All signals were first passed to a PZ-5 pre-amplifier (Tucker-Davis Technologies Inc., Alachua, FL, USA). A 128-channel RZ-2 Neurophysiological Recording System (Tucker-Davis Technologies Inc., Alachua, FL, USA) was then used to acquire tethered electrophysiological recordings. EEG and EMG signals were continuously sampled at 305 Hz and bandpass filtered between 0.1–100 Hz. Signals were then downsampled offline to 256 Hz via spline interpolation. Laminar probe channel signals were sampled at 25 kHz. Two signals were extracted from the laminar probe channels: decimated multiunit activity (MUA) and local field potential (LFP). Multiunit activity is the high frequency component of neural activity that contains the spiking of multiple neurons within the vicinity of an electrode. Decimation is a process for downsampling the MUA whilst retaining spiking activity by storing only the highest amplitude value, either negative or positive, recorded during a set time period. This means that if multiple neurons spike during that period, only the largest is stored, thus the majority of spikes in the decimated signal will originate from nearby neurons. The resulting signal will therefore have a high amplitude when nearby neurons are spiking and a low amplitude during periods of quiescence or when distant neurons are spiking. MUA was generated by bandpass filtering the laminar signals between 300 Hz and 5 kHz then decimating to 498 Hz by splitting the signal into segments of ~ 50 samples and storing the maximum/minimum amplitude of alternating segments as integers. LFP was generated by zero-phase distortion bandpass filtering the laminar signal between 0.1 and 100 Hz and downsampling to 256 Hz via spline interpolation. All offline manipulations and analyses were performed using MATLAB (version R2020a; The MathWorks Inc, Natick, MA, USA). Prior to vigilance state scoring, signals were transformed into European Data Format as previously reported (see [30 (link)]).
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