With the help of functional magnetic resonance imaging, investigators at Brookhaven National Laboratory showed that cocaine users have depressed inner motivational drive and dysfunctional decision making process, probably due to drug-induced changes in the prefrontal cortex:
“Our findings provide the first evidence that the brain’s threshold for responding to monetary rewards is modified in drug-addicted people, and is directly linked to changes in the responsiveness of the prefrontal cortex, a part of the brain essential for monitoring and controlling behavior,” said Rita Goldstein, a psychologist at Brookhaven Lab…
Goldstein’s experiments were designed to test a theoretical model, called the Impaired Response Inhibition and Salience Attribution (I-RISA) model, which postulates that drug-addicted individuals disproportionately attribute salience, or value, to their drug of choice at the expense of other potentially but no-longer-rewarding stimuli — with a concomitant decrease in the ability to inhibit maladaptive drug use. In the experiments, the scientists subjected cocaine-addicted and non-drug-addicted individuals to a range of tests of behavior, cognition/thought, and emotion, while simultaneously monitoring their brain activity using functional magnetic resonance imaging (fMRI) and/or recordings of event-related potentials (ERP).
In one study, subjects were given a monetary reward for their performance on an attention task. Subjects were given one of three amounts (no money, one cent, or 45 cents) for each correct response, up to a total reward of $50 for their performance. The researchers also asked the subjects how much they valued different amounts of monetary reward, ranging from $10 to $1000.
More than half of the cocaine abusers rated $10 as equally valuable as $1000, “demonstrating a reduced subjective sensitivity to relative monetary reward,” Goldstein said.
“Such a ‘flattened’ sensitivity to gradients in reward may play a role in the inability of drug-addicted individuals to use internal cues and feedback from the environment to inhibit inappropriate behavior, and may also predispose these individuals to disadvantageous decisions — for example, trading a car for a couple of cocaine hits. Without a relative context, drug use and its intense effects — craving, anticipation, and high — could become all the more overpowering,” she said.
Picture caption: In control subjects (left), brain regions that play a part in experiencing reward were activated in a graded fashion in response to increasing monetary rewards. These regions were not activated in cocaine-addicted subjects offered the same rewards (right). This indicates that cocaine-addicted subjects’ ability to respond to non-drug rewards is compromised.