Observational Learning Figure 1. Schematic Overview of the Task Structure. (A) Each trial consisted of 4 repetitions (S1 – S4) of an observation interval (6 s) and subsequent pause interval (5 s). Following these 4 repetitions, subjects either tried to imitate the movement sequence from memory (imitation task) or responded if they had observed a catch movement in the action sequence (detection task). (B) Each observation epoch consisted of six button press movements, followed by a 5000 ms pause interval. Each button press movement consisted of a 500 ms display showing the left and right hand at the respective left and right start buttons (S) and a subsequent 500 ms apparent motion display (M) showing either the left or the right hand pressing an ipsi- or contralateral target button. (C) Examples of stimuli: Start display (S) showing the two hands pressing the start buttons; Example of an ipsilateral movement (M1); Example of a contralateral movement (M2); During the Pause display the central red button on the response box was illuminated. doi:10.1371/journal.pone.0014482.g001 between participants SD = 23.3%). (mean percentage correct = 61.5%, Power analysis Statistical analysis of power in the alpha/mu frequency range was tested using a 2626264 within-subject ANOVA, with the factors Location (posterior electrodes pairs, central electrode pairs), Task (imitation, detection), Period (observation, pause), and Repetition (S1, S2, S3, S4). The percentage of correct sequence repetitions of each subject was added as a covariate to investigate if individual variations in alpha and mu power were systematically related to individual differences in imitation ability. Figure 2 presents the grand average activity in the mu/alpha frequency range over central and posterior sites. Alpha power over extrastriate visual areas at electrodes PO3 and PO4 was found to be stronger than mu power over motor regions at electrodes C3 and C4, as reflected by an effect of Location, F(1, 13) = 21.6, p,.001. Furthermore, a main effect of Task was found, indicating that both mu and alpha were more strongly desynchronized in the imitation task than in the detection task, F(1, 13) = 18.8, p,.001. Mu suppression over central areas was stronger during pauses than during actual movement observation, while over posterior electrodes the reverse was found, i.e. alpha was suppressed more strongly during observation than pause intervals, reflected by an interaction between Location and Period, F(1, 13) = 29.6, p,.001. Importantly, the power increase in alpha over visual areas during the pause was stronger in the detection task than in the imitation task. On the contrary, over motor regions a stronger suppression of mu power was found during the pause of the imitation task as compared to the pause of the detection task, reflecting the interaction between Location, Task, and Period, F(1, 13) = 4.4, p = .056. This latter effect points to a functional difference between mu and alpha in their contribution to observation and retention of movements in the two tasks. In order to further specify this three- way interaction two post-hoc analyses were conducted focusing on posterior electrodes PO3/PO4 and central electrodes C3/C4, separately. The results of these analyses are presented hereafter. The first ANOVA repeated measures contrast analysis directed at posterior alpha power revealed a main effect of Task reflecting stronger alpha desynchronization during the imitation task as compared to the detection task, F(1, 13) = 14.9, p,.01. In both tasks alpha desynchronized during observations of movements as compared to the subsequent pauses, as indicated by a main effect of Period, F(1, 13) = 11.2, p,.005. The topographical distribution of this effect is shown in the bottom left of Figure 2. Furthermore, a significant interaction between Task and Period was found, F(1, PLoS ONE | www.plosone.org 3 December 2010 | Volume 5 | Issue 12 | e14482