Cortico-basal ganglia loop circuits play important roles in the acquisition, performance, and switching of learning behaviors. The loop circuits are composed of some subloop structures mediating cognitive, motor, and limbic functions, and these substructures are considered to interact with each other and shift in response to environmental changes. Our research group used positron emission tomography (PET) for small animals to monitor the time course of brain activity during the learning processes, and found that the activity in the anterior dorsolateral striatum (aDLS) was increased at the early phase of auditory discrimination learning and then the activity of the posterior ventrolateral striatum (pVLS) was elevated at the late phase of the learning. Pharmacological inhibition of each subregion at the corresponding timing impaired the acquisition of learning, and multiunit recording of neural activity indicated event-related changes in the firing between the aDLS and pVLS at different learning phases. These data suggest that the aDLS and pVLS have distinct roles in different phases of auditory discrimination and that learning-related activity shift from the aDLS to the pVLS during the progress of learning. Although we previously reported the importance of striatal direct and indirect pathways in the dorsal striatum in reaction time modulation and response accuracy in stimulus discrimination learning, we need to reconsider the function of striatal projections in the subregions of the striatum. In this symposium, we will present the roles of striatal projections originating from each subregion in the acquisition of discrimination learning.