Automatic Imitation in Rhythmical Actions: Kinematic Fidelity and the Effects of Compatibility, Delay, and Visual Monitoring Daniel L. Eaves1,2*, Martine Turgeon2*, Stefan Vogt2* 1 Sport and Exercise Science Section, Teesside University, Middlesbrough, United Kingdom, 2 Centre for Research in Human Development and Learning, Department of Psychology, Lancaster University, Lancaster, United Kingdom Abstract We demonstrate that observation of everyday rhythmical actions biases subsequent motor execution of the same and of different actions, using a paradigm where the observed actions were irrelevant for action execution. The cycle time of the distractor actions was subtly manipulated across trials, and the cycle time of motor responses served as the main dependent measure. Although distractor frequencies reliably biased response cycle times, this imitation bias was only a small fraction of the modulations in distractor speed, as well as of the modulations produced when participants intentionally imitated the observed rhythms. Importantly, this bias was not only present for compatible actions, but was also found, though numerically reduced, when distractor and executed actions were different (e.g., tooth brushing vs. window wiping), or when the dominant plane of movement was different (horizontal vs. vertical). In addition, these effects were equally pronounced for execution at 0, 4, and 8 s after action observation, a finding that contrasts with the more short-lived effects reported in earlier studies. The imitation bias was also unaffected when vision of the hand was occluded during execution, indicating that this effect most likely resulted from visuomotor interactions during distractor observation, rather than from visual monitoring and guidance during execution. Finally, when the distractor was incompatible in both dimensions (action type and plane) the imitation bias was not reduced further, in an additive way, relative to the single-incompatible conditions. This points to a mechanism whereby the observed action’s impact on motor processing is generally reduced whenever this is not useful for motor planning. We interpret these findings in the framework of biased competition, where intended and distractor actions can be represented as competing and quasi-encapsulated sensorimotor streams. Citation: Eaves DL, Turgeon M, Vogt S (2012) Automatic Imitation in Rhythmical Actions: Kinematic Fidelity and the Effects of Compatibility, Delay, and Visual Monitoring. PLoS ONE 7(10): e46728. doi:10.1371/journal.pone.0046728 Editor: Thomas Boraud, Centre national de la recherche scientifique, France Received March 29, 2012; Accepted September 6, 2012; Published October 5, 2012 Copyright: 2012 Eaves et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: There was no external research funding in place to support this research. Competing Interests: The authors have declared that no competing interests exist. * E-mail: d.eaves@tees.ac.uk (DLE); mturgeon@stfx.ca (MT); s.vogt@lancaster.ac.uk (SV) Introduction In everyday life, we often mimic the postures and gestures of the people we interact with, typically without knowing or meaning to do so. This behavioural mimicry is also known to impact upon cooperative social attitudes [1]. There is now a substantial body of research into the neurocognitive mechanisms of such imitative phenomena, which essentially indicates that observing another person’s actions primes similar actions in the observer (visuomotor priming, for a review see [2]). More recently, this phenomenon has been termed ‘automatic imitation’ [3]: a type of stimulus-response compatibility (SRC) effect, wherein pre-instructed actions are initiated faster when observing a congruent compared to an incongruent distractor action (e.g., [4–7]). Using this approach, researchers have tackled a number of important issues, such as the distinctness of automatic imitation from spatial compatibility effects, and different criteria of automaticity [3]. A further proposal is that automatic imitation might represent a laboratory model of the behavioural mimicry found in more naturalistic social settings [3]. Unlike behavioural mimicry research, however, the reaction time (RT) methodology typically used in studies on automatic imitation does not convey information about the degree of similarity between an observed action and the related response. We address this under-researched issue by studying a core aspect of imitative alignment, namely rhythmicity. While rhythmic alignment is also likely to represent one of the main behavioural manifestations of ‘motor resonance mechanisms’ in the brain [2,8], in the present study we focus on observable behaviour at first. Imitation is a multi-level phenomenon. On the one hand, research has demonstrated that high-level behavioural goals, and not necessarily the detailed means to achieve them, are central to imitation. This is not only the case in imitation learning [9–12], but also in instantaneous imitative behaviour (e.g., [13]). On the other hand, behavioural studies also indicate that certain fine kinematic details of an observed action, often irrelevant for goal achievement, are automatically encoded and bias motor execution (e.g., [14,15]). The present study focuses on this latter aspect. In addition, we assessed the extent to which such low-level imitation depends on goal congruency, as well as on the spatial correspon- dence between observed and executed actions. Perhaps some of the clearest examples of low-level sensorimotor couplings come from research adopting a dynamical systems approach to rhythmical actions. In online synchronisation para- digms (i.e., simultaneous action observation and execution), rhythmical actions exhibit a spontaneous tendency to phase- PLOS ONE | www.plosone.org 1 October 2012 | Volume 7 | Issue 10 | e46728