Researchers at Johns Hopkins University have uncovered new insights into how the brain navigates using a combination of external landmarks and self-motion cues, thanks to a sophisticated virtual reality system called the Dome. The study, published in Nature Neuroscience, reveals that place cells in the hippocampus—neurons responsible for creating mental maps of space—use multiplexed theta wave activity to process different types of spatial information simultaneously. Place cells fire when an animal is in a specific location, and their activity is synchronized with theta oscillations, rhythmic brain waves between 7 and 9 Hz. While past research has shown that these cells rely on both external cues (like visual landmarks) and internal cues (such as self-motion or movement tracking), the precise role of these inputs in shaping the timing of place cell firing—known as phase coding—remained unclear. To investigate, the team used the Dome, a planetarium-style VR setup that allows rodents to move through a virtual environment while their real-world motion is decoupled from what they see. This enabled researchers to manipulate visual landmarks independently of physical movement, creating illusions where the rat’s perceived speed in the virtual world differed from its actual movement. By recording neural activity with fine-wire microelectrodes, the scientists observed two key components of theta phase coding: phase precession and phase procession. Phase precession occurs during the late part of a theta cycle and reflects a forward prediction of where the animal will be next. Phase procession, occurring in the early part of the cycle, replays past locations. The study found that phase precession persisted regardless of whether the virtual and physical movement matched. However, phase procession disappeared when the virtual motion did not align with real movement, indicating that this process depends on accurate self-motion signals. This suggests that the brain uses different mechanisms for predicting future locations and encoding past ones, with each mode influenced by distinct types of spatial input. These findings support a long-standing theory from the 1990s that the late and early phases of the theta cycle serve predictive and encoding functions, respectively. The hippocampus appears to cycle through these modes roughly every 125 milliseconds, integrating both landmark and self-motion information to maintain an accurate spatial representation. The researchers, led by James Knierim and first author Yotaro Sueoka, plan to extend their work by recording from upstream brain regions to understand how spatial information is processed before reaching the hippocampus. This could reveal the broader neural circuitry behind navigation. The study also has broader implications. Understanding how the brain builds mental maps may inform the design of more effective AI and robotic navigation systems. Additionally, since the hippocampus is vital for episodic memory in humans and is affected early in Alzheimer’s disease, these findings could help explain why patients struggle with both navigation and memory retention. The work was conducted in collaboration with the engineering lab of Noah Cowan at Johns Hopkins, highlighting the interdisciplinary nature of modern neuroscience. The research underscores the brain’s remarkable ability to integrate multiple sources of information to guide movement and memory, offering a deeper view into one of the mind’s most essential functions.