Compared to most other people, addicted individuals typically place a much higher premium on immediate gratification compared to future rewards. One scientific theory attributes this trait to weakness in a brain system that weighs alternative courses of action in light of their likely impact on long-term goals. A recent NIDA-funded study supports this view and suggests that the weakness prevents individuals from recalling experiences that would enable them to better appreciate the value of delayed rewards.
In the study, memory training increased stimulant-dependent individuals’ willingness to wait for a larger sum of money rather than accept a smaller one right away. If the finding is replicated, memory training may have a place in substance abuse treatment as a way to help patients reject readily available drug highs in favor of the longer-term satisfactions of drug-free living.
Discounting, Memory, and the Brain
Everyone agrees that money in hand is worth more than money promised for the future, but people differ on how much more. When researchers offer individuals real or hypothetical payoffs, some are willing to forgo larger portions than others to have the money right away instead of at some specified later time. The portion of the payoff that the individual forgoes—referred to as the delay discounting—is considered a good indicator of his or her general preference for immediate versus future rewards.
Drug abusers are extreme delay discounters. They also have relatively weak working memories. Dr. Warren K. Bickel and colleagues at the University of Arkansas for Medical Sciences hypothesized that the latter trait underlies the former. This hypothesis brought together work by researchers who theorized that delay discounting represents the sum of two competing brain systems and others who showed that people who have difficulty delaying gratification also demonstrate poor memories.
The idea that delay discounting represents the result of competing brain systems was first proposed by Dr. Antoine Bechara of the University of Southern California. It posits that when an opportunity for a reward occurs, it initiates a contest between:
- A present-oriented brain system, involving the striatum and amygdala, that simply responds positively, generating impulses to seek the reward, and
- A future-oriented system, located primarily within the prefrontal cortex (PFC), that inhibits those impulses as it brings into play the tradeoffs and consequences that pursuing the reward will entail.
Depending on which system is stronger, the person will go after the reward or pass it up.
Dr. Bickel’s team reasoned that drug abusers’ memory limitations might reduce their PFC’s access to lessons from past experience. Lacking that information, the future-oriented system loses some of its ability to predict consequences and accordingly restrain the impulsive system.
The Past as Key to the Future
Dr. Bickel and colleagues recruited 27 patients in treatment for stimulant abuse. To assess the participants’ delay discounting, they used a computer program that presented a choice between a sum of money immediately or a larger amount at a later date. If the participant chose the smaller, immediate amount, the program presented the choice again with a still-lower immediate payoff. The cycle repeated until the participant indicated no preference between the immediate and delayed payoffs. The difference between the two amounts on offer at this point was the measure of the participant’s delay discounting—the premium that he or she put on immediate versus delayed payoff.
The researchers assessed each patient’s delay discounting of hypothetical rewards of $100 and $1,000 at seven delays ranging from 1 day to 25 years. In four additional trials, participants were offered $100 in real money at delays ranging from 1 day to 6 months or a smaller amount immediately. The researchers then randomly chose one of these four trials and paid the selected amount.
After the discounting assessment, the researchers randomly assigned the participants to active or sham memory training. During active-training sessions, participants worked on four memory tasks from a computer-assisted cognitive rehabilitation program. One task, for example, was to view and remember a set of four-letter words and, after a delay, identify the words among a longer list (see Memory Work-out). Three words comprised the initial set; those who mastered it went on to a four-word set, and so on, up to an 11-word set. Participants who failed to find the right words five times in a row were considered to have reached the limit of their working memory ability.
The sham training featured the same tasks but did not exercise working memory because the computer program provided the correct responses. During the word recognition task, these participants saw the words and were then presented, after a delay, with the correct set without having to generate a response.
After the memory training, the researchers repeated their assessment of the participants’ delay discounting. Those who had received the active memory training discounted future rewards, whether hypothetical or real, only about half as much as before. For example, a participant who initially placed equal value on $50 immediately and $100 in 1 month might, after the training, regard $50 immediately as equivalent in value to $100 in 2 months. In contrast, participants who received sham memory training discounted the same amount after it as before. The researchers also observed that participants whose performance on the active memory task improved the most showed the greatest reductions in discounting.
Possible Role in Treatment
Dr. Bickel, who is now at the Virginia Tech Carilion Research Institute, plans to test whether memory training can improve addiction treatment outcomes. He and colleagues hypothesize that improving memory might help substance abusers learn and apply lessons from therapy.
Prior studies have found that substance abusers with greater delay discounting before treatment also demonstrated poorer outcomes of addiction therapy. “If this correlation is due to a causal relationship, then reducing delay discounting with memory training would boost addiction treatment,” Dr. Bickel says.
Dr. Bechara says, “The approach that Dr. Bickel and colleagues have taken is exciting and promising because it involves training aimed at improving a particular function instead of blocking or disrupting one. But we still don’t know whether this improvement will translate to an actual drop in substance use.”
Although it had not been applied to addiction prior to Dr. Bickel’s study, training to improve executive functions as an adjunct to psychiatric treatment is not new. Researchers have used training to improve memory, attention, and learning skills to reduce symptoms and increase functioning among people with schizophrenia.
Dr. Will M. Aklin of NIDA’s Division of Clinical Neuroscience and Behavioral Research agrees that Dr. Bickel’s finding is compelling: “Working-memory training with monetary incentives is a promising adjunct to integrate into substance abuse treatment.” However, he notes that further research is required to determine the power and limitations of the approach. Because the participants in the study had already attained abstinence, Dr. Aklin points out that abstinence may be necessary for memory training to be effective.
Bickel, W.K., et al. Remember the future: Working memory training decreases delay discounting among stimulant addicts. Biological Psychiatry 69(3):260–265, 2011. [Full Text (PDF, 112KB)]