Figure 3. Progressive theta phase shift expressed along layers but not across layers in MEC.

(A) Custom multi-shank electrode array with tips positioned to allow simultaneous recording along and across the cortical layers of the MEC. Arrow heads indicate the positions of recording sites on the adjacent shank. (B) Reconstructed position of the electrode array based on histology, probe geometry, and electrophysiological markers. Color coding of individual sites indicates whether they were excluded for either being beyond the bounds of the MEC (white) or having high impedance (green), and whether they were found to have a phase reversal (red) relative to the remaining sites (black). (C) Cycle-average theta waves for all electrodes, triggered off of peaks on the most dorsal electrode of layer l. The progressive theta phase shift is observed along the same layer, but not across the layer, note the phase inversion of 180° between layer I and layers III–V. (D) The theta phase difference for each electrode pair plotted as a function of the distance between the corresponding electrodes along the cortical layers (as shown in panel [B]) reveals a bimodal distribution that separated cleanly based upon whether the electrode pair spans the phase reversal expected between layers II and I (red) or not (black). (E) Same as panel (D) but phase differences were plotted as a function of distance across the cortical layers (as shown in panel B). (F) Applying a 180° offset to the red dots shown in panel (D) collapses the multi-modal distribution into a single modal distribution with a strong positive correlation between distance along the cortical layer and phase difference. (G) Applying a 180° offset to the red dots shown in panel (E) also collapses the multi-modal distribution into a single modal distribution but with no notable correlation between distance across layers and phase difference.