Laterality of Brain Activation for Risk Factors of Addiction

BACKGROUND: Laterality of brain activation is reported for tests of risk factors of addiction - impulsivity and craving - but authors rarely address the potential significance of those asymmetries. OBJECTIVE: The purpose of this study is to demonstrate this laterality and discuss its relevance to cognitive and neurophysiological asymmetries associated with drug abuse vulnerability in order to provide new insights for future research in drug abuse. METHOD: From published reports, brain areas of activation for two tests of response inhibition or craving for drugs of abuse were compiled from fMRI activation peaks and were tabulated for eight sections (octants) in each hemisphere. Percent asymmetries were calculated (R-L/R+L) across studies for each area. RESULTS: For impulsivity, most activation peaks favored the right hemisphere. Overall, the percent difference was 32% (Χ(2) = 16.026; p < 0.0001) with the greater asymmetry for anterior peaks (46.8%; Χ(2) = 17.329; p < 0.0001). The asymmetries for cue-induced craving were opposite, favoring the left hemisphere by 6.7% (Χ(2) = 4.028; p < 0.05). The consistency of left asymmetry was found for almost all drugs. For nicotine, studies where subjects were not allowed to smoke (deprived) prior to measurement had the same left hemisphere activation but those who smoked (satiated) before the fMRI measure showed right asymmetry. CONCLUSION: Brain activation studies demonstrate different left/right hemispheric contributions for impulsivity versus craving - factors related to addiction. Failure to take laterality into consideration is a missed opportunity in designing studies and gaining insight into the etiology of drug abuse and pathways for treatment.