2.4. Cholinergic gamma oscillations
Prior to perfusion of slices with carbachol, the temperature of the recording chamber was increased to 35°C. After stable gamma oscillations were established (~45-70 min carbachol perfusion), glass electrodes were placed at the pyramidal layer of CA3 and CA1 and data were recorded for ~5 min. Gamma oscillation were analysed with power spectra generated from artifact-free 2 min data by Fast Fourier Transformation using a frequency resolution of 0.8192 Hz using custom-made spike2 scripts (Cambridge Electronic Design, Cambridge, UK). Three parameters of the power spectra were extracted: 1) Peak frequency (Hz): the frequency value at the maximum power value. 2) Peak power (mV2): the power value at the maximum power. 3) Integrated power (mV2): the sum of power from 20 to 80 Hz. For further statistical analysis, power values were converted to log10 values due to variable nature of gamma oscillation power in slice recordings. Furthermore, for local gamma correlations (CA3-CA3 or CA1-CA1 autocorrelations) and CA3-CA1 cross-correlations, correlograms were obtained from corresponding low-pass-filtered data (< 100 Hz) using Spike2 software. For local gamma correlations, the 2nd positive peak value of auto-correlations was used for further statistical analysis (See Fig. 1k). For the CA3-CA1 cross-correlation the peak value with lowest latency to time point 0 was used for further statistical comparison. As a cut off criteria for the CA3 recordings, slices with peak powers lower than 30 µV2, peak frequencies lower than 25 Hz and auto-correlation values lower than 0.1 were discarded. For the CA1 recordings the same criteria were used except the peak power criteria was lowered to 10 µV2 due to generally lower gamma oscillation strength observed in the CA1 in comparison to the CA3 subregion.