Test pulse (TP) threshold modulated by high-frequency biphasic (HFB) stimulation at different intensities. A: TP threshold changes with the intensity of HFB stimulation. The axonal activity at the 3 stimulation conditions marked by symbols “B”, “C”, and “D” in A is shown in B, C, and D respectively. B: TP induced an action potential while HFB stimulation did not. HFB/TP=3 mA/1.6 mA. For clarity, the TP and HFB stimulation are also schematically plotted in B. C: The HFB stimulation induced repetitive firing. HFB/TP=6 mA/0 mA. D: TP induced an action potential while HFB stimulation only induced an initial action potential. HFB/TP=9 mA/0.9 mA. The short arrows in B, C, and D mark the electrode locations along the axon. TP width: 0.1 ms; HFB stimulation frequency: 10 kHz; Axon diameter: 2 μm.