Fig. 5.

Assessing preretrieval theta coupling between the PHG and hippocampus. (A) In an example subject, phase locking was assessed between electrodes localized to the hippocampus (CA1) and PHG (perirhinal cortex [PRC]), indicated as red crosses. (B) As in the power analyses, LFP is extracted from each electrode, and theta coupling is assessed in the 1-s intervals leading up to vocalized recalls. A score is assigned to each transition, capturing the closeness of successive recalls in semantic or temporal dimensions. (C) Phase locking is measured by assessing the consistency of theta phase between the electrodes, for each transition. (Left) A close semantic transition is depicted, where the peak of the CA1 oscillation (dashed lines) consistently aligns with the falling phase of the PRC oscillation. (Right) A far semantic transition is shown, with inconsistent phase alignment. The concentration of phase differences over time is quantified as the PLV, visualized as polar histograms. For the example trials shown, the close semantic transition has a higher PLV (0.62) than the far transition (0.30). (D) Hipp-PHG theta coupling mediates semantic search. (Left) Hippocampal–PHG theta coupling significantly correlated with ensuing semantic transition distance in a 1D space (1-sample t test, t(51) = 2.07, P = 0.044). No effect was observed for phase locking in the gamma band. (Right) Phase locking was not predictive of temporal transition distance in either frequency band (P > 0.05). (E) Theta phase locking was marginally significantly correlated with 2D semantic transition (t(51) = 1.73, P = 0.089), in addition to 1D transitions, as noted in D. *P < 0.05, ^†P < 0.1. Error bars show ±1 SEM.