At birth, human infants and newborns of other primate species demonstrate the capacity to attend and to respond to facial stimuli provided by a caregiver. Newborn infants are also capable of exhibiting a range of facial expressions. Identification of the neural underpinnings of these capacities represents a formidable challenge in understanding social development. One possible neuronal substrate is the mirror-neuron system assumed to activate shared motor cortical representations for both observation and production of actions. We tested this hypothesis by recording scalp EEG from 1- to 7-day-old newborn rhesus macaques who were observing and producing facial gestures. We found that 5-6 Hz EEG activity was suppressed both when the infants produced facial gestures and while they were observing facial gestures of a human experimenter, but not when they were observing nonbiological stimuli. These findings demonstrate the presence of neural reactivity for biological, communicatively relevant stimuli, which may be a likely signature of neuronal mirroring. The basic elements of the mirror-neuron system appear to operate from the very first days of life and contribute to the encoding of socially relevant stimuli.
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