Figure 4.

Laminar structure of extracellular recordings reveals electrode position. A, Tracing of wide-band LFPs across all channels aligned to the 31 recording positions of the silicon Edge probe along the vertical axis (200-μm spacing) represented by the gray cartoon on the left. The signal was referenced to the skull ground screw. Multiunit spiking activity can be seen in a dorsal band indicating stratum pyramidale (approximately channel 4). Note regular bursts of synchronized activity that occur approximately every 300 ms, potentially representing sharp wave-like activity under isoflurane anesthesia (Lustig et al., 2015). Arrows point to the phase reversal of the LFP above and below stratum radiatum as well as at dentate gyrus middle molecular layer. CSD map shows rhythmic large sink in stratum radiatum. B, Power spectra of classic theta (orange, 4–10 Hz), low gamma (blue, 25–55 Hz), high gamma (purple, 55–90 Hz), and the ripple range (red, 150–250 Hz) bands are shown aligned to the channels in A. Each power band is presented as mean ± SEM (n = 4). Note that the theta band (orange) has maximal power in stratum radiatum. Although we found maximal power in the ripple band in stratum pyramidale (by definition the channel of maximal multiunit spiking activity; see Fig. 3D), we did not observe any definitive, discrete ripple oscillatory events. C, Responses to perforant path stimulation (red arrow) were averaged over a 50-ms time period (20 stimulations, 2 s apart, stimulation artifact removed for clarity). The laminar profile of these evoked potentials is shown in one animal. Phase reversals were observed above and below stratum radiatum (arrows).