Fig 2. The utility of an action depends on the reinforcement size and physiological state.

A) The same reinforcement can yield a positive or negative utility depending on whether the difference between the current and new physiological state is positive or negative. B) A large reinforcement may have an utility of zero even if the animal was initially in a depleted state. U = utility, m = motivation. S* = the desired state and S1 = state before action, S2 = state after action. Arrow length indicates the size of the reinforcement (r). Changes in state resulting in an increase and decrease in desirability are indicated with green and red arrows, respectively.