Abstract
Recent human functional magnetic resonance imaging (fMRI) and animal electrophysiology studies suggest that grid cells in entorhinal cortex are an efficient neural mechanism for encoding knowledge about the world, not only for spatial location but also for more abstract cognitive information. The world, be it physical or abstract, is often high-dimensional. However, grid cells have been mainly studied on a simple two-dimensional (2D) plane and little is known about how grid cells encode the three-dimensional (3D) physical space in which we live. This dearth of knowledge is partly due to the technical difficulty of recording freely moving animals in 3D. Here, we first developed interactive software to help researchers visualize 3D grid fields and predict the grid activity in 3D. This relies on the known property of grid cells where their activity is modulated by the movement direction of animals relative to the grid axis. We then searched for 3D grid-like signals in human entorhinal cortex using a novel 3D virtual reality paradigm and a new fMRI analysis method. We found that signals in the left entorhinal cortex were explained by a face-centred cubic lattice model. This is the first empirical evidence for 3D grid codes in the human brain, notwithstanding the inherent methodological limitations of fMRI. We believe that our findings and software serve as a useful initial stepping-stone for studying grid cells in realistic 3D worlds and also, potentially, for interrogating abstract high-dimensional cognitive processes.
Highlights We present software and an analysis method to probe 3D grid codes using human fMRI Based on an alignment score between 3D movement direction and grid orientation We then tested this using a 3D virtual environment and fMRI Signals in entorhinal cortex were explained by a face-centred cubic lattice model