The ability to flexibly change our behavior according to situations is important for living life, and many neuropsychiatric and developmental disorders are associated with a decline in this ability. In our laboratory, we train monkeys to switch the direction of eye movements in response to visual stimuli according to rules and investigate the neural activity in various brain regions. We have discovered many neurons in the thalamus, basal ganglia, and cerebellum that change their activity according to rules and have shown that inactivation of these regions with drugs increases impulsive behavior. More recently, we found that the functional coupling between the cerebral cortex and striatum dynamically change according to the situation, and that the cerebellum and frontal lobes are involved in error detection, which is necessary for updating behavioral strategies.