Figure 2: Spike sorting and assessment of long-term stability of single-unit recordings by single microwire array over 7 days.

(a) Example of spike sorting from single microwire array in 7 days, showing the superimposed spike waveforms (upper panel) and the inter-spike-interval histogram (ISIH, lower panel), and the corresponding identified clusters in the PCs space (far right panel). Clear isolation of units from a given recording channel is indicated by high, F statistic of MANOVA (F), J3, Dunn validity (Dn) and low Davis-Bouldin (DB) index (see Methods). Note the excluded unit in red, whose spike waveforms changed cross days, and had shifted ISI histogram and cluster location in PCs. (b) Long-term stability of identified single-units shown in a over 7 days. The unit shown in red with drifting of cluster was excluded. (c) Autocorrelograms of the three isolated units and their cross-correlogram (white). The presence of refractory periods in the auto-correlograms and absence of refractoriness in the cross-correlogram indicated spikes with clusters marked in yellow, green and blue were generated by three distinct neurons. The short latency sharp peak in the cross-correlogram (arrow) between the putative pyramidal neuron (yellow, reference of the cross-correlogram) and the interneuron (green) may indicate mono-synaptic activation. (d) Example of units exhibiting stable (top, unit 1 in a) or unstable (bottom, unit 4 in a) spike waveform are shown in d. (e) Gaussian mixture distributions fitted to combinations of the four similarity scores (see Methods) computed from spikes recorded from same neuron (black dots, representing true positive values computed using recording acquired in difference sessions on the same day, see Methods) or distinct ones (grey dots, computed from recordings from different channels simultaneously) on the same day corresponding contours: 50% (red), 95% (blue), 99.9% (orange), 99.97% (black) of the distribution. Red and blues crosses represented recordings classified as stably corresponding to the same or arising from distinct neurons respectively, based on combining the use of multiple similarity scores with quadratic classifiers (green lines). (f) Cross-day stability of single-unit isolation quality assessed by L-ratio and isolation distance (n=158 included units shown in red in e). (g) Cross-day stability of four cluster similarity scores (n=158 included units shown in red in e, day 1 session 1 recordings were used as reference).