place vs. to eat), i.e. reflecting the capacity of the MNS to anticipate and integrate successive motor acts (grasping and placing) in an action chain. Interestingly, the anticipatory effects of action chaining were found both during execution and observation of similar grasping acts by an experimenter, confirming the functional contribution of the MNS to both action production and action observation. Regarding the present study, correlation analyses between imitation accuracy and fronto-parietal coherence per repetition and period, indicated that performance differences were most clearly expressed during the first observation of the movement sequence and the third pause interval. This pattern suggests that, in addition to the general difference in fronto-parietal coherence between good and bad imitators, good imitators pay better attention to, or are better able to integrate information, about action goals (left/right button) and action means (left/right hand) during the first sequence presentation. Furthermore, the correla- tion during the third pause interval suggests that the functional contribution of fronto-parietal coherence is not limited to action observation only, but may also support the active rehearsal of a movement sequence in memory [45]. The positive correlation between imitation accuracy and fronto-parietal coherence during the third pause interval suggests that good imitators were better able to activate a correct representation of consecutive goal-means relations in memory for subsequent reproduction. An interesting dissociation is that imitation accuracy correlated with fronto-parietal coherence, but not with mu and alpha power. The absence of a correlation between imitation accuracy and power measures confirms the interpretation that both alpha and mu power represent activation in relative peripheral systems, i.e visual extrastriate regions and sensorimotor cortex, that may not be directly involved in representing the order of an action sequence or the integration between goals and means, but seem to provide low level support during the observation and motor imagery of action sequences in memory. Instead, both the topography of fronto-parietal coherence overlying higher-order motor systems in frontal and parietal cortices and the relation between coherence strength and imitation accuracy point towards a central role in imitation learning. An important question to ask is to what extent the fronto- parietal coherence that we found to support observational learning is representative for the learning of new motor repertoire in more naturalistic settings. One property of the present task is that was quite difficult for subjects to accomplish faultless imitation of the entire sequence. The complexity and difficulty of the present sequence learning task may not, however, be that different from natural conditions in which individuals e.g. have to learn a new sequence of dance steps, or learn the correct order of actions (switching gears, use indicator, looking, etc.) when approaching a crossroad during driving lessons. It is still unclear however to what extent fronto-parietal coherence may support other types of observational learning where the focus is not on combining established movement elements in a new arrangement, but on the learning of an entirely new movement primitive, for instance when children in elementary school have to imitate a teacher’s posture of holding a pen to allow writing. In conclusion, the present study was successful in uncovering different neural mechanisms and their potential roles in observa- tional learning. Observational learning was found to be supported by a pair of fronto-parietal coherence networks, one overlaying the left and one overlaying the right hemisphere. Coherence in these fronto-parietal networks likely provides a direct measure of activation and coherence within the human MNS. Fronto-parietal coherence predicted the accuracy of subsequent imitation, Observational Learning indicating a central role for these structures in imitation learning. In addition, observational learning was found to be supported by activation in visual extrastriate and central motor regions, reflected by power modulations in alpha and mu components. Although visual and motor processes reflected in alpha and mu power may be jointly activated in support of action observation and motor imagery, both the variance in mu and alpha power over time and the absence of coherence between occipito-parietal and central motor regions argue against the global entrainment hypothesis and the hypothesized coupling between alpha and mu oscillations in support of observational learning. Although the present study indicates a role for visual processes in imitation learning, the current findings generally confirm the direct-matching hypothesis of imitation learning which states that observational learning depends on motor resonance in high-level motor structures making up the core MNS. In addition the results obtained during the pause intervals make clear that imitation learning, for a good part, relies on motor imagery and that studies investigating observational learning will need to distinguish between motor activation resulting from action observation and motor imagery for subsequent imitation. We hope that the current study will inspire other researchers to further investigate the neural and functional basis of one of most central of human abilities, i.e. the capacity to learn new behavioral repertoire, through action observation. Materials and Methods Participants The study was approved by the ethics committee of the Faculty of Behavioural Sciences from the University of Twente. Fifteen university students (9 females) participated in the experiment and were given course credits as reward. All participants were between 18 and 26 years of age, with a mean age of 20.5 years (SD = 2y). Subjects were right handed as assessed by the Annett Handedness Inventory [46] and provided written informed consent before the study. Apparatus A custom made response-box was used, consisting of four buttons that were arranged in a square shape and a fifth button located in the middle (see Figure 1). Each button had a built-in red LED which could be illuminated. The response-box was connected to a PC and positioned in front of a 17 inch computer monitor (Philips 107T5, refresh-rate 85 Hz) that was used for presenting visual stimuli to subjects. The experiment was programmed and controlled in E-prime (Version 1.2., Psychology Software Tools, Inc. Pittsburgh, USA). Procedure In each trial, participants were presented with a sequence of 6 button presses on the computer screen. For each button press movement two subsequent pictures were presented, the first one showing the index fingers of both hands at the left and right (proximal) starting buttons of the response box, and a second picture showing one of the hands pressing one of two (distal) target button (see Figure 1). Six individual button press movements were presented in succession with each movement lasting 1000 ms (12 pictures of 500 ms each). The end-locations of each movement were selected from a total of four pictures that reflected the different combinations of hand (left, right) and button (left, right). Each of these four movements was presented at least once in every sequence of movements, but never thrice, and the same movement was never repeated more than once in succession. Directly following the sequence of six movements, a 5000 ms pause was PLoS ONE | www.plosone.org 8 December 2010 | Volume 5 | Issue 12 | e14482