Figure 8.

Effects of perithreshold operation on spike-time coding. A, Maximal conductance (ḡ_M) and voltage dependency (β_z and γ_z) were varied to adjust how strongly I_M was activated at voltages below threshold. Voltage threshold (V*) can vary but its approximate value is indicated on the graph. B, Increasing subthreshold activation of I_M extended the range of I_DC over which the neuron operated in the perithreshold regimen, as indicated by arrows. Arrows are shown again on left panel of C, together with arrow from Figure 6F. Bifurcation diagrams shown here were generated as explained in Figure 6F,G. C, Extension of the range of perithreshold operation demonstrated in B was paralleled by an extension of the range of I_DC over which spike-timing reliability remained high (left); relationship between reliability and firing rate remained unchanged (right), consistent with results in Figure 5E. D, Increasing subthreshold activation of I_M resulted in increased shifting of I*, which is seen most clearly when comparing f–I curves generated by constant stimulation (dashed gray curves). Notice also that increased activation of I_M caused dashed curves to become discontinuous, i.e., curves did not extend to 0 spikes/s; the implications of this are discussed in Figure 9. Inclusion of high-frequency noise/signal caused fluctuation-driven spiking that was manifested on f–I curves as a leftward tail (solid curves).