From control to compulsion

In a broad-brush model of behaviour, we can see control being mediated by two parallel systems. One is goal-directed, relates action to outcome, and is mediated by the ventral striatum. The other is determined by stimulus-response associations, is habitual – even to the extent of being automated on occasions – is divorced from the value of future outcomes, and associated with the dorsal striatum and motor areas.

We know pretty automatically what to do on the red light, and what to do on green; and we know that when the light glows orange we have to think about it.

Shifting our activities to the latter system is generally adaptive since our brain systems have limited capacity, and habits conserve attentional resources for more demanding tasks. Murat Yücel (Monash University, Victoria, Australia) described how we experience this shift when we learn to drive.

We know pretty automatically what to do on the red light, and what to do on green; and we know that when the light glows orange we have to think about it. This analogy shows that at most times we can readily move from one control system to the other. But in substance abuse disorder, including alcohol dependence, this flexibility seems to be lost. Dependent drinkers cannot switch back to goal-directed behaviour.

The way one system comes to dominate the other can be seen in how brain circuitry responds to alcohol-related cues. If you show social drinkers images of drinking (and compare response to neutral images), the ventral striatum lights up on functional magnetic resonance imaging (fMRI). In problem drinkers, however, alcohol stimuli activate the dorsal system. This suggests that control of behaviour has shifted from reward pathways to those involved in relieving negative affect.

In the same symposium impulsivity, along with stress, were described as factors making people vulnerable to developing drug dependence.

Maartje Luiten (Radboud University, the Netherlands) presented data from various psychomotor tasks showing that people with a range of dependence disorders had difficulties with impulse control associated with hypoactivation of prefrontal brain areas measured via fMRI. And she wondered whether in the future it might be possible to train vulnerable people in impulse control.

Marc Potenza (Yale University, New Haven, USA) described how impulsivity is linked to striatal dopamine (DA) function both in healthy subjects and in those with dependence disorders, and suggested the DA D3 receptor as a potential pharmacological target. His research group is also moving towards imaging neurochemical changes in the brain as a way of assessing progress among patients undertaking treatment programmes such as cognitive behaviour therapy.