Mirror system deficit in ADHD? Study of Mu rhythm suppression during observation and imitation of emotion-related facial movements

Abstract

Mu rhythm is an EEG oscillatory measure (9 -13 Hz), whose suppression during movement execution or action observation is considered an indicator of the activity of the human analog of the mirror neuron system. Detection of facial emotional expressions involves detailed observation of facial movements. ADHD is known to be associated with social cognition impairments, including emotional face perception, theory of mind deficits and reduced empathy. The present work compared Mu suppression in 22 ADHD children and 18 matched control participants (32 % female, ages 9 to 14, Mean 10.8 SD: 1.5), while observing and later imitating the movements involved in the dynamic unfolding of facial emotion expressions. Participants observed and imitated facial expressions performed by actors in 96 short video clips, composed by two seconds of a neutral face and two seconds where a happy, sad, angry, or fear expression unfolded. The clip was preceded by a 1 second fixation cross and followed by a 1 second blank screen. In each trial, after the observation stage, they were asked for the presence of a specific emotion, (present in 50% of the trials). Then, they had to press a key to start co-acting the same facial movements. EEG (40 channels) was recorded using a NeuroScan NuAmps system. Eye movements were recorded using an Eyelink 1000 system. After artifact rejection, epochs of -500 ms to 3000 ms were extracted. The Morlets’ wavelets procedure was used for time frequency analysis and a -500 to 0 ms window was used as a baseline for Z scoring. Change in power in the 9-13 Hz band during the 2000 ms window where movement occurred was used for statistical analysis. Results showed significant Mu suppression over central regions, more marked over the left hemisphere, during observation and imitation (F (1, 38) = 3.07, p b 0.05). Movement execution produced larger suppressions than observation in Controls. Suppression in the Control Group was significantly larger than in the ADHD group in both conditions (F (1, 18) = 6.68, p b 0.01) for all emotional expressions. Furthermore, observation and imitation did not significantly differed in the ADHD group. Nevertheless, when comparing the groups using only those trials in which a significant Mu suppression occurred no between groups difference was observed. Lack of suppression in other networks has been reported in ADHD. This pattern is compatible with a functional, but not structural, deficit of the mirror system in ADHD.