Motion class dependency in observers' motor areas revealed by functional magnetic resonance imaging.

Human and animal data suggest that the mere observation of biological motion activates those premotor areas that also underlie the initiation of the same motion. However, data also indicate that the human premotor cortex (PM), in contrast to the monkey PM, responds not only to the observation of goal-directed (transitive) motion but also to intransitive motion. The present study used functional magnetic resonance imaging to test this hypothesis directly. Participants were presented cycles of intransitive motion specified as belonging to the distal (fingers and mouth), proximal (knee, ankle, elbow, and wrist), or axial (trunk and shoulder) motion class. Attention to motion was behaviorally tested by a forced-choice task on motion acceleration and deceleration. Results revealed extended PM activation for each motion condition. However, direct contrasts showed that the most significant activations were elicited in ventrolateral PM by distal motion, in dorsolateral PM by proximal motion, and medial PM (supplementary motor area) by axial motion. Findings confirm observed intransitive motions to engage premotor areas along a gross-scaled somatotopy.