studying discrepancies between alpha and mu power during the different phases of imitation learning. Consistent with the prediction from the global entrainment hypothesis that imitation learning would be supported by both visual and motor processes [15] alpha and mu rhythms were both found to desynchronize when participants observed and remembered sequences of movements for subsequent imitation, as compared to a control task in which subjects detected deviating (thumb) movements. This suggests that both visual and motor processes contributed to the current task of observational learning. Importantly however, no functional coherence was detected between visual and motor processes within the alpha/mu band as was predicted by the global entrainment hypothesis [15] and modulations in alpha and mu power were found to be largely independent from each other, arguing against the functional integration of alpha and mu oscillations in support of action observation and observational learning. The independence between visual and motor oscillations in the mu/alpha range differs from other studies that found posterior – central coherence in the 8–13 Hz frequency range [35,36]. In an experiment by Classen and others [36], for instance, subjects had to exert an oscillating force with their right index finger on a strain gauge. It was shown that central-posterior alpha coherence was highest when the force exerted had to be synchronized with a visually presented oscillating target dot. Another example is provided by Hummel and Gerloff [35] who simultaneously presented a tactile Braille letter to the right index finger tip and a visual Braille letter on a computer screen, resulting in a phasic increase in alpha coherence between central and posterior electrodes that was higher if subjects were successful in verifying if the visual and tactile letter matched or not. The main difference between these studies and the current study is probably that task properties of these other studies demand the immediate integra- tion between visual and sensorimotor information, whereas for observational learning the requirement for temporal integration between visual and motor information is less demanding. One other possibility may be that for observational learning to be effective, motor representations are to be protected from visual input that may interfere with the retention or rehearsal of action sequences, as was indicated by behavioral studies that investigated interference effects during learning of whole body movement patterns [37]. Irrespective, however, of the precise reason for the absence of visuomotor integration during imitation learning, the coherence data of the current experiment suggest that such an integration of visual and sensorimotor information is not a necessary requirement for observational learning to take place. In line with the conclusion that visual and motor processes operated largely independently from each other, several dissoci- ations between visual and motor processes as reflected in alpha and mu power were found. One prominent difference in mu and alpha power was that alpha desynchronization was strongest during the observation interval, suggesting maximal sensitivity to visual input, whereas mu desynchronization was strongest during the pause intervals, suggesting active involvement in the rehearsal of action sequences for subsequent reproduction. The latter finding of enhanced mu desynchronization during pause intervals is particularly interesting because it suggests that subjects engaged in active motor imagery supporting the maintenance and/or encoding of action sequences in memory for subsequent reproduction. Consistent with this interpretation, desynchroniza- tion effects in mu power were found to be stronger in the imitation task as compared to the detection task. Source localization of mu desynchronization using a minimum norm approach showed activation to originate from pre- and post-central gyri around the Observational Learning location of the hand area in the left and right central sulcus. Consistent with the interpretation that mu desynchronization during pauses may be a measure of motor imagery, EEG studies have reliably found motor imagery accompanied by desynchro- nization in mu oscillations originating from primary sensorimotor areas [38,39]. Previous studies investigating action observation of finger movements, facial expressions and goal directed grasping [17,18,40] have firmly implicated mu desynchronization as a measure of motor resonance. This view has furthermore been strengthened by studies that found stronger mu desynchronization to the observation of actions that were already established in the observer’s motor repertoire (e.g. dancing moves by professional dancers [9], or infants’ ability to crawl or walk [10]) as compared to actions that were not mastered yet. The present findings further extend the functional role of mu desynchronization beyond action observation by showing that mu is not only involved in the observation of already learned actions but also directly supports the acquisition of new actions. Importantly, although finger movements are probably well established in the observers’ motor repertoire, covert motor activation was stronger during the pause intervals than during actual action observation. This result points towards the involvement of mu in both action observation and motor imagery, and suggests that, when studying imitation and learning of new motor repertoire, one should be careful to distinguish between motor activation resulting from action observation and motor activation reflecting motor imagery. Consistent with the direct matching hypothesis, coherence analysis rendered two ipsilateral fronto-parietal networks, one over the left and one over the right hemisphere, revealing stronger fronto-parietal coherence during the observation of movement sequences than during subsequent pauses. The properties of these two networks, i.e. their fronto-parietal distribution, their ipsilateral organization and their sensitivity to the observation of movements, matches closely with the known properties of the MNS as uncovered by anatomical and electrophysiological studies in the macaque brain [41]. These studies have found strong parallel and reciprocal connections between ipsilateral regions of the frontal and parietal lobes supporting specific sensorimotor transforma- tions for action execution and action observation [42]. The present study is the first to find a possible correlate of the MNS reflected in fronto-parietal coherence. These findings call for further confir- mation, e.g. by studies that combine EEG and fMRI. Importantly, the strength of fronto-parietal coherence during imitation learning was found to be positively related to the accuracy of subsequent imitation. This suggests that the strength of the functional coupling between parietal and frontal parts of the network for imitation learning is decisive for the accuracy of newly acquired action sequences. Neuroimaging studies have suggested different functional roles for frontal and parietal parts of the MNS [43,44], for processing the goals of actions (i.e. the end-location or the object being targeted) and the means of observed actions (i.e. the selected grasp or movement trajectory). In the present sequence learning task participants needed to integrate informa- tion about the hand that was used (action means) and the specific target button visited (action goal) for each consecutive pointing action. A reasonable inference is that enhanced coherence between frontal and parietal MNS regions may support the integration of information about action goals and means during action observation and rehearsal for subsequent imitation. Consistent with this interpretation, Fogassi et al [45] found that the discharge rates of cells supporting the execution and observation of grasping acts in the macaque inferior parietal lobe were sensitive to the final goals of grasping actions (i.e. grasping to PLoS ONE | www.plosone.org 7 December 2010 | Volume 5 | Issue 12 | e14482