Abstract:

Action understanding enables us to predict others' actions and interact with them smoothly; however, its impairment makes patients unable to take care of themselves leading to cases such as autism. Despite the central importance of action understanding, the cognitive mechanisms involved in it remain highly controversial. Two alternative accounts have been advanced. Simulation theory assumes that we understand actions by simulating the observed behavior through a direct matching process, which has been suggested to usually activate the mirror-neuron circuit. The alternative interpretive account (i.e., theory) assumes that action understanding is based on specialized inferential processes, in which a goal is assigned to an action by evaluating its efficiency as an optimal means of obtaining the goal within the specific constraints of the situation. Each account was supported by previous studies, but due to mythological drawbacks, the evidence can be explained by both accounts. Hence, it remains unclear how action understanding is implemented in our cognition.

The simulation theory claims that action understanding is sensitive to minor differences in the kinematics of actions, and the theory emphasizes the role of contextual information in action understanding. Namely, according to the theory, even for identical actions, contextual information modulates action understanding. Hence, to examine which account is involved in action understanding, we created a chasing action wherein a chaser pursues a movable target, but the chasing action occurred in different contexts. Specifically, in a constrained context, the chaser had to bypass the obstacles to approach the target, and the chaser's action was efficient in catching the target, while in an unconstrained context, the obstacles were removed but the chaser still implemented the same action as in the constrained context. All actions lasted for 3 seconds. In both contexts, the chaser and the target had exactly the same kinematics, but were assigned different goals if the inferential process is involved. To identify the outcome of action understanding, we measured μ suppression (electroencephalogram oscillations within the 8-13 Hz range in the sensorimotor regions; namely, C3 and C4 channels) related to action understanding. Participants were asked to count the fillers (i.e., incomplete chasing action) when watching actions presented on the screen.

It was found that the chasing action occurred in the constrained context (M = -1.955 μV²) induced more μ suppression than the action occurred in the unconstrained context (M = -1.913 μV²), but in both contexts, the evaluated familiarity for them was not significantly different (Experiment 1). Importantly, the occipital α with the same frequency band as μ was not modulated by the contextual information, but this component was suggested to be functional with the attentional mechanisms. In Experiment 2, to further test whether the effect in Experiment 1 was specific to the inferential process, the target was set to be still but the chaser still moved in the same way, which cannot be attributed to an analytical goal. In this case, the possible simulation difference between constrained and unconstrained contexts when the target was still was almost the same as when the target was moving; however, we found that the difference in μ suppression between constrained and unconstrained contexts was insignificant.

Our findings showed that contextual information modulates μ suppression, suggesting that the action understanding is sensitive to the context, and the assigned goal for the actions depends on the contextual information. Hence, our findings support the assumption that action understanding is primarily mediated by an inferential interpretive system rather than a simulation process.

Key words: action understanding, chasing action, context, simulation theory, theory theory, μ suppression