Brain-imaging studies have shown that the human Broca's region and precentral motor cortex are activated both during execution of hand actions and during observation of similar actions performed by other individuals. We aimed to clarify the temporal dynamics of this cortical activation by neuromagnetic recordings during execution, on-line imitation, and observation of right-hand reaching movements that ended with a precision pinch of the tip of a manipulandum. During execution, the left inferior frontal cortex [Brodmann's area (BA) 44] was activated first (peak Ϸ250 ms before the pinching); this activation was followed within 100 -200 ms by activation in the left primary motor area (BA4) and 150 -250 ms later in the right BA4. During imitation and observation, the sequence was otherwise similar, but it started from the left occipital cortex (BA19). Activation was always strongest during action imitation. Only the occipital activation was detected when the subject observed the experimenter reaching his hand without pinching. These results suggest that the left BA44 is the orchestrator of the human ''mirror neuron system'' and is strongly involved in action imitation. The mirror system matches action observation and execution and probably contributes to our understanding of actions made by others.

Subjects with Asperger's syndrome (AS) are impaired in social interaction and imitation, but the underlying brain mechanisms are poorly understood. Because the mirror-neuron system (MNS) that matches observed and executed actions has been suggested to play an important role in imitation and in reading of other people's intentions, we assessed MNS functions in 8 adult AS subjects and in 10 healthy control subjects during imitation of still pictures of lip forms. In the control subjects, cortical activation progressed in 30 to 80-millisecond steps from the occipital cortex to the superior temporal sulcus, to the inferior parietal lobe, and to the inferior frontal lobe, and finally, 75 to 90 milliseconds later, to the primary motor cortex of both hemispheres. Similar activation sites were found in AS subjects but with slightly larger scatter. Activation of the inferior frontal lobe was delayed by 45 to 60 milliseconds and activations in the inferior frontal lobe and in the primary motor cortex were weaker than in control subjects. The observed abnormal premotor and motor processing could account for a part of imitation and social impairments in subjects with AS.

Broca's region, classically considered a motor speech-production area, is involved in action understanding and imitation. It also seems to help in sequencing of actions. Broca's region might have evolved for interindividual communication, both by gestures and speech.

Aggressive behavior is widely observed throughout the animal kingdom because of its adaptiveness for social animals. However, when aggressive behavior exceeds the species-typical level, it is no longer adaptive, so there should be a mechanism to control excessive aggression to keep it within the adaptive range. Using optogenetics, we demonstrate that activation of excitatory neurons in the medial prefrontal cortex (mPFC), but not the orbitofrontal cortex (OFC), inhibits inter-male aggression in mice. At the same time, optogenetic silencing of mPFC neurons causes an escalation of aggressive behavior both quantitatively and qualitatively. Activation of the mPFC suppresses aggressive bursts and reduces the intensity of aggressive behavior, but does not change the duration of the aggressive bursts. Our findings suggest that mPFC activity has an inhibitory role in the initiation and execution, but not the termination, of aggressive behavior, and maintains such behavior within the adaptive range.