Action understanding within and outside the motor system: the role of task difficulty

When we observe actions, we activate parietal and premotor areas that are also recruited when we perform actions ourselves. It has been suggested that this action mirroring is causally involved in the process of action understanding. Alternatively, it might reflect the outcome of action understanding, with the underlying cognitive processes taking place elsewhere. To identify and characterize areas involved in action understanding, we presented participants with point-light displays depicting human actions and engaged them in tasks that required identifying the effector (arm/leg) or the goal of an action. We observed a stronger blood oxygen level-dependent signal during the Goal in comparison to the Effector Task not only in premotor areas, but also in the middle temporal gyrus (MTG) and the anterior ventrolateral prefrontal cortex. In the MTG, the Goal Task led to a signal higher than the Effector Task only when actions were easy to understand, whereas frontal areas showed this difference also when the task was difficult, a finding that is not caused by a ceiling effect. Our results suggest an interplay between temporal and frontal areas that is modulated by task difficulty and thus provide important constraints for biologically plausible models of action understanding.