(A) Representative coronal sections of the brains of all three gerbils. Tetrode tracks are visible in right cingulate cortex. In addition, stains from DiI coating of the tetrodes are visible in (A1, A2), and a big postmortem electrolytic lesion in (A3). Background staining was done with DAPI and Neurotracer (A1, A2) and Neutralred (A3). A section from a gerbil brain atlas is overlaid in (A1) (Radtke-Schuller et al., 2016). M1, M2, primary and secondary motor cortex; CG1, cingulate cortex, area 1; PrL, prelimbic cortex; IL, infralimbic cortex; fmi, forceps minor of corpus callosum; CPu, caudate putamen. (B) Spike clusters in feature space of signals recorded at one tetrode; data from three different example sessions. (B1) Cell from Figure 2A in the main text. (B2) cell from Figure 2B in the main text. (B3) cell from Figure 2C (orange) in the main text and Figure 2—figure supplement 3A (green). Peak signals are shown for two different channels of one tetrode as an example projection. Other spikes and voltage deflections are displayed as gray dots. Insets: average waveforms (1 ms length) corresponding to the colored clusters. (C) Three different projections of the feature space from the tetrode recoding in (B1). All eight spike clusters that could be separated are marked (color coded). Other spikes and voltage deflections are again displayed as gray dots. The table lists quality measures for each cluster. Clusters 6–8 had too large amplitude cutoff and were excluded from further analysis. (D) Distributions of isolation distance and signal-to-noise ratio (SNR) for the clusters of all 1766 cells (i.e., excluding clusters/cells of insufficient quality). Both values were weakly correlated with the average firing rate (bottom panels). Isolation distance was calculated as described in Schmitzer-Torbert et al., 2005 for clusters of the peak amplitude (C). SNR was calculated as the ratio of the peak of the average spike waveform to the standard deviation of the background noise, that is, spikes that do not belong to a separated cluster. This was done for every channel individually and then averaged for all channels of a tetrode. Interspike interval (ISI) violations and amplitude cutoff were calculated as described in the Materials and methods section.