Research Articles, Systems/Circuits Patrick A. LaChance, Julie R. Dumont, Pelin Ozel, Jennifer L. Marcroft and Jeffrey S. Taube Journal of Neuroscience 8 April 2020, 40 (15) 3035-3051; DOI: https://doi.org/10.1523/JNEUROSCI.2789-19.2020 Abstract Navigation often requires movement in three-dimensional (3D) space. Recent studies have postulated two different models for how head direction (HD) cells encode 3D space: the rotational plane hypothesis and the dual-axis model.