Recursion in Action: An fMRI study on the Generation of new Hierarchical Levels in Motor Sequences

Generation of hierarchical structures, such as the embedding of subordinate elements into larger structures, is a core feature of human cognition. Processing of hierarchies is thought to rely on lateral prefrontal cortex (PFC). However, the neural underpinnings supporting active generation of new hierarchical levels remain poorly understood. Here, we created a new motor paradigm to isolate this active generative process by means of fMRI. Participants planned and executed identical movement sequences by using different rules: a ‘recursive’ hierarchical embedding rule, generating new hierarchical levels; an ‘iterative’ rule linearly adding items to existing hierarchical levels, without generating new levels; and a ‘repetition’ condition tapping into short term memory, without a transformation rule. We found that planning involving generation of new hierarchical levels (‘recursive’ condition vs. both ‘iterative’ and ‘repetition’) activated a bilateral motor imagery network, including cortical and subcortical structures. No evidence was found for lateral PFC involvement in the generation of new hierarchical levels. Activity in basal ganglia persisted through execution of the motor sequences in the contrast ‘recursive’ vs. ‘iteration’, but also ‘repetition’ vs ‘iteration’, suggesting a role of these structures in motor short term memory. These results showed that the motor network is involved in the generation of new hierarchical levels during motor sequence planning, while lateral PFC activity was neither robust nor specific. We hypothesize that lateral PFC might be important to parse hierarchical sequences in a multi-domain fashion but not to generate new hierarchical levels.

Keywords: fMRI, Prefrontal Cortex, Hierarchy, Motor, Recursion