Human Hippocampal Theta Oscillations Organise Distance to Goal Coding

The rodent hippocampal local field potential is dominated by 6-12 Hz theta oscillations during active behaviour that are strongly implicated in spatial coding and memory function across species. Invasive electrophysiology in both rodents and humans has shown increases in hippocampal theta power immediately before the onset of translational movement that persists throughout subsequent motion, and the magnitude of this increase correlates with the distance subsequently travelled. Using non-invasive magnetoencephalography (MEG) and an abstract navigation task, we observed increased theta power during both spatial planning and subsequent navigation. Importantly, theta power in the right hippocampus covaried with subsequent path distance during planning, only when participants were aware of the distance to their goal. During subsequent navigation, hippocampal theta power decreased dynamically as participants approached the goal, only when they were aware of how far they still needed to travel. In addition, theta phase during navigation modulated 70-140 Hz fast gamma amplitude in the entorhinal cortex while traversing novel paths; and 30-70 Hz slow gamma amplitude in the right hippocampus while traversing previously experienced paths. In both cases, theta-gamma phase-amplitude coupling increased with proximity to the goal during navigation, consistent with the hypothesis that sequences of upcoming locations were represented by gamma bursts occurring at successive theta phases. In sum, these findings are consistent with the proposed role of hippocampal theta oscillation in flexible planning and goal-directed spatial navigation across mammalian species.