Hippocampal theta phase coherence signals binding during retrieval and novelty processing
John Walker1, Nathan Whitmore1, Stephen VanHaerents1, Christina Zelano1, Jessica Templer1, Stephan Schuele1, Joel Voss1, Donna Bridge1; 1Northwestern University
It has been hypothesized that hippocampal theta phase orchestrates binding during memory formation. We propose that binding happens rapidly and continuously during visual exploration, and is linked to theta phase coherence. To test this hypothesis, we used concurrent eye-movement tracking and intracranial EEG (iEEG) recordings from the hippocampi of 4 individuals with epilepsy while they completed a spatial memory task. Participants studied object-location associations on scene backgrounds and were then re-presented the objects in either their original (match) or novel (mismatch) locations. We examined hippocampal activity time-locked to visual fixations to identify signals corresponding to binding. We found that hippocampal theta phase was time-locked to retrieval fixations (viewing the original location during mismatch), reflecting binding between the retrieved content and the original memory. Similarly, theta phase was locked to viewing the novel object location later in the trial (following retrieval), reflecting binding between the novel location and the original object-location memory. Consistent with prior fMRI findings, we found that hippocampal activity can reflect binding in the service of opposing long-term memory outcomes depending on when the binding occurs and the specific information that is bound, producing either memory stability or updating. Importantly theta phase was not linked to all fixations. During the match condition, theta phase coherence was not linked to fixations of objects in their original locations, as these fixations did not provide any new information for binding. We show for the first time that binding-related activity can be isolated by the novel combination of eye tracking and iEEG.
Topic Area: LONG-TERM MEMORY: Episodic