(a) Schematic diagram of STN–GPe circuit model. Red lines with rhombus represent the excitatory projections and black lines with round heads denote inhibitory projections. (b) Block diagram of STN–GPe circuit model, where C₁ and C₂ are the inter-coupling strengths between STN and GPe populations, and C₃ represents the intra-coupling (self-feedback) strength of GPe population, and y_STN and y_GPe are the outputs of STN and GPe populations, respectively. Both STN and GPe populations are denoted by their linear convolution operators (T_STN and T_GPe) and sigmoid nonlinear elements (S_STN and S_GPe).