Project

Fractals in Action: the Brain on Motor Hierarchies

Goal: To describe the neural mechanims underlying the generation of new hierarchical levels using recursive 'fractal' rules.

Methods: fMRI Analysis

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Mauricio J Martins
added a research item
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 versus Iteration, but also Repetition versus 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.
Mauricio J Martins
added an update
How do we generate new hierarchical levels in the motor domain? Check out our new fMRI study! https://onlinelibrary.wiley.com/doi/full/10.1002/hbm.24549
 
Mauricio J Martins
added a research item
Generation of hierarchical structures, such as the embedding of subordinate elements into larger structures, is a core feature of human cognition. Discrimination of well-formed hierarchies is thought to rely on lateral prefrontal cortex (PFC). However, the brain bases underlying the active generation of new hierarchical levels remain poorly understood. Here, we created a new motor paradigm to isolate this active generative process. In fMRI, participants planned and performed (identical) movement sequences based on three previously learned rules: (1) a hierarchical ‘fractal’ rule that involved generation of new levels, (2) a linear ‘iterative’ rule adding items to existing hierarchical levels, and (3) simple ‘repetition’. We found that generation of new hierarchical levels (using the fractal rule) activated a bilateral motor planning- and imagery network, but did not involve lateral PFC. Conversely, adding items to existing hierarchical levels required M1 directly during execution. These results show that the generation of new hierarchical levels can be achieved without involvement of putative domain-general systems such as those ascribed to lateral PFC. We hypothesize that these systems might be important to parse hierarchical sequences in a multi-domain fashion but not necessarily to generate new hierarchical levels.
Mauricio J Martins
added a research item
The capacity to represent and generate hierarchical structures is a core feature of human cognition. While discrimination of well-formed hierarchical structures is thought to rely on lateral prefrontal cortex (lPFC) in several domains, the brain bases of active generation of new hierarchical levels remains relatively unexplored. Here we introduce a novel motor paradigm addressing this question. Participants were tested with fMRI after they learned to execute (identical) sequences of finger movements following three different rules: (1) simple ‘repetition’, (2) a linear ‘iterative’ rule adding items to existing hierarchical levels without generating new levels, and (3) a hierarchical ‘fractal’ rule that involved generation of new levels. By comparing ‘iterative’ and ‘fractal’ planning, we demonstrated that the motor generation of new hierarchical levels activates a bilateral motor network (sensorimotor and premotor cortices, cerebellum, lateral occipital cortex, and left pallidum) also involved in motor planning and imagery. Furthermore, executing the final sequence when formed by ‘fractal’ or ‘repetition’ rules, entailed an additional fronto-striatal interaction with the hand area in comparison with ‘iteration’, which reduced activity within M1. We speculate that this finding reflects buffering of previously completed motor programs in both “fractal” and “repetition”, whereas “iterative” sequences might be completed on the fly. lPFC was not recruited by fractal sequences, neither during planning nor execution. Within the broader framework of human hierarchical processing, these results contribute to the idea that expert generation of new hierarchical levels might be achieved without involvement of putative domain-general systems such as those involving lPFC.
Mauricio J Martins
added a project goal
To describe the neural mechanims underlying the generation of new hierarchical levels using recursive 'fractal' rules.