Regular drug users can have difficulty choosing between right and wrong, perhaps because the specific parts of their brains used for moral processing and evaluating emotions are damaged by their prolonged drug habits, says a study. There is strong link between drug use and criminal behaviour, but it is not known whether the criminal behaviour is in part a result of the drugs' effects on brain function. "This is the first study to suggest impairments in the neural systems of moral processing in both cocaine and methamphetamine users," said lead author Samantha Fede from University of New Mexico in the US. The findings were published in Springer's journal Psychopharmacology. The researchers examined how the neural networks and brain functioning of chronic cocaine and methamphetamine users in US jails relate to their ability to evaluate and decide about moral situations or scenarios. Poor judgment about moral situations can lead to poor decision making and subsequent antisocial behaviour. The researchers recorded the life history of substance abuse of 131 cocaine and methamphetamine users and 80 non-users incarcerated in New Mexico and Wisconsin prisons. The participants' brains were scanned while they completed a moral decision-making task in which they evaluated whether certain phrases were morally wrong or not. Compared to the non-users, the regular stimulant users had abnormal neural activity in the frontal lobes and limbic regions of their brains during moral processing. Specifically, lifetime stimulant users showed less activity in the amygdala, a group of neurons in the brain that helps to regulate and understand emotions. The researchers also observed a relationship in the level of engagement of the anterior cingulate cortex: the longer people had been using stimulants, the less activity in this region. This is an area of the brain that coordinates reinforcement, effect and executive action needed in moral decision making.