A small imaging study tailored monetary choices to each participant’s preferences. It found altered recruitment for easy versus hard choices in methamphetamine-dependent participants, but the recruitment difference did not explain their greater delay discounting.
Monterosso and colleagues studied 12 methamphetamine-dependent participants and 17 controls. Initial hypothetical-money choices estimated how steeply each person discounted delayed rewards. Those estimates then helped construct an individualized fMRI task.
Hard choices paired smaller-sooner and larger-later options with similar estimated subjective values. Easy choices presented options differing substantially in value. The same nominal monetary comparison need not be equally difficult for two people with different preferences.
The methamphetamine group discounted delayed rewards more steeply. Selected frontoparietal regions also differed between groups in the hard-versus-easy contrast. Controls generally recruited these regions less for easy choices; the methamphetamine group showed less differentiation.
Correlational analyses did not establish a relationship between that recruitment anomaly and the greater discounting in the methamphetamine group. Coexisting group differences therefore do not prove that one caused or statistically accounted for the other.
This was observational human research, not controlled methamphetamine administration. Small groups, hypothetical rewards and participant differences limit generalization and causal inference. Proposed inefficient processing remains an interpretation of the task pattern.
The individualized task is useful because it separates nominal reward size from estimated decision difficulty. Its fMRI contrasts are not drug concentrations, and neither steeper discounting nor altered recruitment provides a validated measure of intoxication, driving impairment or a person’s capacity to make a particular real-world decision.
See task-specific monetary-cue imaging.
Frontoparietal cortical activity of methamphetamine-dependent and comparison subjects performing a delay discounting task. doi:10.1002/hbm.20281.