Figure 7.

Spectral profile of the slow oscillation and theta during urethane anesthesia. Sequential laminar profiles of field activity were recorded in 0.5 mm increments through the dorsal to ventral axis of the brain, to a final depth of 4.5 mm using a multichannel probe; each line represents one laminar recording spanning 1.4 mm with the multiprobe. A histological photomicrograph of the multiprobe track is shown in A. Spectral analysis was performed on these signals in addition to simultaneously recorded field activity from fixed sites in the HPC and nCTX. B, Average spectral values, including autopower, phase, and coherence are plotted as a function of depth and aligned to the histology in A. Phase and coherence were computed with respect to the fixed neocortical signal during the SO and the fixed hippocampal signal during theta. The SO (red lines) null zone occurred in the nCTX at a depth of 0.9 mm (approximately layer V). It also showed a significant phase shift (∼20°) aligned with a maximal value within the HPC. This depth coincided with the position of the theta (black lines) maximum [approximately at the level of the hippocampal fissure (fiss)]. Coherence for both the SO and theta remained high throughout the profile except at positions that corresponded to phase reversals. The coherence of the SO within the HPC, however, was more variable, especially at locations near the power maximum. Raw field samples of theta (C) and the SO (D) from the numbered probe contacts are shown. Fast components of the SO, in addition to theta, reverse phase at a depth of 2.6 mm (trace 5). E, An average voltage profile clearly demonstrates the SO phase shift within the HPC. The corresponding average CSD shows a large sink at ∼3.0 mm and smaller sinks at ∼2.7 mm. The CSD scale is −13.0 to 13.0 mV/mm².