| Summary: | Brain structures that support spatial cognition by encoding one’s position and direction have been extensively studied for decades. In the majority of studies, neural substrates have been investigated on a horizontal two-dimensional plane, whereas humans and other animals also move vertically in a three-dimensional (3D) world. In this thesis, I investigated how 3D spatial information is represented in the human brain using functional MRI experiments and custom-built 3D virtual environments. In the first experiment, participants moved on flat, tilted-up or tilted-down pathways in a 3D lattice structure. Multivoxel pattern analysis revealed that the anterior hippocampus expressed 3D location information that was similarly sensitive to the vertical and horizontal axes. The retrosplenial cortex and posterior hippocampus represented direction information that was only sensitive to the vertical axis. In the second experiment, participants moved in a virtual building with multiple levels and rooms. Using an fMRI repetition suppression analysis, I observed a hierarchical representation of this 3D space, with anterior hippocampus representing local information within a room, while retrosplenial cortex, parahippocampal cortex and posterior hippocampus represented room information within the wider building. As in the first experiment, vertical and horizontal location information was similarly encoded. In the last experiments, participants were placed into a virtual zero-gravity environment where they could move freely along all 3 axes. The thalamus and subiculum expressed horizontal heading information, whereas retrosplenial cortex showed dominant encoding of vertical heading. Using novel fMRI analyses, I also found preliminary evidence of a 3D grid code in the entorhinal cortex. Overall, these experiments demonstrate the capacity of the human brain to implement a flexible and efficient representation of 3D space. The work in this thesis will, I hope, serve as a stepping-stone in our understanding of how we navigate in the real – 3D – world.
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