MEG adaptation reveals action representations in posterior occipitotemporal regions.

When we observe other people's actions, a number of parietal and precentral regions known to be involved in the planning and execution of actions are recruited for example seen as power decreases in alpha and beta frequencies indicative of increased activation. It has been argued that this recruitment reflects the process of simulating the observed action, thereby providing access to the meaning of the action. Alternatively, it has been suggested that rather than providing access to the meaning of an action, parietal and precentral regions might be recruited as a consequence of action understanding. A way to distinguish between these alternatives is to examine where in the brain and at which time point it is possible to discriminate between different types of actions (e.g., pointing or grasping) irrespective of the way these are performed. To this aim, we presented participants with videos of simple hand actions performed with the left or right hand towards a target on the left or the right side while recording magnetoencephalography (MEG) data. In each trial, participants were presented with two subsequent videos (S1, S2) depicting either the same (repeat trials) or different (non-repeat trials) actions. We predicted that areas that are sensitive to the type of action should show stronger adaptation (i.e., a smaller decrease in alpha and beta power) in repeat in comparison to non-repeat trials. Indeed, we observed less alpha and beta power decreases during the presentation of S2 when the action was repeated compared to when two different actions were presented indicating adaptation of neuronal populations that are selective for the type of action. Sources were obtained exclusively in posterior occipitotemporal regions, supporting the notion that an early differentiation of actions occurs outside the motor system.