Unseen Drug Cues Can Still Induce Cravings
February 3, 2008
People generally look at addiction in one of two ways. Some understand the extreme difficulty inherent in battling a craving, whether it be for drugs, food, cigarettes, etc. This is especially the case with those who have experienced addiction, and to a lesser extent with those who have helped someone else through it or studied it extensively. Others may attribute addiction to a personal choice, as if a drug user is able to simply sit down and decide whether or not he or she wants to do drugs today, and then acts on that decision.
The science doesn’t support the latter view. A number of neuroimaging studies demonstrate abnormal activity in the brains of drug addicts. This aberrant activity can involve decision-making areas (pre-frontal lobes), areas correlated with compulsive behavior (anterior cingulate and orbitofrontal cortices), and reward processing centers (mesolimbic dopamine pathway and limbic system). Now consider the genetic influences on tendencies toward addictive behavior and it begins to seem that, if a personal decision is involved, the deck is stacked pretty heavily against an addict making the right one. Drawing an analogy of addiction to personal choice is like telling someone who is clinically depressed they just need to “snap out of it”.
Recent research by a group at University of Pennsylvania has shown addiction to be even more difficult to overcome than previously thought. The group used functional MRI (fMRI) to study brain activity in cocaine-addicted patients as they viewed images with different themes. Some pictures were drug stimuli (e.g. a crack pipe), others were sexual in nature, and the rest were either aversive (i.e. involving pain) or neutral. The images were flashed on a screen for 33 milliseconds, and then immediately followed by another image used to distract the participant and ensure the first picture could not be registered—a technique known as backward masking. Previous studies have shown that this procedure results in participants not consciously recognizing the content of the more briefly displayed image.
The patients exhibited brain activity consistent with drug craving when the drug images were shown, even though the images didn't seem to be consciously processed. This activity occurred throughout limbic structures previously implicated in reward and drug addiction, such as the striatum, amygdala, orbitofrontal cortex, insula, and prefrontal cortex. The activity was similar to that which occurred when the sexual images were viewed.
The Penn research group has hypothesized that the recognition of the drug cues is the result of pure Pavlovian conditioning. According to this view, drug craving becomes a learned reflexive response. With exposure to relevant stimuli, its onset is automatic. The group also suggests that the similarity between how the drug stimuli and sexual stimuli were processed indicates these patients’ brains are viewing drugs in the same way they are viewing biologically rewarding stimuli. Many scientists believe our brains have evolved to view life-promoting goals like procreation and food as rewarding because they are essential to our survival as a species (thus the desire to achieve them was naturally selected for). It seems drugs can hijack the brain’s reward circuitry and convince it drugs are as important as the rewards we have been genetically programmed to pursue. This presents a grim picture of addiction. In my opinion, however, studies such as this one are important in coaxing our society to view addiction as a disorder that requires treatment, instead of a social stigma or a behavior that in and of itself necessitates incarceration.