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. Author manuscript; available in PMC: 2011 May 24.
Published in final edited form as: IEEE Trans Circuits Syst I Regul Pap. 2011;58(5):1034–1043. doi: 10.1109/TCSI.2010.2089556

Fig. 1.

Fig. 1

Measured silicon neuron membrane traces (x, normalized) rise like a resistor-capacitor circuit, with a positive feedback spike for several step-input current levels (r=0.48, 0.57, 0.69, 0.82, 0.98, and 1.2). Below a threshold input, the neuron reaches a steady state at which the input and positive feedback are insufficient to overpower the leak. Above threshold, larger inputs enable the positive feedback to overcome the leak more quickly, resulting in a shorter time to spike. Inset The neuron’s phase plot shows ẋ versus x (black), fit with a cubic (gray). The neuron has two fixed points (circles) when its input is small, r < 2/3, one stable (filled) and one unstable (open). As r increases above 2/3, the fixed points merge and destroy each other, undergoing a saddle-node bifurcation, the transition from resting to spiking. Phase plot traces filtered by a 5 ms halfwidth gaussian.