A choice that is difficult for one person may be easy for another. Comparing brain responses to identical monetary offers can therefore confound group membership with subjective difficulty. A 2007 methamphetamine imaging study addressed this problem by tailoring choices to each participant’s preferences.
Establishing individual reward preferences
Monterosso and colleagues studied 12 methamphetamine-dependent adults and 17 comparison adults. They first used hypothetical monetary choices outside the scanner to estimate how each participant discounted delayed rewards. Larger discounting meant a greater willingness to accept less money immediately instead of more money later.
The methamphetamine group showed greater discounting. Its geometric mean discounting parameter was 0.045 versus 0.013 in controls. Those are model parameters, not drug concentrations or a percentage of participants judged impaired. Response times in the initial assessment did not differ significantly.
The dependent participants stayed in a research center for five to seven days of abstinence before testing, verified with urine testing. Prior use varied in amount and route. The study did not administer a fixed methamphetamine dose or establish a concentration-response threshold.
How were easy and hard choices constructed?
The researchers used each person’s estimated preferences to generate three scanner conditions. Hard choices offered immediate and delayed alternatives of approximately equal subjective value. Easy choices offered alternatives with markedly different estimated values. A no-choice condition presented only one option.
This design matters because a greater preference for immediate money does not itself mean that every immediate-versus-delayed choice is more difficult. Personalizing value helped compare the demands of deciding, rather than merely comparing identical dollar amounts.
What did the imaging show?
Across groups, hard choices versus no-choice trials recruited several frontal and parietal regions. Within those task-defined regions, the hard-versus-easy contrast differed between groups in the left dorsolateral prefrontal cortex and right intraparietal sulcus.
Controls showed less recruitment for easy than hard choices. The methamphetamine group generally showed less differentiation between the two difficulty levels. Similar patterns in other tested clusters did not reach significance and should not be described as confirmed differences everywhere.
Did that pattern explain greater discounting?
No significant relationship was found between the hard-versus-easy activation difference and the degree of delay discounting. The imaging group difference therefore did not establish the proposed explanation for the behavioral group difference.
A separate relationship appeared between signal in a ventrolateral prefrontal region and discounting for the hard-versus-no-choice contrast. That result concerns a different comparison and cannot replace the null finding for difficulty-related differentiation.
The investigators also did not detect a significant imaging difference between hard trials on which participants selected the larger delayed reward and those on which they selected the smaller immediate reward. Neither preference was identified by a validated neural signature.
Limits of a small historical study
The sample included only 29 people, with smoking and prior other-drug exposure relevant to interpretation. Monetary outcomes were hypothetical, and the observational design did not establish whether group differences preceded or followed methamphetamine use.
The useful contribution is the method: subjective difficulty deserves explicit control. The study does not validate a brain scan for an individual’s impulsivity, functional incapacity or driving impairment.
Historical primary source
Monterosso JR, Ainslie G, Xu J, Cordova X, Domier CP, London ED. Frontoparietal Cortical Activity of Methamphetamine-Dependent and Comparison Subjects Performing a Delay Discounting Task. Human Brain Mapping. 2007;28:383–393. doi:10.1002/hbm.20281.
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