Exp Brain Res (2011) 211:601–612 required, and the colour of the index ﬁnger changed to green), and on 50% of trials, the ﬁnger that changed colour was incompatible with the required response (e.g. an index ﬁnger response was required, and the middle ﬁnger chan- ged colour). The RT difference between incompatible and compatible trials generated an index of effector priming (Effector Priming Effect). Stimuli In both the Effector Priming Control Task and Imitation Task, stimuli were presented on a computer screen at a distance of approximately 57 cm. Stimulus videos in the Imitation task were comprised of ﬁve frames (Fig. 1a). The ﬁrst was a photograph of a hand at rest against a blue worktop. This frame was displayed for a variable interval with range 800–2,400 ms and acted as a warning signal. The second frame was a photograph of the index or middle ﬁnger 1/3rd of the way through a lifting action and a number 1 or 2. The third was a photograph of the same ﬁnger 2/3 of the way through a lifting action. These frames were displayed for 34 ms. The fourth frame was a photograph of the ﬁnger in the fully lifted position, which was displayed for 500 ms. These display durations ensured the appearance of a short video clip of a ﬁnger 605 being lifted with the concurrent appearance of a number. The ﬁfth frame was a blank screen. This frame remained on screen until the duration of the trial had reached 3,000 ms and until the participant had returned both ﬁngers to the letters V and B on the keyboard. Stimulus videos in the Effector Priming Control task were comprised of three frames (Fig. 1b). The ﬁrst was a photograph of a hand at rest, displayed for a variable interval with range 800–2,400 ms. For the second frame, the ﬁnger in the photograph was coloured green, and a number 1 or 2 was added to the image. This frame was displayed for 568 ms. The third frame was a blank screen the same blue as the background in frames 1 and 2. This frame remained on screen until the duration of the trial had reached 3,000 ms and until the participant had returned both ﬁngers to the response keys; hence, both Imitation and Effector Priming Control Trial types were of the same duration. Testing procedure Participants were tested in groups of approximately 10–20 individuals under exam conditions; adolescents were tested in their school computer cluster room, and adults were tested in a computer cluster room at UCL. Participants Fig. 1 a The ﬁve frames of an Imitation Task trial. The frames shown here constitute a compatible video where the participant must lift their index ﬁnger whilst watching an index ﬁnger lift. Video duration was 3,000 ms. b The three frames of an Effector Priming Control trial. The frames shown here constitute a compatible Effector Priming trial where the participant must lift their index ﬁnger, whilst viewing an index ﬁnger highlighted by a green mask. Video duration was 3,000 ms 123