Correlation of VSD signals and intracellular granule cell activity. (a) VSD signals with a small slope or large slope were obtained from ROIs corresponding to the somata of GrCs recorded intracellularly and compared to electrical traces. The recording from a GrC in a low-responding region shows a prevalence of EPSPs (10 consecutive sweeps are superimposed). The recording from a GrC in a high-responding region shows larger EPSPs and EPSP-spike complexes (10 consecutive sweeps). (b) The histogram shows the number of spikes emitted by GrCs during 10 consecutive sweeps in ROIs belonging to group 1 or group 2. (c) Correlation between normalized peak amplitude and the probability of eliciting at least one action potential per GrC during 10 consecutive sweeps. Linear regression lines are shown for both clusters of VSD signals emerging from PCA (dashed black line, group 1 ROIs, R
2 = 0.2, Fisher's F-test p = 0.04; dashed gray line, group 2 ROIs, R
2 = 0.37, Fisher's F-test p = 0.02). (d) Correlation between normalized time-to-peak and the probability of eliciting at least one action potential per GrC during 10 consecutive sweeps. Linear regressions are shown for both clusters of VSD signals emerging from PCA (dashed black line, group 1 ROIs, R
2 = 0.15, Fisher's F-test p = 0.06; dashed gray line, group 2 ROIs, R
2 = 0.16, Fisher's F-test p = 0.03). (e) Correlation between the vector slope of VSD signals and the probability of eliciting at least one action potential per GrC. Linear regressions are shown for both clusters of VSD signals emerging from PCA (dashed black line, group 1 ROIs, R
2 = 0.85, Fisher's F-test p = 10−14; dashed gray line, group 2 ROIs, R
2 = 0.89, Fisher's F-test p = 10−15). (f) Correlation between the slope of the VSD signals and the total number of emitted spikes per GrC. Linear regressions are shown for both clusters of VSD signals emerging from PCA (dashed black line, group 1 ROIs, R
2 = 0.82, Fisher's F-test p = 10−11; dashed gray line, group 2 ROIs, R
2 = 0.84, Fisher's F-test p = 10−13). (g) Correlation between the initial slope of the VSD signals and the probability of eliciting at least one action potential per GrC. Linear regressions are shown for both groups of VSD signals emerging from PCA (dashed black line, group 1 ROIs, R
2 = 0.91, Fisher's F-test p = 10−16; dashed gray line, group 2 ROIs, R
2 = 0.78, Fisher's F-test p = 10−9). (h) Correlation between the initial slope of the VSD signals and the total number of emitted spikes per GrC. Linear regressions are shown for both groups of VSD signals emerging from PCA (dashed black line, group 1 ROIs, R
2 = 0.92, Fisher's F-test p = 10−14; dashed gray line, group 2 ROIs, R
2 = 0.8, Fisher's F-test p = 10−7).