Self-Destructive Acts Linked to Limited View of Future

Borrowed from behavioral economics, an important tool Bickel uses in his addiction research is temporal discounting, which measures how a person weighs the relative values of immediate and delayed rewards. The test subject is asked to choose between two hypothetical rewards: either receiving $1,000 in a month or a smaller amount now. If the current prize is also $1,000, an average person would choose the prize now. As the current reward is reduced to $950, $900, $850, and so on, while the future prize is held constant, a person will eventually switch from the smaller current prize to the larger future prize. The trade-off amount is a discount against the future constant. The larger the discount due to delay would indicate a greater unwillingness to wait and can be used to measure how impulsive a person is.

Temporal discounting ties addiction to abnormally high impulsivity and a severely narrowed view of the future, Bickel said. The link has been demonstrated in human as well as animal research, which shows that chronic self-administration of certain substances such as amphetamine can alter the extent of discounting. In a review published in the September 2007 Drug and Alcohol Dependence (Supplement), Bickel and colleagues noted that previous studies have found that people with opioid and cocaine dependence, drinking and gambling problems, and a smoking habit discount the future significantly more than nonaddicted controls. In a study they published in the July 2008 Drug and Alcohol Dependence, cigarette smokers discounted the hypothetical future, as well as past monetary gains significantly more than nonsmoking controls.

To understand the physiology of behavioral patterns, scientists use neuroeconomics to uncover the mechanisms underlying how people perceive the cost of their choices and make trade-off decisions in the context of time. A number of new studies have found evidence to support the connection between addiction and impulsivity.

Two systems in the brain compete in the temporal discounting process: the impulsive system that wants the reward immediately and the executive system that evaluates rationally the temporal consequences and restrains the urge of instant gratification, Bickel explained.

In a study published in the October 15, 2004, Science, Samuel McClure and colleagues used functional magnetic resonance imaging scans to demonstrate the two systems involved in the time-discounting decisions. Parts of the limbic system associated with the midbrain dopamine system are activated by immediate rewards. This is consistent with knowledge about dopamine's role in addiction and the overall reward system. However, choosing a delayed reward required greater activities in the lateral prefrontal cortex and posterior parietal cortex, McClure found.

Bickel calls the theory of two systems the “competing neurobehavioral decision systems hypothesis.” Impulses are primarily driven by activities in the limbic region, ventral striatum, and nucleus accumbens. The executive system, in contrast, is known to be carried out in the prefrontal cortex and dorsolateral prefrontal cortex, which are central to learning, planning, regulating impulses, and making rational decisions. In other words, the executive system regulates the urges for immediate reward and considers whether a delayed reward is more valuable.

In individuals with addiction, this balance is tipped in the direction of the impulsive system, and a person's reason and future outlook are severely weakened.

Are individuals with addiction born with a vulnerable executive neurocircuitry that make them more susceptible to substance abuse? Or is substance abuse responsible for damaged brain functions? There is evidence to support both scenarios, and the interaction between innate predisposition and the environment is a plausible explanation.

A recent study, published in the December 31, 2008, Journal of Neuroscience, provides a glimpse of one biochemical pathway for risk-taking behavior and addiction. David Zald, Ph.D., and colleagues at Vanderbilt University found that these individuals have lower density of dopamine autoreceptors in the brain than control individuals. These autoreceptors normally “wipe up” the amount of free dopamine after it is initially released. With fewer autoreceptors, these people are more likely to have a greater “rush” after a dopamine-triggering stimulus, leading to greater temptation for seeking the next reward.

In addition, neuroimaging research in recent years undertaken by Nora Volkow, M.D., director of the National Institute on Drug Abuse, and other scientists suggests that long-term abuse of substances is associated with normal neurocircuits being compromised and the cognitive and executive functions being damaged (Psychiatric News, July 6, 2007).

The chicken-and-egg question is less important than what can be done to prevent or reverse the nature-environment feedback loop that pushes people into a downward spiral, according to Bickel. He suggested that public policies to promote a long-term view may help children and adolescents to extend their temporal horizon and reduce the discounting of future reward.

Brain-imaging research has shown that adolescence is a period of incomplete development of the executive function accompanied by peak activities in the nucleus accumbens, the brain's center for reward and pleasure. It is no coincidence that the majority of alcohol and substance use as well as smoking begin in teenage years and peak during adolescence and young adulthood, Bickel said. A similar time profile is seen in crime rates and impulsive, risk-taking behaviors such as unsafe sexual practices. “Risk is a biologically normal event among adolescents,” said Bickel, because they have a“ shortened temporal view of life events and choices.”

“It may be important to train executive and cognitive function in children or strengthen their capacity in this area,” Bickel said. Early training for children and adolescents specifically to extend their long-term outlook from a young age could make a big difference in preventing addiction and other risky, self-destructive behaviors, he suggested.

Fortifying executive function and restraining impulsivity are possible with active interventions. “The prefrontal cortex is very plastic,” Bickel said. It is possible to escape “the tyranny of small decisions” even for victims of addiction whose brain functions have already been eroded, he believes, pointing to the effectiveness of contingency management of addiction and replacement medications in tempering a hyperactive impulsivity and in treating addiction. Future medications may boost a person's hypoactive executive functions.

There is ongoing research into whether cognitive and executive functions can be restored in those with brain damage from traumatic brain injuries or schizophrenia. It remains to be seen whether neurological deficits related to substance use can be similarly reversed. ▪