Corresponding author: Ruth Raymaekers, Ghent University, Department of Experimental-Clinical and Health Psychology, Research Group Developmental Disorders; Henri Dunantlaan 2, 9000 Gent; Selection Procedure  Clinical diagnosis  Questionnaires for parents and teacher: Disruptive Behavior Disorder rating scale Child Behavior Checklist / Teacher Report Form Social Communication Questionnaire  Diagnostic interview parents Autism Diagnostic Interview – Revised Diagnostic Interview Schedule for Children-IV  WISC-III: Total IQ > 85 (Vocabulary, Similarities, Block Design, Picture Arrangement) In the current study, children between 8 and 13 years participated. The HFA group contained 20 children and was matched on age and IQ to a group of typically developing children. All children were asked to participate in a task that consisted of 4 conditions. In three of them, a video had to be watched: (1) a video of a hand movement, (2) two balls that moved to and fro each other, and (3) a video of white noise or static. In the other condition (4), the children had to imitate the hand movement. All conditions were counterbalanced with the restriction that the self movement condition always came after the video of the moving hand. Throughout the experiment, children were asked to sit as quietly as possible and watch the videos carefully. Mu suppression was calculated as the natural log of the ratio of the power in the experimental conditions relative to the power during the baseline condition (white noise). The ratios were measured by three electrodes close to the sensorimotor cortex: C3, Cz and C4. diPelligrino (1992), Rizzolatti and colleagues (2001) discovered a unique set of premotor neurons in macaque monkeys called mirror neurons. These neurons appear to respond both when a primate performs an action and when it sits motionless observing another individual performing the same action. There is indirect evidence that a comparable neuronal system is present in humans, most likely also situated in the premotor cortex. Thus, the mirror neuron system (MNS) is a type of observation/execution matching system that is primarily thought to be involved in perception and comprehension of motor acts. In addition, the human MNS has been implicated in a variety of higher order cognitive processes: imitation, TOM abilities, empathy, social cognition and language. Autism Spectrum Disorders (ASD) are characterized by deficits in these skills. MNS as neural substrate for deficits in social interaction and communication in ASD? Several studies have investigated the MNS in ASD leading to the finding that this system is dysfunctional in ASD. Unfortunately, the number of participants was mostly small and the age range large. Furthermore different neurophysiological methodologies have been used: fMRI (Williams et al., 2006; Dapretto et al., 2005) or EEG (Oberman et al., 2005, 2006; Bernier et al., 2007). The MNS can be examined with the aid of EEG measurements, by monitoring the so-called mu frequency band oscillations. At rest, motor neurons spontaneously fire in synchrony leading to large amplitude EEG oscillations in the 8 through 13 Hertz frequency band (mu band). When subjects perform an action, these neurons fire asynchronously, thereby decreasing the power of this mu band. During observed motor actions, the MNS is the only network that has been identified to be active in the area of the premotor and sensorimotor cortices, suggesting that mu wave suppression to observed actions could be used as a selective measure of the functioning of the MNS. The manipulation was successful, the adopted methodology therefore proved suitable to probe the functioning of the MNS. However, we have not found a confirmation of the results of other MNS studies in ASD. Our results indicate that the MNS is functioning in the control group as well as in children with HFA. A quick check of correlations with other variables showed that the observed effects might depend on these variables. Inverse correlations were identified between mu suppression and IQ for HFA and control groups. So, it appears that these correlates have to be taken actively into account when setting up a paradigm and definitely call for further research. Introduction Discussion Results Method EEG study of the mirror neuron system in children with HFA Ruth Raymaekers, Jan Roelf Wiersema & Herbert Roeyers HFA N = 20 Control N = 15 CA(years) M sd IQ M sd Gender M : F 18:212:3 ControlsHFA Frequency (Hz) C3 CzC4 White noise Video hand Hand Control HFA Control groupASD group Self-condition: mu suppression is significant in this condition as compared to the white noise baseline. There is no significant group difference. Video displaying a moving hand: relative to the baseline, the mu frequency is again suppressed. But suppression is not that strong as compared to the self condition. For the ASD group, the MNS is apparently also working. All but for electrode C4, there is a clear mu suppression with respect to the baseline. Furthermore, the differences between the groups are not significant for electrodes Cz and C4. For electrode C3, at best a trend is observed.