? We first provide an account based on action and plane compatibility as separable factors, and then propose a more integrative account based on competing sensori- motor representations. Given the crucial role of action goals in action observation and imitation, it is plausible that a distractor action with the same goal as the imperative action is not only more likely to be imitated as an action category (as demonstrated in RT studies on automatic imitation; [3]), compared to a DA/SP distractor, but also that low-level distractor features, such as its rhythm, are imitated more accurately. A similar case could be made for our plane compatibility manipulation, as sensorimotor synchronisation improves with compatible spatial information [36,42]. Although the present offline paradigm did not involve overt synchronisation, it is likely that participants covertly synchronised the observed distractor action with their own motor planning, as described next. Figure 7 shows the three main events of the present paradigm, along with hypothetical visual and sensorimotor processes. Although the imperative picture stimulus per se did not specify a particular rhythm, participants had been instructed to perform the habitually slow and fast actions at different speeds. This was reinforced throughout the experiment by the fact that, for example, habitually slow imperative actions were always followed by a distractor movie of a habitually slow action (performed at 60 or 90 bpm). We therefore assume that, upon seeing the imperative stimulus (Event 1), participants formed a sensorimotor representation which included that action’s Automatic Imitation in Rhythmical Actions habitual rhythm (slow or fast). During presentation of the distractor movie (Event 2), participants were then able to simulate the instructed action in real-time [43], and to synchronise this internal simulation with the observed distractor action (overlap- ping boxes in Figure 7). According to Hove et al. [36], for example, a tighter synchronisation would then be predicted for spatially compatible actions. As a result, during motor execution (Event 3), cycle time C would be more strongly biased by distractor actions with matching, compared to non-matching, planes. Whereas the above account of reduced distractor effects in the incompatible conditions is well in line with the existing literature (see Introduction), two of our present findings do not fit. First, this account should in principle also apply to intentional imitation, but we found no related differences between compatibility conditions in intentional imitation. In addition, we found significant interactions between intentionality of imitation and each of the compatibility factors: results which underline that the attenuation by incompatible distractor actions was specific for automatic imitation. Second, the effects of action and of plane compatibility in automatic imitation were not additive. That is, no further attenuation was observed when both action and plane were incompatible (DA/DP), relative to the other two incompatible conditions. If, however, one assumes separable contributions of action and plane compatibility, the DA/DP condition should show the strongest attenuation, and there should be no interaction between action and plane compatibility - neither of which was the case in the present study. We are therefore inclined to favour a more integrative account, according to which the distractor’s impact on motor processing is generally reduced whenever this is not functional for the observer’s own motor planning, as developed further in the following. It is useful to conceptualise automatic imitation effects, such as in the present paradigm, in the framework of Cisek and Kalaska’s [44] biased competition hypothesis. During Event 2 (distractor presentation), the instructed and distractor actions can be modelled as two parallel, and potentially competing sensorimotor streams. In case that the distractor action is identified as action- irrelevant (all incompatible conditions), the competition between these two streams is strongly biased towards the instructed action. Consequently, attempts to covertly synchronise the two streams will be relatively sparse. Motor preparation then proceeds largely (but not necessarily completely) decoupled from the available visual input, which would result in relatively small distractor effects during subsequent execution, as observed in the present study. In contrast, when the distractor action widely overlaps with the intended action (compatible SA/SP condition), the two corre- sponding sensorimotor streams merge, rather than compete. Covert synchronisation between intended and observed actions will be enhanced relative to the incompatible conditions, and this will, in turn, result in stronger distractor effects during execution. In summary, in this biased competition account, the strength of distractor effects depends on the general usefulness of the concurrent visual input for supporting motor preparation, whereas our earlier account assumed separate effects of matching goals and planes. Further research will be required to identify the conditions under which the imitation bias might (a) approach those in intentional imitation (e.g., by manipulating the ratio of compatible and incompatible trials, or the action-relevance of incompatible distractor trials), and (b) might be further reduced or even annihilated. In addition, the proposed role of covert synchroni- sation between distractor action and simulation of the to-be- executed action, as a core mechanism of the observed imitation bias, also requires further investigation. PLOS ONE | www.plosone.org 9 October 2012 | Volume 7 | Issue 10 | e46728