Abstract
Time-stable personality traits, such as impulsivity and its relationship with functional and structural brain alterations, have gained much attention in the recent literature. Evidence from functional neuroimaging data implies an association between impulsivity and cortical as well as subcortical areas of the reward system. Discounting future rewards during impulsive decisions can be related to activation in the orbitofrontal cortex and striatum. Cortical structural changes in prefrontal regions have been found for introspective impulsivity measures. The present study focuses on brain regions associated with delay discounting to investigate structural manifestations of trait impulsivity. To test this, seventy subjects underwent structural magnetic resonance imaging (MRI) followed by a behavioral delay discounting task outside of the scanner to measure impulsivity with questions like: “Would you like to have 3€ immediately or 10€ in 5 days?”. The amount of smaller-but-sooner decisions was calculated and used as a measure of behavioral impulsivity. Furthermore, we estimated subject’s individual delay discounting parameter K reflecting the tendency to discount future rewards. Behaviorally, we found strong evidence in favor of a discounting utility model compared to a standard hyperbolic model of choice valuation. Neuronally, we focused on cortical and subcortical brain structure and investigated the association of behavioral impulsivity with delay discounting tendencies and gray matter volume. Voxel-based morphometric analyses showed positive correlations between delay discounting and gray matter volume in the striatum. Additional analyses using Freesurfer provided evidence for a positive correlation between delay discounting and gray matter volume of the caudate. Taken together, our study provides strong evidence for a structural manifestation of time-stable trait impulsivity in the human brain.
Highlights
Impulsivity is a stable psychometric trait defined as ‘‘a predisposition toward rapid, unplanned reactions to internal or external stimuli without regard to the negative consequences of these reactions to the impulsive individual or to other’’ (Moeller et al, 2001, p. 1784)
The present study investigates the relationship between gray matter volume in cortical and subcortical regions implicated in reward processing and individual impulsivity traits assessed implicitly using a DD paradigm adopted from Pine et al (2009) in healthy individuals
We found that the amount of SbS decisions as well as the estimated K-parameters from the winning model, which reflect behavioral impulsivity as well as individual tendencies to devalue future reward, correlated with striatal gray matter volume
Summary
Impulsivity is a stable psychometric trait defined as ‘‘a predisposition toward rapid, unplanned reactions to internal or external stimuli without regard to the negative consequences of these reactions to the impulsive individual or to other’’ (Moeller et al, 2001, p. 1784). High impulsivity is associated with strong discounting of (negative) future consequences as well as an inability to wait for future rewards (for a review, see Green and Myerson, 2004). A common task to measure individual impulsivity is the socalled delay-discounting (DD) task, in which subjects have to decide between a smaller-but-sooner (SbS) and a larger-but-later reward (e.g., ‘‘Would you like to have 3e immediately or 10e in 5 days?’’). High impulsivity is associated with enhanced DD, such that impulsive individuals reliably prefer SbS over larger-butlater rewards (Simpson and Vuchinich, 2000; Pine et al, 2010; Dalley et al, 2011; Broos et al, 2012). The DD paradigm is an implicit measure of impulsivity. Such measures have one major advantage over (explicit) introspective questionnaires, such as the Barratt impulsiveness scale: they provide a direct assessment of the specific processes associated with impulsivity and are harder to manipulate than self-report scales (Reynolds et al, 2006)
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