The mirror-neuron system and handedness: a "right" world?

To assess the relationship between the mirror-neuron system (MNS), an observation-execution matching system, and handedness, we acquired functional magnetic resonance imaging from 11 right-handed (RH) and eight left-handed (LH) subjects to identify regions involved in processing action (execution and observation) of the right and left upper limbs. During the execution tasks, LH subjects had a more bilateral pattern of activation than RH. An interaction between handedness and hand observed during the observation conditions was detected in several areas of the MNS and the motor system. The within- and between-groups analyses confirmed different lateralizations of the MNS and motor system activations in RH and LH subjects during the observation tasks of the dominant and nondominant limbs. The comparison of the execution vs. observation task demonstrated that during the execution task with their dominant limbs, RH subjects activated areas of the motor system in the left hemisphere, whereas LH subjects also activated areas of the MNS. During the execution task with the nondominant limbs, both groups activated regions of the MNS and motor system. Albeit this study is based on a small sample, the patterns of MNS activations observed in RH and LH subjects support the theory that suggests that this system is involved in brain functions lateralization. In LH people, this system might contribute to their adaptation to a world essentially built for right-handers through a mechanism of mirroring and imitation.