Frontoparietal cortical activity of methamphetamine-dependent and comparison subjects performing a delay discounting task

Abstract

Relative to individuals who do not have addictive disorders, drug abusers exhibit greater devaluation of rewards as a function of their delay ("delay discounting"). The present study sought to extend this finding to methamphetamine (MA) abusers and to help understand its neural basis. MA abusers (n = 12) and control subjects who did not use illicit drugs (n = 17) participated in tests of delay discounting with hypothetical money rewards. We then used a derived estimate of each individual's delay discounting to generate a functional magnetic resonance imaging probe task consisting of three conditions: "hard choices," requiring selections between "smaller, sooner" and "larger, later" alternatives that were similarly valued given the individual's delay discounting; "easy choices," in which alternatives differed dramatically in value; and a "no choice" control condition. MA abusers exhibited more delay discounting than control subjects (P < 0.05). Across groups, the "hard choice > no choice" contrast revealed significant effects in the ventrolateral prefrontal cortex, dorsolateral prefrontal cortex (DLPFC), dorsal anterior cingulate cortex, and areas surrounding the intraparietal sulcus (IPS). With group comparisons limited to these clusters, the "hard choice > easy choice" contrast indicated significant group differences in task-related activity within the left DLPFC and right IPS; qualitatively similar nonsignificant effects were present in the other clusters tested. Whereas control subjects showed less recruitment associated with easy than with hard choices, MA abusers generally did not. Correlational analysis did not indicate a relationship between this anomaly in frontoparietal recruitment and greater degree of delay discounting exhibited by MA abusers. Therefore, while apparent inefficiency of cortical processing related to decision-making in MA abusers may contribute to the neural basis of enhanced delay discounting by this population, other factors remain to be identified.

Figure 1

Radiological presentation of areas in which significantly greater signal was observed in hard‐choice blocks relative to nonchoice blocks (P < 0.05 cluster extent, whole brain analysis). The six clusters are characterized in Table I.

Figure 2

A presents the signal change (mean ± standard error) for each group for both the easy‐choice condition and the hard‐choice condition, both relative to the no‐choice trials. Among control subjects, very little signal change is present in the easy‐choice condition relative to what is observed in the hard‐choice condition. For MA abusers, however, similar signal change was observed in both conditions. B directly presents the differences between the hard‐choice and easy‐choice conditions, highlighting the group effect. Although the effect was only significant in two clusters, the tendency for signal change in these two conditions to be more different among control participants than among MA participants appears to be uniform across the six clusters.

Figure 3

Radiological presentation of cluster in right VLPFC in which greater signal change (P < 0.05 cluster extent) in the “hard choice > no choice” contrast was associated with less trading of amount for immediacy (less delay discounting). Analysis was limited to clusters identified as active in the same contrast (Table I).