Human behavior is driven by two major systems: the goal-directed system and the habitual system. When the control of the goal-directed system over behavior weakens, the individual will exhibit habitual behaviors dominated by the habitual system. The learning theory of addiction suggests that compulsive drug-seeking—one of the core symptoms of drug addiction—is an example of abnormal habitual behavior: repetitive drug use weakens the goal-directed system and strengthens the habitual system, leading to stimulus-driven and outcome-insensitive drug-seeking behavior. Furthermore, systems’ abnormalities in addicts may also lead to habitual behaviors in other contexts. This theory has been supported by a large amount of animal research. However, in human studies, only research on cocaine abusers has supported the existence of abnormal habitual behavior in addicts. To verify the learning theory in addiction, this study utilized a self-report inventory and a behavioral paradigm to measure habitual behaviors in patients with methamphetamine use disorders (PwMUD), the largest addiction group in China. Further, by leveraging computational modeling to separate goal-directed and habitual systems, this study attempted to provide mechanistic evidence for the learning theory in addiction. Finally, correlational analysis and longitudinal follow-up are employed to explore the potential causal relationships between addiction with habitual behavior and its related systems.
Experiment 1 measured the habitual tendencies of PwMUD using the Creature of Habit Scale. Results indicated that PwMUD have a stronger tendency towards automaticity in daily habitual behavior compared to the healthy control group, but this abnormal automatization tendency showed no significant correlation with drug use characteristics, suggesting that abnormal habituation tendency in PwMUD mostly occurs unconsciously and may not be caused by drug use.
Experiment 2 examined the habitual behaviors of PwMUD using the probabilistic reversal learning task. The results showed that the habitual behaviors of methamphetamine addicts were more pronounced compared to the healthy control group during behavior paradigm completion. Computational modeling was used to separate the effects of the habitual and goal-directed systems, revealing that the habitual system in methamphetamine addicts was significantly stronger than that in healthy controls, while there was no significant difference in the goal-directed system. The abnormality of the habitual system also showed no significant correlation with drug use characteristics. Experiment 2 corroborated the results of Experiment 1 and suggested that the abnormal habitual behavior of methamphetamine addicts may stem from an enhanced habitual system and that this abnormality may not induced by drug abuse.
Experiment 3 conducted follow-up research on PwMUD during abstinence. The results showed no significant differences in habitual behaviors between short-term and long-term abstainers, but the habitual system was significantly stronger in short-term abstainers. However, the habitual system did not recover with abstention in addicts. The results of this experiment reconfirmed that abnormalities in the habituation system may not be caused by drug abuse.
In summary, this study discovered that the abnormal habitual behavior of PwMUD is evident across different contexts, and at the system level, there is only an abnormality in the habituation system. These abnormalities are unrelated to drug use and do not recover with abstention, thus it may be a potential risk factor for compulsive drugseeking.
Theoretically, this study is the first to validate the applicability of learning theory in addiction among PwMUD and tests its hypotheses of the causal relationship between habituation and addiction, contributing to the further refinement and improvement of learning theory in addiction. Methodologically, this study is the first to verify the goodness of fit of the Value-free model for real behavioral data across populations and over time, providing a new research tool for subsequent studies to separate the goaldirected system and habitual system. In terms of practical application, this study has found promising implications for the prevention and intervention of addiction in China.
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