Neural correlates of action understanding in infants: influence of motor experience

Mirror neurons are recognized as a crucial aspect of motor and social learning yet we know little about their origins and development. Two competing hypotheses are highlighted in the literature. One suggests that mirror neurons may be innate and are an adaptation for action understanding. The alternative, proposes that mirror neurons develop through sensorimotor experience. To date, there has been little direct evidence from infant studies to support either argument. In the present study, we explored the temporal dynamics and spatial distribution of electroencephalography (EEG) brain responses in young infants during the observation of three distinct types of actions: (a) actions that are within the motor repertoire of infants, (b) actions that are not within the motor repertoire of infants, and (c) object motion. We show that young infants had significant motor resonance to all types of actions in the sensorimotor regions. Only observation of human goal-directed actions led to significant responses in the parietal regions. Importantly, there was no significant mu desychronization observed in the temporal regions under any observation condition. In addition, the onset of mu desychronization occurred earliest in response to object motion, followed by reaching, and finally walking. Our results suggest that the infants may have a basic, experience-independent sensorimotor mechanism optimized to detect all coherent motion that is modulated by experience.