The serotonin and dopamine systems are neuromodulator systems that change how other circuits behave by increasing or decreasing the probability of certain circuits being active.

“But there's something so profound about the serotonin system and the dopamine system, because the way I define a neuromodulator is, it's a modulator. It changes the way that other circuits behave. And essentially, it increases the probability that certain circuits will be active and decreases the probability that other circuits will be active,”