Fig. 5. Depth profile of cortico-hippocampal LFPs. LFPs were recorded simultaneously at four different depths in the neocortex and from the underlying dorsal CA1 using a Multisite probe. Channel 1 (blue) was in the superficial parietal cortex (layer 2/3), channel four (cyan) was judged to be in layer six of parietal cortex, and channel five (violet) was in dorsal hippocampus. Channels two and three were at intermediate depths in parietal cortex. Cortical LFP at all depths showed clear and synchronous UDS oscillations. Hippocampal LFP also showed clear and synchronous UDS.

Fig. 6. Correlation between action potentials (AP) and LFP. The LFP was recorded in layer 2/3 of the parietal cortex. Spike times were extracted from the whole-cell recordings in the dentate gyrus granule cells (green), CA3 pyramidal neurons (blue), and CA1 pyramidal neurons (red). The AP-LFP correlation coefficient was computed for each cell and averaged across all cells in that class. The median value of the population-averaged AP-LFP correlation was not significant for the granule cells. The main reason for this low correlation presumably were the utterly low firing rates of those neurons. The median value of the population averaged AP-LFP correlation was not significant for CA3 neurons, whereas it was significantly negative for CA1 pyramidal neurons (-0.0009 ± 0.0004, P = 0.006). Thus, the ensemble of CA1 pyramidal neurons reduced their firing during the cortical up state, whereas no significant UDS modulation of spiking activity was found in the ensemble of dentate gyrus granule cells or CA3 pyramidal neurons.