Direct method for the estimation of the PRC.

<p>(A) A direct estimate of the PRC, obtained for the same PC, is plotted after data smoothing (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004112#sec002" target="_blank">Methods</a>) 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 <i>Z</i>(<i>φ</i>) = 0, while the continuous black thick traces are the PRCs, estimated at distinct firing rates (<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004112#pcbi.1004112.g001" target="_blank">Fig. 1</a>). Black circles indicate the location of the extrema for each of the two halves of the curves (i.e., in <i>φ</i> ∈ [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 <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004112#sec002" target="_blank">Methods</a>). 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 + <i>e</i>^{−(<i>F</i>−<i>a</i>)/<i>b</i>})⁻¹, with best-fit parameters <i>a</i> = 44.1, <i>b</i> = 20.5. The inset further displays the location of the PRC peak, relative to the time of the AP following the stimulus (i.e., <i>τ</i>_<i>peak</i> = (<i>τ</i>_<i>peak</i> − 1) ⋅ ⟨<i>ISI</i>⟩), for the same five cells.