4. Conclusions and future directions
Adaptive behavior in a fluctuating and unpredictable environment relies on flexible and accurate inhibition of prepotent responses, cognitive sets, and emotions. Various forms of inhibition have been described, including response inhibition (e.g., inhibition of prepotent or reflexive behavioral responses), cognitive inhibition (e.g., inhibition of irrelevant information), and emotional inhibition (e.g., inhibition of fear responses). The goal of the present review was to summarize and critically discuss the neural bases of inhibitory function through an integration of experimental tasks and approaches, including functional neuroimaging and lesion studies in humans and neurophysiological data in animals. Several important points emerged. First, although the prefrontal cortex plays a pivotal role in inhibitory functions, it is clear that specific facets of inhibition rely on partially non-overlapping neural pathways. Specifically, response inhibition, cognitive inhibition, and emotional inhibition are supported by a right-lateralized frontobasal ganglia circuitry, the OFC, and interactions between the VMPFC and the amygdala, respectively. Accordingly, from both a psychological and neurobiological perspective, inhibition is a heterogeneous construct, and findings from the present review support recent taxonomic approaches to inhibition-related functions (Friedman & Miyake, 2004; Nigg, 2000). Critically, recent advances in experimental psychology and affective neuroscience have allowed researchers to “dissect” inhibitory functions and identify its critical sub-components, opening new avenues for a more precise characterization of various disorders featuring impairments in inhibition-related processes, including ADHD (e.g., Nigg & Casey, 2005), schizophrenia (e.g., Fukushima et al., 1988), PTSD (e.g., Bremner et al., 1995; Rauch et al., 2006), depression (e.g., Goeleven, De Raedt, Baert, & Koster, 2006), and personality disorders (Nigg, Silk, Stavro, & Miller, 2005). In ADHD research, for example, this approach has allowed researchers to identify dysfunctions in response inhibition, but generally normative cognitive inhibition (see Nigg, 2000, for a review). Future research is warranted to evaluate whether dysfunctions in neural pathways subserving separable inhibition-related processes might serve as endophenotypes for various psychopathological conditions (Almasy & Blangero, 2001).
