An Exploration of the Relationship Between Eye Movements and the Human Hippocampal Theta Rhythm

The ability to remember various locations and aspects of the environment is important for navigating and interacting with the world. Humans primarily rely on vision to engage with their surroundings, prompting research that attempts to uncover the relationship between eye movements and neural activity in the hippocampus, a structure in the brain associated with forming memories. Previous research found evidence that the hippocampus may actively guide eye movements during memory formation, and other work has shown that the human hippocampus exhibits neural activity that oscillates at 4-8 Hz (called the hippocampal theta rhythm) and is associated with memory. However, it is unclear how hippocampal theta rhythms relate to eye movements and the connection this might have with memory accuracy. Thus, the research described here addresses this question using recordings of hippocampal theta acquired via intracranial EEG from neurosurgical patients performing a visual scene recognition memory task that incorporated eye movement tracking. This setup allowed the measurement of hippocampal theta activity, the number of eye movements per image, and memory accuracy per trial (whether memory of an image was correct or incorrect). An oscillation detection algorithm was applied to the hippocampal recordings to identify theta oscillations, which occurred in low (1-4 Hz) and high (4-10 Hz) frequency ranges. Linear mixed effects models were used to assess relationships among theta oscillations, eye movements, and memory accuracy. Results demonstrate that a significant relationship exists between eye movements and higher frequency hippocampal theta rhythms (4-10Hz), but that this association does not exist for lower frequency hippocampal theta (1-4Hz). Importantly, the association between eye movements and theta activity did not significantly vary with memory accuracy. These results provide knowledge of how visual behavior relates to the neural mechanisms of memory in that the number of eye movements is linked to higher frequency hippocampal theta rhythms, but it does not contribute to memory performance.