Figure 1

Description of the mass-pendulum arrangement. (A) An electromechanical pendulum system supported by a force balance topology that maintains the mass M position stable in the equilibrium position. Nodes including R₁ and C serve the recovery of the mass position following fast disturbances of the system, while integrator τ and the in-series resistor R₂ ensures the long-term stabilization of the mass. k₃ is the stiffness constant of the system, and k₂ is the constant that defines the gain of the device that senses the deviation of the pendulum around the equilibrium position. k₁ is the force constant (motor constant) which defines the amount of the applied correction in response to the external excitations of the system. (B) In analogy, a mood pendulum system is illustrated where the mood plays the role of the mass seen in the previous block and the feedback nodes are substituted by analogous ‘therapeutic actuators’ that keep the mood stable. The upper node compensates the fast and transient mood episodes (mood oscillations) and usually is implemented by the use of some antiepileptic drugs. The lower node exhibits integrative behavior and compensates mood episodes with long time evolution acting as long-term mood stabilizer. Usually, this type of regulatory response is served by the action of lithium.