Response inhibition and cognitive flexibility in schizophrenia with and without comorbid substance use disorder.

Addiction is a frequent comorbid disorder in schizophrenia related to dopaminergic dysfunction in fronto-subcortical circuits. These brain networks are relevant for both (executive) cognition and the neuropathology of schizophrenia. The aim of the present study was to elucidate whether response inhibition and cognitive flexibility - executive abilities relevant for achieving and maintaining abstinence - are differentially impaired in schizophrenia patients with or without comorbid substance use disorder. Patients suffering from major depression or alcoholism as well as healthy controls served as comparison groups. The ability to inhibit predominant response tendencies during response conflict and to efficiently shift the focus of attention between different task requirements was assessed by verbal and non-verbal cognitive tasks. Contrary to expectation, non-addicted schizophrenia patients showed the most pronounced executive function impairments relative to the control groups, affecting both response suppression and cognitive flexibility. Dual diagnosis patients did not differ significantly from non-addicted schizophrenia patients or from the alcoholic group, but were impaired at cognitive flexibility relative to the depression subgroup and healthy controls. Whether the relative preservation of response inhibition and cognitive flexibility in the dual disorder patients is due to high premorbid functioning, beneficial self-medication effects or compensatory brain activation remains to be elucidated. The relatively intact executive abilities in young, addicted schizophrenia patients might represent a beneficial resource for treatment strategies.