Periodic Report Summary 2 - NEURALCHUNK (Neural bases of action chunking in basal ganglia subcircuits)
We can learn facts and events, but also how to perform new actions to obtain desirable outcomes. Many of these actions are complex sequences of movements. As we learn these sequences of movements, they become organized in modules or chinks. We found that as animals learn to organize individual movements in sequences or chunks, activity related to the initiation/termination of these chunks, or to the concatenation of the entire chunk, emerges in basal ganglia circuits. Furthermore, we uncovered that these different types of neural activity are differentially expressed in the so-called direct and indirect basal ganglia circuits. We found that both projection pathways are concurrently active during sequence initiation, but are differentially modulated during sequence performance. In order to determine the functional roles of these pathways, we used optogenetics to inhibit or activate with high temporal precision the activity of each striatal projection pathway simultaneously or independently, while animals initiated or executed a learned action sequence. We found that activity in both pathways is necessary for smooth initiation and execution of learned action sequences. These findings are not compatible with a rate model of competition between these pathways to promote more or less movement, but rather suggest a model where complementary activity patterns in these pathways are important to coordinate the different behavioral aspects required for smooth action initiation and execution. They also suggest a more complex organization of basal ganglia output than previously thought. Our preliminary data indicate that different cell types in Substantia Nigra pars reticulate target different midbrain and brainstem nuclei. These data shed new light into the mechanisms of action selection and action chunking.