4. Does automatic imitation rely on visual monitoring of the hand during execution? Methods We assessed automatic imitation in Experiment 1, which consisted of two sessions run on consecutive days (see Method, Sections 1 to 4). Intentional imitation was assessed in Experiment 2, which consisted of one session that was run 7 to 14 days after Experiment 1 (see Method, Section 5). 1. Participants Twenty participants (13 male, mean age 27.2 yrs; SD = 5.9 yrs) volunteered for the study. All had normal (n = 13) or corrected-to- normal vision. Participants were na¨ıve to the study’s purpose, right-hand dominant (Edinburgh Handedness Inventory: M = 87; [37]), and without physical injuries. They were randomly allocated to one of two vision conditions, either full vision throughout all procedures, or were asked to close their eyes during motor execution. Written informed consent was obtained prior to participation, and ethical approval had been granted by Teesside University and Lancaster University. 2. Task and Design In each experimental trial, participants watched a picture of a to-be-pantomimed everyday rhythmical action (imperative stimu- lus, see Figure 1 and Method, Section 3), followed by a short, action-irrelevant distractor movie of the same or a different action. They then executed the imperative pantomime action. We studied actions that are typically performed relatively slow (‘habitually slow actions’) as well as habitually fast actions. Within each habitual speed category, slow and fast versions of each distractor action were used. Experiment 1 consisted of six blocks of 32 trials, with three blocks run in each of the two sessions. A six-factorial mixed design Automatic Imitation in Rhythmical Actions was used. The availability of vision during execution was manipulated between subjects (vision vs. no-vision). The delay between action observation and execution was manipulated across the three blocks run in each session (0, 4, or 8 s), in a counterbalanced order across participants. The other four factors were manipulated within each block of trials: habitual action speed (slow or fast), distractor speed (slow or fast), action type compatibility (same or different action: SA or DA), and plane compatibility (same or different plane: SP or DP) between imperative and distractor actions. The two compatibility factors thus yielded four levels of compatibility: SA/SP, SA/DP, DA/SP, and DA/DP. Note that the two factors of action type compatibility and plane compatibility were derived from pooling the data from their four constituent factors, namely: imperative action type (face- or surface-oriented, see Figure 1 and Method, Section 3), imperative action plane (horizontal or vertical), distractor action type, and distractor action plane. The full combination of these four factors with habitual action speed and distractor speed resulted in 64 trials for each of the three delay conditions, half of which were presented in a quasi-random order within each block of session one, and the other half in session two. As a result of the pooling, each cell of the effective six-factorial design consisted of an average across four trials. 3. Stimuli and Apparatus A conventional digital video camera (Panasonic NV-MX500B) was used to create the imperative picture and distractor movie stimuli. Figure 1 shows the eight imperative stimuli: The two habitually slow actions were face washing and paint brushing, and the two habitually fast actions were tooth brushing and window wiping. Within each habitual speed category, one face-orientated and one surface-orientated action was used, each of which could be performed in either the vertical or in the horizontal plane. Given the relatively complex design, and given that we were only interested in the compatibility between imperative and distractor Figure 1. Imperative action stimuli with the factors plane, habitual speed, and orientation. doi:10.1371/journal.pone.0046728.g001 PLOS ONE | www.plosone.org 3 October 2012 | Volume 7 | Issue 10 | e46728