Start/stop signals emerge in nigrostriatal circuits during sequence learning

Learning new action sequences subserves a plethora of different abilities such as escaping a predator, playing the piano, or producing fluent speech. Proper initiation and termination of each action sequence is critical for the organization of behaviour, and is compromised in nigrostriatal disorders like Parkinson’s and Huntington’s diseases. Using a self-paced operant task in which mice learn to perform a particular sequence of actions to obtain an outcome, we found neural activity in nigrostriatal circuits specifically signalling the initiation or the termination of each action sequence. This start/stop activity emerged during sequence learning, was specific for particular actions, and did not reflect interval timing, movement speed or action value. Furthermore, genetically altering the function of striatal circuits disrupted the development of start/stop activity and selectively impaired sequence learning. These results have important implications for understanding the functional organization of actions and the sequence initiation and termination impairments observed in basal ganglia disorders. Brain function: knowing when to stop When executing a behaviour, appropriate initiation and termination of the action sequence is vital; this process is compromised in nigrostriatal disorders such as Parkinson's and Huntington's disease. The neural mechanisms that underlie the learning and execution of a fixed behavioural pattern are not well understood, but Xin Jin and Rui Costa reveal start/stop neuronal activation patterns in basal ganglia circuits that emerge during task training. Mice can learn to perform a self-paced sequence of a set number of repetitive actions — lever pressing for a sucrose reward — very precisely. Using this task, Jin and Costa found that many neurons in nigrostriatal circuits can selectively signal the self-initiation or self-termination of each executed sequence. Genetic alteration of these circuits disrupted the start/stop activity and impaired performance, providing evidence for a causal relationship between the specific neuronal activity and task learning. The appropriate initiation and termination of behavioural action sequences is imperative, but the neural mechanisms underlying the learning and execution of fixed behavioural patterns are poorly understood. Here the authors reveal start/stop neuronal activity in basal ganglia circuits that emerge during task training in mice. Genetically altering these circuits disrupted the acti