Direct method for the estimation of the PRC.

(A) A direct estimate of the PRC, obtained for the same PC, is plotted after data smoothing (see Methods) while altering the cell’s firing rate in the range 20 − 160 Hz): a strong dependency on the firing rate is apparent. The transition from an approximately flat to a phase-dependent PRC profile does not occur abruptly, but smoothly: in each subplot, the horizontal gray dashed lines represent Z(φ) = 0, while the continuous black thick traces are the PRCs, estimated at distinct firing rates (Fig. 1). Black circles indicate the location of the extrema for each of the two halves of the curves (i.e., in φ ∈ [0; 0.5] or [0.5; 1], emphasized by the vertical thin black line), used to concisely characterize the PRC shape according to its peak-to-baseline ratio (see Methods). The graded PRC shape dependency on the firing rate is confirmed in three other PCs (B, markers) and quantified by their peak-to-baseline ratio. The black curve represents the function (1 + e^−(F−a)/b)⁻¹, with best-fit parameters a = 44.1, b = 20.5. The inset further displays the location of the PRC peak, relative to the time of the AP following the stimulus (i.e., τ_peak = (τ_peak − 1) ⋅ ⟨ISI⟩), for the same five cells.