Figure 3.
CPG mechanism with neuromodulation. (A) Wiring diagram of the CPG circuit. The extrinsic modulatory input MI alters the synaptic weights of the CPG, thereby modulating the CPG outputs. The synaptic weights are set as W11,22 = 1.4, W12m = 0.18 + MI, W21m = −0.18 − MI. (B) The resulting eigenfrequency of the outputs of the CPG (black solid line, left scale) and the walking speed of AMOS II (blue dashed line, right scale) with respect to MI. Here MI is increased by 0.01. If MI is smaller than 0.0 the network dynamics exhibits only fixed point attractors; i.e., oscillations are switched off. Recall that the CPG network is updated with a frequency of approximately 27 Hz (i.e., one time step is ≈0.037 s). (C) Examples of the asymmetrical periodic outputs of the CPG (top) where MI is set to 0.02, 0.08, and 0.16. The signals differ in phase by π/2 and are shaped by neural CPG postprocessing such that smooth ascending and descending signals are obtained for motor control (bottom). This kind of asymmetrical periodic signals is appropriate for walking found in insects where swing (ascending slope) and stance (descending slope) phases differ in duration, being intrinsically asymmetry (Wilson, 1966).