"Chunking allows the brain to efficiently organize memories and actions. Although basal ganglia circuits have been implicated in action chunking, little is known about how individual elements are concatenated into a behavioral unit at the neuronal level. Using a differential reinforcement procedure where mice learn to chunk rapid action sequences, we uncovered neuronal activity encoding entire sequences as single actions in basal ganglia circuits. Besides activity signaling sequence initiation (start), we found neurons with sustained or inhibited activity throughout the execution of an entire sequence. These findings clearly show that basal ganglia circuits display neural activity related to the execution of whole action sequences, rather than unitary elements. Neurons with start, sustained and inhibited sequence-related activity were observed throughout the basal ganglia, namely in the main input (striatum), and output (substantia nigra reticulata) nuclei of the basal ganglia. However, the basal ganglia have different cell types/subcircuits linking input to output, the so called direct ves. indirect pathways. Furthermore, basal ganglia output projects to different target areas. Here we will 1) determine if these correlates of motor concatenation are differentially expressed in direct versus indirect basal ganglia pathways by optogenetic identification of cell types in the striatum and in vivo imaging, 2) test the necessity and sufficiency of these two pathways in action sequence initiation and performance, and 3) test if different basal ganglia output circuits express and mediate different aspects of action chunking. These experiments will dissect with unprecedented spatial and temporal precision the role of basal ganglia subcircuits in the initiation and performance of action chunks."