Corticospinal Neurons in Macaque Ventral Premotor Cortex with Mirror Properties: A Potential Mechanism for Action Suppression?

The discovery of "mirror neurons" in area F5 of the ventral premotor cortex has prompted many theories as to their possible function. However, the identity of mirror neurons remains unknown. Here, we investigated whether identified pyramidal tract neurons (PTNs) in area F5 of two adult macaques exhibited "mirror-like" activity. About half of the 64 PTNs tested showed significant modulation of their activity while monkeys observed precision grip of an object carried out by an experimenter, with somewhat fewer showing modulation during precision grip without an object or grasping concealed from the monkey. Therefore, mirror-like activity can be transmitted directly to the spinal cord via PTNs. A novel finding is that many PTNs (17/64) showed complete suppression of discharge during action observation, while firing actively when the monkey grasped food rewards. We speculate that this suppression of PTN discharge might be involved in the inhibition of self-movement during action observation.