In the preceding section, we reviewed some of the evidence relating measures of action inhibition and waiting (collectively thought to measure aspects of inhibitory response control) to impulsivity and substance abuse behaviors. We also discussed the underlying neural circuitry and neuropharmacology of these measures. What is clear is that action inhibition and tests of waiting do not relate to two distinct patterns of underlying neural circuitry (Figure 1), nor do they exhibit two internally consistent but differentiable patterns of pharmacological response (Table 2). Instead, measures of both action inhibition and waiting can depend upon similar brain regions and neurotransmitter systems, while individually exhibiting idiosyncratic responses to manipulations of particular receptor subtypes. From a big picture perspective, this means that the conceptually-attractive organization of phenotypes related to inhibitory response control show neither strong within category consistency, nor strong between category differences in these biological substrates. Perhaps by comparing this collection of tasks to other even more conceptually distinctive measures of impulsivity (e.g., impulsive choice), further support for the validity of these categories will be revealed; this is the subject of the next two sections of this article.
