Automatic Imitation in Rhythmical Actions Figure 7. Hypothetical early-visual and sensorimotor processes during the three main events of an automatic imitation trial (for further details, see text). Event 1: Although the imperative picture did not specify execution speed, it is likely that the sensorimotor representation of the to-be-performed action included the habitual rhythm (cycle time A). Event 2: During presentation of the distractor movie, instructed and distractor actions are represented as two parallel and potentially competing sensorimotor streams. Event 3: The cycle time C during motor execution reflects the result of the biased competition for those two representations in Event 2. The dotted route indicates an alternative explanation of the imitation bias, via visual monitoring and related corrections during execution, for which no evidence was found in the present study. doi:10.1371/journal.pone.0046728.g007 Automatic Imitation Experiment: Effects of Delay and Visual Monitoring The third aim of the present study was to assess if the imitation bias would reduce when a short delay is inserted between Events 2 and 3, that is, between the end of the distractor presentation and movement onset. In previous RT studies, automatic imitation effects have been shown to decay after delays shorter than 1 s [7,45–46]. In reach trajectory, a similarly rapid decay was found after the self-priming from one’s own earlier reach path [47]. In contrast, Edwards et al. [30] demonstrated an imitation bias in reach-grasp kinematics following a longer (3 s) delay between action observation and execution. Relative to the generally short- lived priming effects in the above studies, we found a remarkable stability of the distractor effects across the three delay conditions (0, 4 and 8 s, see Figure 4). The only exception was a pocket of slightly enhanced distractor effects for immediate execution in a small subset of the data, namely fully-compatible, habitually slow actions. Whilst this finding requires further study (and fits well to the above framework), the dominant explanandum in the present results is certainly the relative longevity of the imitation bias compared to previous results. Since participants were not prevented from covert rehearsal of the instructed action in the delay period, the most likely explanation is that they continued to simulate this action, including its rhythm, in the delay period up until execution. In support of this interpretation, Vogt [41] documented visual and motor imagery as effective rehearsal strategies for maintenance of temporal information in short-term memory over 12 s. Thus, one likely factor distinguishing the present study from studies showing rapid decay of automatic imitation is the opportunity for covert rehearsal. Further factors could be the relatively long presentation duration of the distractor in the present study (4 s), as well as the nature of the dependent variable, given that Vogt et al. [7] and Gowen et al. [45] used reaction times as dependent measures, whereas the present study employed cycle times. The fourth aim of this study was to assess possible effects of visual monitoring of one’s own hand during execution, primarily in automatic imitation. If the imitation bias had been present only when vision of the hand was available during execution, then an explanation in terms of a purely visual (rather than visuomotor) representation of the distractor action could have accounted for the findings. Exploring performance without vision of the effector (see [21,24]) is particularly relevant in a relatively slow experi- mental paradigm such as ours, which provides the opportunity for vision-based corrections during execution. In contrast to inten- tional imitation, where visual monitoring indeed affected cycle time ratios (see Discussion above), automatic imitation effects were equally pronounced in the vision and no-vision groups (Figure 3). Importantly, this finding allows us to rule out a ‘visual’ interpretation of the observed imitation bias, according to which a purely visual representation of the distractor would subsequently PLOS ONE | www.plosone.org 10 October 2012 | Volume 7 | Issue 10 | e46728