Brain bases of meaningful and meaningless action imitation: a neurostimulation and two-person motion-tracking approach

Humans can imitate both meaningful and meaningless actions. However, the behavioural and neural processes underlying the imitation of these different action types are still not fully understood. It is difficult to link imitation performance to brain regions commonly associated with imitation due to the limitations on movement imposed by the brain scanning environment. Furthermore, claims made about areas commonly associated with imitation are often imitation-general, despite experiments generally testing single action types. However, evidence from apraxia (a disorder of complex movement) suggests both common and specific brain regions may be involved in meaningful and meaningless action imitation. For example, damage to the left inferior parietal lobule (IPL) is frequently associated with deficits in meaningless action imitation. In order to address these concerns, we used motion-tracking, transcranial magnetic stimulation (TMS), and ecologically valid two-person experimental paradigms. In experiment 1, we found that imitating meaningless actions increased the reaction time for performing a subsequent action, supporting theories of different processing routes to meaningless and meaningful action imitation. In addition, we discovered that the hallmark of meaningless action imitation is a strategy designed to maximise correction time at the end of the action, ensuring accurate completion of actions that may be unfamiliar. Using TMS and two-person motion-tracking, we then examined three brain areas often associated with imitation. In experiment 2 we found that the left IPL is involved in matching observed kinematics. In experiment 3 we found that the left ventral premotor cortex (PMv) has a general role in hand shaping for imitation. In experiment 4 we found that the left posterior middle temporal gyrus (pMTG) is involved in meaningful, but not meaningless, hand gesture imitation. Overall, these results provide causal evidence for previous claims that meaningful and meaningless action imitation are subserved by common (IPL, PMv) and specific (pMTG) neural pathways