The Purkinje cell soma is surrounded by the axons of basket cells (diagram in Supplementary Fig. 1a). The propagation of the action potential of the interneurone in this perisomatic basket could itself affect the somatic voltage of Purkinje cell and thus its firing, though the control cells in Fig. 1i,k,m,o suggest this action is weak, if present. To gain insight into this possible mechanism, we modelled changes of extracellular voltage at the soma (black) by changing V SE and compared the result to the effects of the pinceau already modelled (orange) and of action potentials in both compartments (green). Spike propagation in the basket can be active, because an extracellular negativity could be observed there (Fig. 2c, traces 9 and 10). To estimate an upper limit on the contribution of an action potential in the basket we set VSE to twice the recorded trace with the maximal negativity (Fig. 2c trace 7), thus assuming a spatially uniform negativity of −70 μV around the soma. (both the amplitude and uniformity of this signal cause overestimation of its effect). This change of extracellular potential capacitivelyhyperpolarises the Purkinje cell soma (b) by 20 μV. The somatic transmembrane potential is depolarised (c). The intracellular hyperpolarisation propagates to the axon (d) with very little decrement. These changes do not affect the intra-pinceau potential (e), producing a net transmembranehyperpolarisation of the Purkinje cell axon (f). However, even in this overestimate, the hyperpolarisation caused in the Purkinje cell axon by the basket is small compared to the pinceau effect and the firing of the Purkinje cell is only weakly modulated (g). The basket-induced signals described above would render impossible the intracellular detection of the pinceau signal, which we showed in Fig. 4 would in any case be undetectably small. The only method able to demonstrate the pinceau effect on the Purkinje cell is therefore measurement of its effect on firing.