actions regarding action type and plane, rather than in the separate effects for each individual action, we pooled the data across imperative action types and planes (as described above). The model performed all actions with the left hand to provide mirror images of the participants’ subsequent actions, who always used their right hand. This arrangement provided spatial compatibility between displayed and performed actions, which has been shown to facilitate imitation relative to an anatomically matched but spatially incompatible arrangement (e.g., [11,38]). Sixteen distractor movies were used in the main experiment, one slow and one fast version of each of the eight imperative actions. During filming, the model’s performance had been paced by a metronome to achieve the exact distractor speeds shown in Table 1, whereas throughout the experiment all stimuli were displayed without sound. Importantly, imperative stimuli were always paired with a distractor stimulus from within the same habitual speed category. We used two habitual speeds for two reasons: First, we wanted to assess the imitation bias of the distractor movies on motor execution across a range of cycle times and not just for one speed. Second, the fact that participants executed, in quasi-random order, rhythmical actions with two substantially different habitual speeds served to divert their attention away from the more subtle manipulation of the distractor speeds. Finally, note that each imperative action was displayed with the relevant object (sponge, paintbrush, toothbrush, or cloth), which enabled quick discrimination between the actions, whereas participants performed pantomimed actions (without objects). The latter was done to avoid participants having to select the relevant object in the beginning of each trial. The distractor movies showed pantomimed actions to allow participants to better distinguish between imperative and distractor stimuli, and to potentially strengthen the impact of the distractor stimuli on the subsequent pantomimed execution. Participants sat at a wooden desk in a dimly-lit room facing a 17-in LCD computer monitor (Apple Studio Display) positioned approx. 80 cm away from their head. All stimuli were displayed against a light grey background via PsyScript 2.3 software (http:// www.psych.lancs.ac.uk/software/psyScript.html) running on a Power Macintosh G4 computer fitted with a digital I/O board. The start location for the participants’ right index finger and thumb was on an electro-conductive plate mounted on top of a 23 cm-tall wooden post, 20 cm ahead of them on the desk. A magnetic motion sensor was fitted to the distal end of the second metacarpal bone of the right hand. Participants’ kinematic data were sampled at 103 Hz in 3-D space for 4 s periods using a Minibird Magnetic Tracking System (Ascension Technology Corp.), and were stored on a separate PC. At the end of each trial, kinematic data plots were displayed on a second monitor, unseen by participants. Table 1. Distractor stimuli specifications. Parameters Distractor speed Beats per min Cycle times (ms) Total cycles in 4 s Slow:fast ratio (%) doi:10.1371/journal.pone.0046728.t001 Habitually slow actions Habitually fast actions Slow 60 1000 4 150 Fast 90 667 6 Slow 120 500 8 150 Fast 180 333 12 Automatic Imitation in Rhythmical Actions 4. Procedure Familiarisation. In Phase 1 of the familiarisation period, participants learned to pantomime each action from a set of eight familiarisation movies (eight actions with two attempts each). These movies were identical to the movies for the main experiment, except that the cycle times were mid-way between the distractor speeds shown in Table 1, that is, 75 bpm for the habitually slow actions, and 150 bpm for the habitually fast actions. Participants were given verbal feedback about their movement based on the kinematic plots visible to the experiment- er. This ensured that their movement amplitude and cycle time aligned closely with the medium-paced stimuli. In Phase 2, participants saw a picture of each action while simultaneously pantomiming the same action for 4 s (16 trials). In Phase 3, they experienced the structure of trials in the main experiment, including the four compatibility conditions, but using the medium-paced distractors (16 trials). In Phases 2 and 3, verbal feedback was only given if movements occasionally drifted away from the criterion amplitude (10 cm for all actions) or cycle times. Short versions of this familiarisation procedure were run on each new day of testing. Main experiment. When participants placed their fingers in the start location, a green circle appeared on the monitor for 1 s to mark the beginning of a trial (Event A in Figure 2). (B) Then a picture of the to-be-pantomimed (imperative) action was shown for 1.5 s, followed by (C) a distractor movie of the same model pantomiming either the same or a different action. During movie presentation, participants fixated on the model’s left eye to minimise any visual coupling to the model’s rhythmical arm movements [39]. (D) In blocks with delayed execution, a red circle was shown for either 4 or 8 s. (E) Finally, the imperative action was performed while movement kinematics were tracked in 3-D. The end of the 4 s recording interval was indicated by a computer- generated auditory signal, after which participants could verbally report distractor characteristics (see below) before moving their hand back to the start location. The core manipulation across trials was that of distractor speed, with a ratio of slow:fast movements of 150% (see Table 1). Participants were not informed of the distractor speed changes, and this manipulation was further concealed by the more prominent differences between the two habitual speeds across trials. To focus their attention on the distractor movie, participants were asked to verbally recall the distractor properties (action type and plane) after motor execution on approx. 10% of trials. Experiment 1 was distributed over two sessions to reduce the possibility of physical fatigue from prolonged testing. Each session consisted of three blocks of 32 trials (see Method, Section 2), and each block was preceded by a single warm-up trial and followed by a rest period of 5 min. 5. Experiment 2: Intentional imitation Design and procedures of the intentional imitation experiment were essentially the same as those for the automatic imitation experiment described above, with the following differences: First, participants were asked to execute the imperative actions while imitating the cycle times of the distractor movies as precisely as possible. Second, we only assessed the 0 s delay condition, for which intentional imitation should be optimal. Experiment 2 thus involved only two blocks of 32 trials. As before, participants fixated on the model’s left-eye during distractor observation, but they did not verbally recall the distractor actions between trials. PLOS ONE | www.plosone.org 4 October 2012 | Volume 7 | Issue 10 | e46728