Facial expressions: what the mirror neuron system can and cannot tell us.

Facial expressions contain both motor and emotional components. The inferior frontal gyrus (IFG) and posterior parietal cortex have been considered to compose a mirror neuron system (MNS) for the motor components of facial expressions, while the amygdala and insula may represent an "additional" MNS for emotional states. Together, these systems may contribute to our understanding of facial expressions. Here we further examine this possibility. In three separate event-related fMRI experiment, subjects had to (1) observe (2) discriminate and (3) imitate facial expressions. Stimuli were dynamic neutral, happy, fearful and disgusted facial expressions, and in Experiments 1 and 2, an additional pattern motion condition. Importantly, during each experiment, subjects were unaware of the nature of the next experiments. Results demonstrate that even passive viewing of facial expressions activates a wide network of brain regions that were also involved in the execution of similar expressions, including the IFG/insula and the posterior parietal cortex. Only a subset of these regions responded more during the observation of facial than pattern motion (bilateral ventral IFG, bilateral STS/MTG, bilateral amygdala, SMA). While the viewing of facial expressions recruited similar brain regions in all three experiments, adding an active task (discrimination, imitation) augmented the magnitude of these activations. Brain activations reflected differences in observed facial expressions, with emotional expressions activating relatively more the insula/frontal operculum, and neutral ones (blowing up the cheeks) the somatosensory cortices (SII). Using movies, fear activated the amygdala and disgust the insula, but other emotions activated these structures to a similar degree.