Daniel Reznik & Jacob Bellmund | Grid-like hexadirectional codes in the human default network

Grid cells in the entorhinal cortex are a core component of the brain's cognitive mapping system. In humans, grid-like hexadirectional signals can be measured using fMRI and are considered a proxy measure for grid-cell population responses during navigation in physical as well as in abstract spaces. These hexadirectional signals have been observed in multiple cortical brain regions outside of the entorhinal cortex. These areas include the ventro-medial prefrontal cortex, posterior parietal cortex and the posterior cingulate cortex. Notably, these areas overlap with the canonical default network. Precision fMRI of densely sampled individuals showed that canonical distributed brain networks break down into different closely situated subnetworks. In particular, the default network can be decomposed into at least two subnetworks that are found in anatomically close-by, yet consistently distinct cortical areas. The anatomical separation of the two cortical networks suggests distinct functional specialisation, however the exact nature of this specialisation is unclear. In this project presentation, we propose a precision fMRI study that combines multiple sessions of task-free and task-based fMRI to probe the relationship between the default network and hexadirectional signals. The goal of this dense sampling design is to detect, with unprecedented anatomical precision, the distributed brain regions that are modulated by hexadirectional signals. We hypothesise that hexadirectional signals will specifically overlap with one of the default networks, thus contributing to our understanding of the network-level properties of the grid system and the computational principles underlying default network function.