Observational Learning Figure 2. Effects on Alpha and Mu Power. (A) Alpha power (left) and mu power (right) during observation (O1, O2, O3, O4; black symbols) and pause (P1, P2, P3, P4; white symbols) intervals in the imitation task (squares) and the detection task (circles). Squares labelled with ‘‘Imitate’’ represent power levels during execution of the observed movement sequence. (B) Topographic representation of alpha power (left) maximal at bilateral parieto-occipital electrode sites, and mu power (right) overlying the left and right rolandic fissure. Alpha and mu topographies reflect the difference between pause and observation intervals in the detection task and the imitation task, respectively. doi:10.1371/journal.pone.0014482.g002 13) = 9.3, p,.01, reflecting the stronger rebound effect during the pauses of the detection task than the imitation task. Other effects did not reach significance. A second ANOVA repeated measures contrast analysis was directed at mu power over central electrodes. Effects in mu frequency power at electrodes C3/C4 are presented in the right panel of Figure 2. As on posterior electrodes, stronger overall desynchronization was found in the imitation task as compared to the detection task, reflected by a main effect of Task, F(1, 13) = 26.9, p,.001. Different from the stronger rebound in alpha power during pauses, mu activation was found to be suppressed more strongly during pauses than during observation intervals, as reflected by a main effect of Period, F(1, 13) = 6.3, p,.05. The topographical distribution of this effect is shown in the bottom right of Figure 2. A trend towards stronger desynchronization during pauses of the imitation task than in the detection task was found, as reflected by the interaction between Period and Task, F(1, 13) = 4.0 p = .066. Finally, an interaction between Period and Repetition was found, F(3, 39) = 4.6, p,.05. Subsequent contrast analysis to specify this interaction indicated that mu suppression during the last pause that preceded execution was stronger than in other pauses. In none of the analyses of power data did we find any effect of or interaction with imitation accuracy. Source localization of effects in mu and alpha power Source localization of alpha power, i.e. the difference in power between pause and observation intervals in the detection task, showed maximal effects in the left occipital gyrus, extending to the parieto-occipital sulcus dorsally (see Figure 3 left). Mu power, reflected by the difference between pause and observation intervals in the imitation task, was localized to originate from the location of the hand area in the left and right central sulcus (see Figure 3, PLoS ONE | www.plosone.org 4 December 2010 | Volume 5 | Issue 12 | e14482