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.