An exciting new study published in Science this week attempts to answer the chicken-or-egg question pervasive in drug addiction research of, "Which comes first, drug use or brain abnormalities?" Dr. Karen Ersche from the University of Cambridge* approaches this question with a new perspective, investigating the biological siblings of dependent drug users. And as is the case with most seemingly dichotomous questions in science, the answer is: both.
Dr. Ersche's group studied 50 stimulant-dependent individuals, 50 of their healthy, non-dependent biological siblings, and 50 unrelated control volunteers on a barrage of cognitive tests, personality measures, and brain imaging techniques. Throughout the assessments, there was a striking pattern of similar responding between the drug users and their siblings, significantly differing in their results from the control participants. Specifically, drug users and their siblings were both significantly more impaired on the Stop Signal Reaction Time Task (SSRT), a test of inhibitory control that measures how well an individual can stop an ongoing response when triggered. Impulse control and inhibition are traits known to be impaired in drug-dependent individuals, and poor performance on the SSRT has previously been associated with an increased risk for drug abuse. However, these dysfunctions have long been debated as to whether they can be attributed to accumulated years of drug use and its effects on the brain, or are instead a predisposing factor that places an individual at an increased risk for drug dependence. In the current study, sibling participants performed as poorly on the SSRT as drug-dependent individuals, requiring more time to inhibit their actions. This would suggest that poor impulse control is a shared trait that is present in drug-dependent individuals before the onset of abuse. However, impaired inhibition is clearly not a determining variable, as dysfunction in the siblings did not lead to subsequent drug abuse or dependency.