This hypothesis is also consistent with studies using animal models and fMRI that have implicated the ventral striatum in discounting decision-making, a key region in the corticomesolimbic dopamine system (for reviews, see Cardinal, 2006; Carter, Meyer, & Huettel, 2010). For example, lesioning the ventral striatum in an animal model of discounting has been shown to induce significantly more impulsive delay and probability discounting (Cardinal & Howes, 2005; Cardinal, Pennicott, Sugathapala, Robbins, & Everitt, 2001). Similarly, compared to Fischer rats, Lewis rats have fewer dopamine D2 and D3 receptors in the striatum and also fewer dopamine transporters in these regions (Flores, Wood, Barbeau, Quirion, & Srivastava, 1998), paralleling the behavioral findings. Moreover, a recent meta-analysis of brain activity during fMRI discounting paradigms revealed the ventral striatum was consistently recruited (Carter et al., 2010). However, it is important to note that dopaminergic hypofunction as an underlying mechanism is not consistent with all the published findings (e.g., Koffarnus, Newman, Grundt, Rice, & Woods, 2011; Pine, Shiner, Seymour, & Dolan, 2010). Moreover, other neurotransmitter systems have also been implicated in delay discounting (Bevilacqua et
