Two opposing systems can therefore be postulated: a Go system and a Stop system. The Go system could drive craving and engage habits via the basal ganglia.105,127 For example, steeper delayed discounting and cocaine craving in cocaine dependence is associated with increased connectivity in the network linking the medial prefrontal cortex and anterior cingulate cortex with the ventral striatum and in the network linking the insula with the dorsal striatum.128 Activation of the dorsal anterior cingulate is also associated with fear conditioning in post-traumatic stress disorder.129 The Stop system could control assessment of the incentive value of choices and suppression of affective responses to negative emotional signals.130–132 Under this framework, a Stop system would inhibit the Go craving system and stress system. For example, in post-traumatic stress disorder there is substantial evidence of hypoactivity in the ventromedial prefrontal cortex and hyperactivity in the central nucleus of the amygdala that reflects an inhibitory connection between the ventromedial prefrontal cortex and central nucleus of the amygdala.133,134 Indeed, successful cognitive inhibition of craving in cocaine abusers has been associated with inhibition in the right medial orbitofrontal cortex and activation in the right inferior frontal cortex.135
