Researchers already know that alcohol dependence (AD) is strongly associated with impaired impulse control or, more precisely, the inability to choose large, delayed rewards rather than smaller but more immediate rewards. Findings from a study using functional magnetic resonance imaging (fMRI) to investigate the neural basis of impulsive choice among individuals with alcohol use disorders (AUDs) suggest that impulsive choice in AD may be the result of functional anomalies in widely distributed but interconnected brain regions that are involved in cognitive and emotional control.
The results were published in Alcoholism: Clinical & Experimental Research.
"Individuals with a higher AD score on questionnaires
that measure impulsivity - for example, 'I act without thinking' - are less
able to delay gratification, and are less able to inhibit responses," said Eric D. Claus, a research scientist with The Mind Research Network and first author of the study.
Impaired behavioural control
Given that impulsive choice in AUDs has been associated with impairment of
frontal cortical systems involved in behavioural control, Claus explained, this study was designed to examine the neural correlates of one specific aspect of
impulsivity, the ability to delay immediate gratification and instead
choose rewards in the future.
"We investigated this choice process in
individuals with alcohol use problems, ranging from alcohol abuse to
severe AD that required treatment," said Claus. "This is the
largest study to date that has investigated the neural correlates of
impulsive choice in AD, which enabled us to examine the full range of
AUDs, instead of only examining extreme group differences."
Claus and his colleagues examined 150 individuals (103 males, 47 females) with various degrees of alcohol use. All of the participants completed a delay discounting task - during which two options were presented, a small monetary (eg. $10) reward available immediately, or a larger monetary reward (eg. $30) available in time (eg. two weeks) - while undergoing fMRI. Impulsive choice was defined as the selection of the more immediate option.
"We showed two things," said Claus. "We replicated previous research by showing that AUD severity was associated with a greater tendency to discount future
rewards. In addition, we showed that when individuals with more
severe AUDs did delay gratification, they engaged the insula and
supplementary motor area - regions involved in emotional processing and
response conflict - to a greater degree than individuals with less
severe AUDs. In summary, these findings suggest that the dysfunction in these regions is graded and increases as a function of
AUD severity, rather than operating as an all-or-none function."
Delaying reward difficult for alcoholic brains
"This work showed that the brains of alcoholics don't behave all that differently from the brains of non-alcoholics during delay discounting, but that the alcoholic brain had to work harder when they chose the delayed reward," said Daniel W. Hommer, chief of the Section of Brain Electrophysiology & Imaging at the National Institute on Alcohol Abuse and Alcoholism. "Many different studies have shown similar results, that is, alcoholics have a greater increase in brain blood flow to perform the same task as non-alcoholics."
"The current study suggests that
the neural dysfunction underlying impulsive choice seems to increase
with AD severity," added Claus. "Now that we know that this neural dysfunction is associated with
impulsivity, the next steps are to determine whether this impulsivity
predates the onset of AD and whether neural measures of impulsivity
can predict who will respond best to particular types of treatment. Further, the particular neural dysfunction that we
observed indicates that individuals with more AD may be more impulsive
because their brain is averse to delay gratification, and not because it is rewarding to be impulsive. Clinicians might need to deal directly
with the aversion of choosing future benefits over immediate ones."
"The most important thing about this paper is that it leads you to question what people mean by impulsive behaviour and how should it be measured," said Hommer. "The field has defined increased discounting of time - failure to delay gratification - as a good measure of impulsiveness, but the results reported in this paper say, 'Wait a minute, delay discounting does not correspond to what is usually meant by impulsiveness.' Rather, brain activity during a delay discounting task looks more like how the brain responds during conflicted decision-making than it does during rapid, unconflicted choice of a highly valued goal." Hommer added that this sort of debate is important to researchers, forcing them to think more carefully about what they mean by impulsive choice. - (EurekAlert!, April 2011)
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