Abstract: Impaired self-control is one of the most prominent features of heroin use disorder (HUD). Even when individuals fully understand the long-term risks of drug use, they may be unable to resist the urge to use heroin. Clarifying the mechanisms underlying this loss of self-control is crucial for understanding the development and maintenance of addictive behavior and the risk of relapse, as well as for designing targeted interventions. Contemporary theories of addiction identify deficient behavioral inhibition as a key contributor to impaired self-control in individuals with HUD, a view supported by empirical evidence. However, previous studies have focused primarily on reactive inhibition, with relatively little attention to intentional inhibition. Reactive inhibition refers to the suppression of impulsive actions in response to external cues, whereas intentional inhibition refers to the suppression of impulsive actions initiated by an internal decision. The present study addressed this gap by systematically examining reactive and intentional inhibition in individuals with HUD.
Two experiments were conducted. Experiment 1 examined reactive and intentional inhibition at the behavioral level. Thirty-four abstinent individuals with HUD and 32 matched healthy controls (HCs) completed the recently developed Free Two-Choice Oddball Task while behavioral data were recorded. This task adds free-choice trials to a conventional two-choice oddball task comprising standard and deviant trials. Standard, deviant, and free-choice trials were presented at a ratio of 4:1:2, thereby yielding behavioral indices of both reactive and intentional inhibition. Experiment 2 sought to replicate the behavioral findings in an independent sample and characterize the temporal dynamics of these two forms of inhibition using event-related potentials (ERPs). Twenty-six abstinent individuals with HUD and 30 matched HCs completed the same task during EEG recording.
In Experiment 1, compared with the HC group, the HUD group showed impaired intentional inhibition, as reflected by a significantly greater response-time delay on free-choice trials. Experiment 2 provided converging evidence: the HUD group had a lower inhibition rate on free-choice trials. The ERP results further suggested weaker outcome-evaluation processing during intentional inhibition in the HUD group, as reflected by the absence of a significant late slow wave (LSW) difference between prepotent and non-prepotent responses. Contrary to our hypothesis and many previous findings, Experiment 2 also suggested enhanced reactive inhibition in the HUD group, as reflected by a smaller accuracy cost on forced-choice trials. The ERP findings were interpreted as indicating relatively stronger inhibitory-execution processing during reactive inhibition, indexed by a smaller P3 difference-wave amplitude.
Overall, the findings suggest a dissociable pattern of inhibition in individuals with HUD, characterized by relatively enhanced reactive inhibition but impaired intentional inhibition. These findings complement existing theories of addiction and may inform targeted rehabilitation interventions.

Key words: heroin use disorder, behavioral inhibition, reactive inhibition, intentional inhibition, ERP