Giovanni Buccino, MD, PhD,* Ana Solodkin, PhD,w and Steven L. Small, MD, PhDw
Mirror neurons discharge during the execution of hand object-directed actions and during the observation of the same actions performed by other individuals. These neurons were first identified in the ventral premotor cortex (area F5) and later on in the inferior parietal lobule of monkey brain, thus constituting the mirror neuron system. More recently, mirror neurons for mouth object-directed actions have also been found in the monkey. Several pieces of experimental data demonstrate that a mirror neuron system devoted to hand, mouth, and foot actions is also present in humans. In the present paper we review the experimental evidence on the role of the mirror neuron system in action understanding, imitation learning of novel complex actions, and internal rehearsal (motor imagery) of actions. On the basis of features of the mirror neuron system and its role in action understanding and imitation, we discuss the possible use of action observation and imitation as an approach for systematic training in the rehabilitation of patients with motor impairment of the upper limb after stroke. Hand use in humans plays a critical role for the vast majority of social and cognitive functions of the species, including most types of communication and most activities of the workplace. Hand motor skill, or ‘‘the ability to solve a motor problem correctly, quickly, rationally, and resourcefully,’’1,2 has influenced the survival of the species, as demonstrated both phylogenetically and ontogenetically, and this skill has reached maximum expression in humans. To support this increase in the complexity of hand skill, there has been a parallel increase in the size of the neuropil, reaching a maximum in the human.3 This suggests that the central nervous system has evolved to reach very complex patterns of connectivity among areas. Such connectivity is the basis of the complex neural circuits that support skilled hand motor functions. These integrative circuits incorporate a variety of cerebral cortical regions, which participate to varying degrees in supporting hand motor tasks. The neural context4,5 underlying regional involvement depends on task variables, such as complexity, bimanuality, sensory trigger, and planning requirements, and on individual variables, such as handedness, experience, health, and even emotional state and affect. These circuits must integrate information from visual, auditory, somatosensory, and limbic sources, and are, as recent experimental evidence demonstrates (see below), not only involved in the execution of actions, but also in higher cognitive processes such as action understanding, action imagining (internally generated), or action imitation (externally generated). Motor imitation represents a hand motor task in which there is specific visual and proprioceptive information that may facilitate movement, possibly by way of a specialized premotor circuit.6 Motor imagery can harness visual circuits and/or motor circuits,7–9 depending on the type of imagery performed, and has particular interest because it plays an important role in development of motor skill and can be used in neurologic patients with complete paralysis. A principal tenet of medicine is that an organism must constantly adapt to changes in its external and internal environments.10 Cerebrovascular insults (strokes) lead to important changes to the internal and external environments that can affect the functioning of the human brain with respect to hand motor skill. In this article, we review the neurophysiologic and neuroanatomic basis of action understanding, imitation, and imagery, and propose that it is possible to affect the physiology of the hand motor circuits through interventions aimed at internal and external influences on the generation of skilled hand movements, with a consequent profound effect on hand motor recovery after stroke.