Martins, M. d. J. D. | bioRxiv Science

bioRxiv2022

Distinct hippocampal and cortical contributions in the representation of hierarchies

Humans generate complex hierarchies across a variety of domains, including language and music, and this capacity is often associated with activity in inferior frontal gyrus (IFG). Non-human animals have also been shown to represent simple hierarchies in spatial navigation, and human neuroimaging work has implicated the hippocampus in the encoding of items-in-contexts representations, which constitute 2-level hierarchical dependencies. These fields of research use distinct paradigms, leading to disjoint models and precluding adequate cross-species comparisons. In this study, we developed a paradigm to bring together these two areas of research and show that anterior hippocampus and medial prefrontal cortex encode hierarchical context, mimicking findings from animal spatial navigation. Additionally, we replicated classic neurolinguistic findings of 1) left IFG and posterior temporal cortex in the representation of hierarchies and 2) the association between IFG and processing automaticity. We propose that mammals share an evolutionary ancient system for the generation of simple hierarchies which is complemented in humans by additional capacities. HighlightsO_LIHPC and mPFC activity is specifically modulated by hierarchical context C_LIO_LISyntax-related regions in the left hemisphere encode for hierarchy in general C_LIO_LIIFGop activity is maintained in later trials for hierarchies but not sequences C_LIO_LIThese findings mimic those from animal spatial navigation and neurolinguistics C_LI

neuroscience↗

bioRxiv2017

Fractals 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. 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.

neuroscience↗