When exposed to novel dynamical conditions (e.g., externally imposed forces), neurologically intact subjects easily adjust motor commands on the basis of their own reaching errors. Subjects can also benefit from visual observation of others' kinematic errors. Here, using fMRI, we scanned subjects watching movies depicting another person learning to reach in a novel dynamic environment created by a robotic device. Passive observation of reaching movements (whether or not they were perturbed by the robot) was associated with increased activation in fronto-parietal regions that are normally recruited in active reaching. We found significant clusters in parieto-occipital cortex, intraparietal sulcus, as well as in dorsal premotor cortex. Moreover, it appeared that part of the network that has been shown to be engaged in processing self-generated reach error is also involved in observing reach errors committed by others. Specifically, activity in left intraparietal sulcus and left dorsal premotor cortex, as well as in right cerebellar cortex, was modulated by the amplitude of observed kinematic errors.

Neural representations of novel motor skills can be acquired through visual observation. We used repetitive transcranial magnetic stimulation (rTMS) to test the idea that this “motor learning by observing” is based on engagement of neural processes for learning in the primary motor cortex (M1). Human subjects who observed another person learning to reach in a novel force environment imposed by a robot arm performed better when later tested in the same environment than subjects who observed movements in a different environment. rTMS applied to M1 after observation reduced the beneficial effect of observing congruent forces, and eliminated the detrimental effect of observing incongruent forces. Stimulation of a control site in the frontal cortex had no effect on reaching. Our findings represent the first direct evidence that neural representations of motor skills in M1, a cortical region whose role has been firmly established for active motor learning, also underlie motor learning by observing.

Learning to control movements in different dynamic environments is marked by proactive interference; learning a first skill interferes with the subsequent learning of a second one. The neural basis of this effect is poorly understood. We tested the idea that proactive interference results from persisting neural representations of previously learned skills in the primary motor cortex (M1). We used repetitive transcranial magnetic stimulation (rTMS) of M1 to disrupt retention of a recently learned motor skill. If interference results from the retention of this skill then its disruption should be associated with reduced interference. Subjects reached to targets while interacting with a robotic arm that applied force fields to the limb. Fifteen minutes of 1-Hz rTMS to M1 impaired the retention of a first force field, and more importantly, reduced proactive interference when subjects learned a second one. Our findings suggest that retention and interference are linked at the level of M1.