Analysis and Statistics.

The ΔF/F epifluorescence signals arising from PFs were spread out over a large surface area (SI Appendix, Fig. S1 A and B), even those arising from a single activated fiber (SI Appendix, Fig. S7 A–D). The geometrical arrangement of PFs (perpendicular to the image plane) was advantageously used for analysis: ΔF/F signals exhibited a radial symmetry and could hence be fully described by radial plotting, which was used to measure Φ (SI Appendix, Fig. S1D). Data are presented as mean ± SEM. Significance was determined by an unequal variances two-tailed t test (when appropriate) or a Mann–Whitney U test (M-W as indicated in Table 1). In order to control type II error, we verified for each statistical comparison that the achieved power, given actual alpha and effect size, was above 0.8.